The Physics SPLATs

What are SPLATs? They are explained here.

The principles of matter and energy

• Matter and energy can be neither created nor destroyed, but matter can sometimes be converted into energy, and energy can sometimes be converted to matter.

• The energy of stars comes from a slow conversion of matter to energy as light atoms join to make slightly heavier atoms. Stars do this for billions of years.

• All change requires energy. Energy is what you need to do work. Energy can be electrical, chemical, or of other sorts. Energy is neither created nor destroyed.

• Energy is the cause of all change in all systems, everywhere. Energy can be converted from one form to another. Power is the rate of transfer of energy.

• Heat makes changes of state happen: the processes of boiling: condensing: melting, subliming and solidifying all happen when heat is added or taken away.

• Energy operates within in accordance with the laws of thermodynamics, and can only be understood, used and manipulated within those assumptions.

• Tensioning a bow or any other elastic item increases its potential energy, which is converted back to kinetic energy when it is released again.

• Falling weights, expanding gas and burning material all release energy, and can be used to do work. Most machines convert energy from one form to another.

• The first form of energy assistance for humans came with the use of draught animals, hence the name of the original unit for work done, the horsepower.

• We waste valuable domestic energy by bad housing design and a lack of insulation. This leads to a cost which is not usually borne by the wasters.

• In 1834, Benoit Paul Emile Clapeyron presented a formulation of the second law of thermodynamics, giving us for the first time, the notion of entropy.

• James Joule spent most of his time experimenting. The joule derives from his studies which showed that heat and mechanical movement are forms of the same thing.

• Joule's work proved that heat and movement were both forms of energy, and that was as good as a theoretical proof. It also put a measure on the equivalence.

• Joule's predecessors include Plato who saw 'heat and fire' come from 'impact and friction', and Count Rumford, who studied the heat produced in cannon-boring.

• Maxwell's demon is a fascinating paradox which implied that there might be a way to beat the laws of thermodynamics and achieve perpetual motion. You can't.

• The Maxwell's demon paradox is based on the fact that some gas molecules have more energy than others: if a demon could separate them, this would yield energy.

• When James Clerk Maxwell proposed the paradox, he would have known that there must be an answer, but he could not immediately see what it would be.

• The answer to Maxwell's demon is that the energy needed to separate the molecules would be greater than the energy that could be obtained from the differences. 

The principles of wise energy use

• Energy can be stored in many ways, and it is often convenient to think of it as potential energy when attempting to understand what is happening.

• Alternative energy is widely seen as important to our future but some known alternative energy sources are carbon-neutral, or close to it, while others are not.

• Many alternative energy sources carry with them a severe carbon cost in making steel, concrete or other materials, or in transport and feed stock for sprays.

• In 1609, an attempt was made to harness tidal power in the Bay of Fundy, the first time that tidal power had been brought into use, but it was unsuccessful.

• Energy has been the cause of social change. The developed world lives at 7.5 kilowatts, the undeveloped world at 1 kilowatt. This needs to change.

• The energy we need to live at 7.5 kilowatts involves burning enough fossil fuel to produce 5.5 tonnes of carbon dioxide for each person each year.

• Incandescent lamps are inefficient light producers, because much of the electricity they use is converted to heat first and released as waste heat.

• Energy conservation will prolong the life of the earth as we know it, and of the life forms on the planet, and so prolong the survival of humanity.

• Fossil fuels like coal, gas, oil and peat contain stored solar energy from the past. They can be used far more rapidly than replacements can be formed.

• Most renewable energy requires a certain energy input, but some renewable energy sources consume more energy (mostly fossil fuels) than they deliver.

• In reality, most alternative energy systems carry a major carbon cost in the manufacture of the components or in lost opportunities like lost photosynthesis.

• Some forms of energy are renewable, as we see it in our time frame. In that sense, solar energy is renewable, although in a longer time frame, it is not.

• In 1865 W. S. Jevons warned that coal supplies would eventually run out, though he exaggerated estimates of use, and did not allow for oil being used as a fuel.

• The best solar conversion systems now convert 9% of sunlight to hydrogen, which is getting close to the generally assumed break-even point of 10%.

• Geothermal energy draws heat from hot water and rock, deep underground, and is effectively renewable in our time frame, although not in a longer time frame.

• Alcohol which is made from sugar or corn, vegetable oils, draught animals, biologically generated hydrogen and biogas are all renewable energy sources.

• Wind power is a form of alternative or renewable energy that relies on the sun for the energy input side. Wind generators require backup, as they stop at times.

• Photovoltaic cells convert sunlight to electrical energy but there are problems in getting satisfactorily efficient conversion in mass-produced modules.

• There seems to be a practical limit of 25% on the efficiency of photovoltaic cells. There is no good theoretical reason for this limit that anybody can see.

• Hydrogen makes a clean and effective fuel, but it needs to come from somewhere in the first place, and it presents special storage and transport problems.

• Fuel cells convert the chemical energy of fuel and an oxidant to electrical energy cleanly, but while they show promise, they are not yet fully developed.

• At the moment, more people die around the world as a result of mining coal each year than were killed in nuclear accidents in the whole of the past ten years.

The principles of nuclear energy

• Some nuclei are unstable as a result of an imbalance in the numbers of neutrons and protons in the nucleus. The radioactive decays end when stability is reached

• Radioactivity can be natural or artificial: one important use of nuclear reactors is in making radioisotopes which have important medical applications.

• Radioactive nuclei all have a half-life, the time in which half the nuclei in a sample decay. The half-life of any unstable nucleus can be determined.

• More unstable nuclei have shorter half-lives: the half-life depends on the probability that a given nucleus will undergo fission within a given time.

• Nuclear fusion involves two light nuclei being combined into a heavier nucleus with less mass than the original nuclei and releasing energy as a result.

• Nuclear fission involves a heavy nucleus forming two nuclei lighter (in total) than the original nucleus and releasing energy equivalent to the lost mass.

• As a general rule, the nuclei in the centre of the periodic table have less energy available because energy was released during their formation.

• The mass deficiency at the end of a nuclear reaction is linked to the energy released in accordance with the much-misquoted "e equals mc squared".

• A critical mass is an amount of fissile material formed so that each fission generates products (usually neutrons) that trigger, on average, one more fission.

• The amount of fissile material needed to make a critical mass is least when the fissile material is in the shape of a sphere, as fewer neutrons escape.

• A nuclear chain reaction requires a critical mass of fissile material in a small space, and control systems which need to be highly reliable, except in a bomb.

• Beta particles are energetic electrons ejected from the nucleus during nuclear decay, and they indicate that a neutron has become a proton in the nucleus.

• Alpha particles are the most massive form of radiation. Each alpha particle is made up of two neutrons and two protons, ejected from a fissioning nucleus.

• Radioactivity involves the release of energy, and the release comes in three forms, originally simply called alpha, beta and gamma radiation.

• Gamma radiation is a form of electromagnetic radiation, rather like X-rays, which is emitted as a way of losing energy during some forms of nuclear decay.

• Nuclear energy produces no greenhouse emissions: the damage from continuing greenhouse emissions can be predicted, unlike the damage from nuclear reactors.

• The energetic radiation coming from radioactive material can be harmful to living cells, depending on the radiation produced, and how close the source gets.

• Most nuclear accidents have been caused by poor training and careless operation of facilities and operations by people who feel over-confident with their tasks.

• Nuclear waste can be classified as high, medium or low-grade waste, depending on its half-life, the products of decay and how much of it there is.

• Some nuclear waste will need to be stored safely for many thousands of years, while the radioactive products break down. It might harm our descendants, one day.

• Burning fossil fuels to obtain the energy we all demand is now considered to cause global warming, and that will, without a doubt, harm our descendants, soon.

• Some spent nuclear fuel rods can be recycled to produce new fuel rods. The recycling processes need to be managed and supervised with very great care.

• Nuclear weapons bring a variety of technical problems in maintenance and storage as the fissile materials in them slowly decay, and need to be refurbished.

• Burning one gram of hydrogen gas in the normal way with oxygen provides the energy that is needed to light a 100 watt bulb for about 40 minutes only.

• If the same gram of hydrogen could be converted completely to energy by some form of nuclear reaction, it would power a 100 watt bulb for 56,000 years.

• In 1939, Otto Hahn and Fritz Strassman bombarded uranium salts with thermal neutrons and found barium among the reaction products, indicating fission.

• In 1939, Rudolf Peierls and Otto Frisch worked out the critical mass and theory of the uranium-235 fission bomb, with a critical mass of about 10 kilograms.

• In 1976, Shlyakhter used samarium ratios from a 2 billion-year-old natural fission reactor in Gabon to show the laws of physics have not changed in that time.

• In 1939, Teller, Szilard and Einstein, sent a warning letter to President Roosevelt about the possibilities of the atomic bomb, starting the Manhattan Project.

• In 1932, Leo Szilard realized that nuclear chain reactions may be possible, and by 1934, he had filed a patent on the principles, and gave it to the War Office.

• Early on, Frederick Soddy calculated that the energy liberated in the complete change of 28 grams of radium would be equal to that from burning 10 tons of coal. 

The principles of dynamics in physics

• Raising an object against gravity increases its potential energy, which can be recovered by letting it go, when the potential energy becomes kinetic energy.

• Mass and weight are different: objects in free fall have no weight, but they still have mass, and strictly speaking, they are weightless but not massless.

• Every solid body which has mass has a centre of gravity, a point which sometimes lies outside of the body itself, if the body has an irregular shape.

• When a force operates on a body, it accelerates. This is a change in velocity, involving either speed or direction, so moving in a circle is acceleration.

• Aristotle believed that if a small stone and a large stone were dropped from a tower, the large stone would fall faster than the small stone, which was wrong.

• Around 1350, Jean Buridan and Nicolas Oresme said, contrary to Aristotle, that unequal masses would fall at the same speed, as Galileo Galilei argued later.

• Galileo Galilei may have dropped rocks of different sizes, but also described what we would now call a perfectly good thought experiment to give the answer.

• In 1604 Galileo Galilei showed that the distance travelled by a freely falling object increases as the square of time during which it has been falling.

• Velocity is a vector quantity with both a speed and a direction, so acceleration may involve a change in either or both. Circular motion is accelerated.

• The study of flight is called aerodynamics. Flight depends on the interactions of forces produced by solid surfaces moving with respect to the atmosphere.

• The law of conservation of momentum describes what happens when moving bodies interact in a collision, the main effect being that momentum is conserved.

• Much of modern technology depends on devices that convert energy from one form to another. Usually one of those forms is electrical or chemical.

• All movement is subject to three laws called Newton's laws of motion which relate velocity, force, time, displacement, acceleration and mass to each other.

• Newton's 1st law: Every body continues in a state of rest, or uniform motion in a straight line, unless made to change that state by forces impressed upon it.

• Newton's 2nd law: The change of motion is proportional to the force on it, and is made in the direction of the straight line in which the force is impressed.

• Newton's 3rd law: To every action there is an equal and opposite reaction; the mutual actions of two bodies on each other are equal, and directed opposite ways.

• The process of adding vectors to one another may be carried out with the parallelogram of forces, or mathematically, whichever is more convenient.

• Perpetual motion is physically impossible, mainly because of frictional losses in moving parts and the transfer of energy to air surrounding the machine. 

  The principles of force in physics

• The four known and recognized forces of nature are the electromagnetic force, the gravitational force, the strong nuclear force and the weak nuclear force.

• The gravitational force exerted on a standard mass by an object depends on its mass, and on the square of the distance between their two centres of mass.

• The force of gravity obeys the inverse square law, and gravitational forces may be calculated using Newton's law of gravitation which is based on this.

• In 1674 Robert Hooke attempted to explain planetary motion as a balance of centrifugal force and gravitational attraction, but this failed to stand up.

• In 1665 Isaac Newton deduced the inverse-square gravitational force law from the 'falling' of the moon, rather than the apple of all the mythological accounts.

• In 1680 Isaac Newton demonstrated that the operation of the inverse square law on gravity leads directly to the formation of elliptical orbits in space.

• Gravitation is one of the four forces of nature. Although it may seem strong to us as we experience the force, it is a weak force which acts everywhere.

• In 1798 Henry Cavendish measured the gravitational constant with John Michell's torsion balance and from that, was able to determine the mass of the Earth.

• Based on the value of G and the known size of the Earth, Cavendish was able to estimate the density of the Earth at 5.48, close to the current value of 5.52.

• In 1749, Pierre Bouguer attempted to estimate the value of G, the Universal Gravitational Constant, using a mountain as an attracting mass, but it was too weak.

• The centrifugal force is a fictitious force, but there really is a force called the centripetal force. Either can be used in explanations and calculations.

• Under extreme conditions, the operations of gravity may lead to the formation of a black hole, a concentration of mass so great that even light cannot escape.

• Parabolic flight may be used to simulate 'weightlessness' near the Earth's surface, for short periods of time as an aircraft slows, turns over and falls freely.

• A centrifuge may be used to simulate high gravitational forces, relying on the accelerational forces used to keep rotating material moving in a circle.

• The understanding of the pendulum depends on understanding the forces involved, in particular, the restoring forces that operate in all forms of the pendulum. 

  The principles of air and pressure

• The air we breathe is made mainly of two gases. One is oxygen, which most living things need. There are also other gases in small amounts, and water vapour.

• Air has weight, and it exerts a pressure on us. This air pressure can be measured, and it decreases with altitude and as the weather changes.

• In a mixture of gases, each gas exerts a partial pressure, equal to the pressure it would exert on the container if it alone filled the container.

• Pressure measurement in the atmosphere can be done in different ways: with a pressure gauge or with a barometer, but each relies on air exerting pressure.

• Places on the map with the same (sea level-corrected) air pressure are linked by lines called isobars to reveal weather patterns that involve pressure changes.

• In 1632, Galileo Galilei said that he had been told by a workman that there was no way suction could raise water a hair's breadth more than eighteen cubits.

• You cannot suck air up more than 10 metres or siphon over a rise greater than 10 metres. This limit is imposed by the pressure of the atmosphere.

• Warm air is less dense than cooler air, so it rises. As it rises to areas of lower pressure, it expands and so gets cooler. This is called adiabatic cooling.

• In 1646, Blaise Pascal made a barometer using a mixture of water and wine, which rose under atmospheric pressure, to twenty cubits, more than Galileo's report.

• In 1648, Blaise Pascal took his wine and water barometer up a mountain and discovered that the atmospheric pressure varies with altitude in a systematic way.

• In 1660, Otto von Guericke used a barometer and the trends shown (whether it was rising, falling, and the rate) in order to develop forecasts of future weather.

• Flotation effects happen when a solid is placed in a more dense fluid and displaces some of it. The law of flotation depends on Archimedes' principle.

• When one object floats in a fluid it floats because the mass of fluid it displaces, pushes out of the way, is equal to the mass of the floating object.

• A hot air balloon floats in the air because the total mass of the balloon and the air in it is less than the mass of cool surrounding air displaced by it.

• An object only floats in a fluid when it is less dense. A steel ship floats in water because its overall average density is less than the density of water.

• Before the launch of the first iron ship, many people predicted that it would plummet to the bottom of the ocean, because iron always sinks in water.

• You can make a vacuum with a suitable air pump, or by boiling water to drive out air, then sealing and condensing the water vapour. Other ways exist as well.

• Getting a good vacuum requires a vacuum pump: the Magdeburg hemispheres were an early demonstration that a vacuum could exist, even if it was thought unnatural.

• It is very difficult to obtain a good vacuum, and gases could not really be discovered until a vacuum could be created after an effective pump was invented.

• In 1663 Blaise Pascal proposed isotropy of pressure: pressure acts equally in all directions, a rule which we know today better as Pascal's principle.

• Fluids exert pressure, and the pressure exerted obeys Pascal's principle that the pressure applied is transmitted equally and in all directions.

• Pressurized fluids can be used in many ways. The hydraulic press is an application of Pascal's principle with the advantage that the force direction changes.

• Gas bubbles appear in carbonated drinks when the seal is broken (opened), as the solubility of gases in the blood depends on pressure, which is eased.

• Divers can get the 'bends' as bubbles form if they come up from great depth too fast, as the solubility of gases like nitrogen in the blood depends on pressure.

• In 1738 Charles Dangeau de Labelye developed the caisson, a pressure cabinet to allow workers to operate beneath the water, to build a bridge at Westminster.

• In 1686, Edmond Halley worked out the theory of the trade winds, established the relationship between barometric pressure and height above sea level.

• Robert Boyle's investigations that led to what we now call Boyle's law relied on Robert Hooke constructing an effective air pump for the experiments.

• In 1727, in his Vegetable Staticks, Stephen Hales showed that air was an element which took part in chemical reactions, that it could be 'fixed' in some way.

• In 1771, Joseph Priestley showed by experiment that air in which a candle had burned could be restored by a sprig of mint to let another candle burn in it. 

The principles of surface tension

• Surface tension gives rise to capillary action and this explains why water will soak into a rock, and many other effects, including 'wetting'.

• Surface tension affects many animals, but it usually has a greater effect on small animals which encounter greater pro rata forces on their smaller mass.

• Surface tension effects give rise to the meniscus at a liquid boundary, the curve being shaped by the relative attractions of the molecules for each other.

• Two-dimensional bubble films will always contract and take up a shape to minimize their surface areas in the same way that a three-dimensional bubble does.

• The pressure inside the bubble is greater than the pressure outside, due to the compressive effects of surface tension in the bubble on the air inside.

• Bubbles take the shape which minimizes their surface area: when they are unconstrained, this will normally be a sphere, but other shapes are possible. 

The principles of the nature of the electron

• In 1834, Michael Faraday used the expression 'atoms of electricity', generally taken now as the earliest reference to what we today call the electron.

• George Johnstone Stoney coined the name 'electron' for the unit of electric charge, in 1874. Later, this name was transferred to the cathode ray particles.

• If scientists could measure the charge/mass ratio (e/m) for an electron, that was proof that there was really something fitting the name 'atom of electricity'.

• In 1890, Arthur Schuster measured the e/m ratio for electrons, and found the value was about 1000 times the value for a hydrogen ion. He dismissed it as wrong.

• In 1895, Jean Perrin showed that cathode rays are negative particles, rather than being a form of electromagnetic radiation, as German scientists believed.

• Jean Perrin showed that cathode rays had negative charge, leading the way for J. J. Thomson to measure the ratio e/m, and prove that electrons were particles.

• In 1897, both Walter Kaufmann and J. J. Thomson carried out separate measurements of the electron charge to mass ratio by deflection of cathode rays.

• When J J Thomson measured the charge/mass ratio of the electron, e/m, this proved once and for all that electrons were particles, not electromagnetic radiation.

• R A Millikan succeeded in studying the behaviour of charged oil drops in an electric field, and so deduced the charge on the electron, and that it was uniform.

• In 1924, Louis de Broglie more or less suggested that electrons might be in some ways like waves. Actually, he said that the particles were guided by waves.

• In 1927, Clinton Davisson, Lester Germer, and G. P. Thomson demonstrated electron diffraction by a crystal, showing that electrons have wavelike properties. 

Electricity

• An electric charge on an object is the result of there being electrons removed or added to the object. Friction on a non-conductor can cause this loss or gain.

• In 1786 Luigi Galvani discovered 'animal electricity' and proposed a somewhat confused idea that animal bodies are storehouses of electricity.

• In 1774, the existence of electric eels in South Carolina was described to the Royal Society in considerable detail, introducing the idea of animal electricity.

• Electric current can be generated in a number of ways, some physical (generators), some chemical (batteries), some even biological (electric eels).

• Electricity and magnetism are related: an electric current makes a magnetic field, and a changing electric current makes a changing magnetic field.

• In 1820 Ampère measured the force on an electric current in a magnetic field and Oersted reported that a current in a wire can deflect a compass needle.

• In 1820, Hans Oersted had found that an electric current produced a magnetic field, setting the scene for the development of electric relays and electromagnets.

• In 1821, Michael Faraday discovered both the principle of the electric motor and the generator, and also plotted the magnetic field around a conductor.

• In 1833, Heinrich Lenz stated that an induced current in a closed conducting loop will appear in such a direction that it opposes the change that produced it.

• An electromagnet is formed when an electrical current flows in a coiled conductor surrounding soft iron, aligning the magnetic domains in the soft iron core.

• In 1847, Werner von Siemens suggested the use of gutta-percha as insulation on wiring to protect it from moisture, essential to later electricity transmission.

• An electric current may be generated by the Seebeck effect, where a voltage develops across the junction of two metals or alloys at different temperatures.

• In 1901, Hertha Ayrton had a paper on electric arcs read to the Royal Society by a male friend, as women were not allowed, at that time, to read papers.

• A potential difference may be generated by the piezoelectric effect, when pressure is applied to a crystal. A PD applied to a crystal produces a deformation.  

The principles of electrostatics

• Static electricity shows attraction and repulsion: the forces obey the inverse square law, the force being inversely proportional to the square of the distance.

• Electrostatic charges can accumulate on the outside of insulators, but the charges cannot move freely over the surface, or through the insulators.

• A charge which exists on an object is called a static charge because it does not move, but it is still capable of moving if a path is available.

• Static electricity is most easily generated by friction, but it may also be generated by induction with an electrophorus which has been charged by friction.

• An electrostatic charge may be induced in conducting material: this is the basis of the operation of the electrophorus, an early electrostatic device.

• Objects like a balloon, comb, and other common objects made from insulating materials can be charged, simply by rubbing them against another insulator.

• A Leyden jar was an early form of capacitor, a device for holding static charge and allowing crude experiments on the flow of brief currents.

• A capacitor can be used to store a static charge and the capacitance of a capacitor depends on the dielectric of the medium separating the two charges.

• Lightning is caused by the build-up of static charge, it carries a great deal of energy, and it has good and bad effects, fixing nitrogen and starting fires

• Lightning is a form of static electricity, and thunder is caused by air being heated and expanding suddenly along the flash when the charge breaks down.

• A Faraday cage is a metal screen that can protect somebody from lightning because it isolates them from charge on the outside. The cages also block radio waves.

• In 1660 Otto von Guericke developed an electrostatic machine to generate charge. It was made by charging a ball of sulfur with static electricity.

• In 1675, Jean Picard was carrying a barometer through the darkened streets of Paris, when he noticed a faint glow in the empty space above the mercury.

• In 1702 Francis Hauksbee noticed rarefied air glows during an electrical discharge through a vacuum, and showed this to the Royal Society the following year.

• In 1729, Stephen Gray used string to send an electrostatic signal in a barn, over a distance of 293 feet along a fine thread, the first telegraph.

• In 1746 Abbé Nollet showed that electricity travels at an apparently instantaneous speed around a mile-circumference circle of monks, linked to a Leyden jar.

• In 1775, a Royal Navy gunpowder magazine suffered a lightning strike at Purfleet in England, in spite of the fact that it was fitted with lightning rods.

• We know now that static charge accumulates better at the point of a lightning rod, but the Purfleet strike was used to claim that knobby ends were better.

• In 1785, Charles Coulomb showed that electrostatic repulsion and attraction are related to the product of the charges and the inverse square of the distance.

• If Benjamin Franklin ever flew a kite in a thunderstorm to attract lightning, he did so in 1749: he certainly wrote about doing so in that year, and in 1752.

• The next person to fly a kite in a thunderstorm after Franklin published his account was killed. Sometimes, a thought experiment or a better design is needed. 

The principles of electrodynamics

• An electric current is a flow of electrons along a conductor which happens when there is a higher charge at one end of the conductor, compared with the other.

• When an electric current flows in a conductor, there will be an associated magnetic force near the conductor, and this force can be used in a variety of ways.

• An electrical current requires a potential difference and a conductor or conducting medium through which electrons can flow with little interference.

• A current may be thought of as a flow of electrons in one direction, or a flow of holes in the other. Each can be used to understand electric currents.

• A varying magnetic force or field near a conductor makes electrons in the conductor move, producing an electric current. This is the basis of the generator.

• Dynamic electricity (an ordinary electric current) may be generated by electromagnetic induction, a magnetic field inducing a flow of electrons in a conductor.

• An electrical current may be generated by chemical reactions in a 'dry cell', a form of electrochemical cell which is not entirely dry, but has no loose liquid.

• A current may be generated by electromagnetic induction, when there is relative motion of a conductor and a magnetic field: it does not matter which one moves.

• A dynamo produces direct current. While many devices need DC to operate, it is generally easier to transmit electrical power as alternating current.

• Electrical currents may be alternating current or direct current. In alternating current, the peak voltage is greater than the average voltage.

• Alternating currents are easier to change the voltage of, using a transformer to step the voltage up or down. This is why domestic supplies are all AC.

• Metals make good electrical conductors, non-metals can make good insulators: this related to the availability of free electrons in their structures.

• Electric circuits need to be closed before a current will flow in the circuit: a switch can open and shut a circuit, and switches can be of many sorts.

• Electrical systems are often protected by fuses and circuit breakers, which are designed to stop overload that might burn out expensive wiring and cause fires.

• Many electrical systems are fitted with sensitive detectors that cut the current in the event of any 'leakage to earth', which usually indicates a fault.

• A galvanometer can be used to detect a very small electrical current, using a coil to produce a small magnetic field that interacts with a permanent magnet.

• Electrical currents can be measured: the unit of current is the ampere, and current is measured with a modified galvanometer called an ammeter.

• Potential difference can be measured: the unit of potential is the volt, and voltage is measured with a modified galvanometer called a voltmeter.

• Wattmeters/Joule meters measure the energy transferred, and are more useful when it comes to charging consumers for the electricity they use.

• In 1911, Heike Kammerlingh Onnes discovered superconductivity in extremely cold conductors, having mastered the art of attaining low temperatures.

• In 1957, John Bardeen, Leon Cooper and Robert Schrieffer develop the BCS theory of superconductivity to explain why some substances are superconducting.

• Until superconductors are found that operate at room temperature (or above the boiling point of nitrogen), superconductivity will be of little practical use.

• If there is a magnetic field near a conductor in which a current is flowing, there will be a force on the conductor. This is the basis of the electric motor. 

The principles of electronics

• A capacitor can store a static charge. A Leyden jar was an early form of capacitor, charged by electrostatic means. The unit of capacitance is the farad.

• Conductors may be in a parallel circuit or in a series circuit. At the junction of any circuit, all electric currents must obey Kirchhoff's laws.

• Resistance is measured in ohms, conductance is measured in mhos, each refers to a conductor's capacity to allow electrons to pass through it.

• A potentiometer (or a rheostat) has a variable resistance because a slider can tap in at various points on what is generally a uniform resistor.

• A light dependent resistor can be used to measure light intensity, because the resistance it causes to a current is proportional to the light falling on it.

• A light dependent resistor can be used to measure light intensity in a uniform way, since the resistance varies with the intensity of the incident light.

• A reed switch uses a magnetic effect, changing from one state (with the switch open or closed) to the other when a magnet is moved near it or away from it.

• An image intensifier is a device that allows us to see in what is effectively the dark, by taking the few available photons and amplifying them.

• Electrons can flow through a vacuum, and this is the basis of the thermionic valve, where electrons are 'boiled off' a hot cathode and then travel to an anode.

• Thomas Edison made just one real scientific discovery, the 'Edison effect', which is the key to the thermionic valve. He patented it, but never used it.

• An integrated circuit or chip contains many separate semiconductor devices, all of them incorporated into a single unit, made in a single process.

• Modern electronics relies on semiconductor devices: a diode only allows current to flow in one direction, a transistor can act as a switch or an amplifier.

• The strength of a signal may be increased with an amplifier, a circuit designed for that purpose, and using either thermionic valves or transistors.

• A diode only allows current to pass in one direction, and a set of diodes may be arranged to make a full-wave rectifier, as in a conventional power pack.

• In 1947, the transistor effect was noted, and by 1948, William Shockley, Walter Brattain, and John Bardeen had made and proven the first working transistor. 

The principles of magnetism

• A magnet has the power of attracting magnetic material like iron. A given pole of a magnet will attract an unlike pole and repel a like pole.

• Magnetic forces of attraction and repulsion pass through wood, paper and flesh without any measurable effect, and can operate on the other side.

• Many aspects of magnetism can be explained by lines of force. Lines of force do not exist, but they are a convenient 'fiction' that we continue to use.

• A magnet can induce magnetism in a piece of iron if it is manipulated properly. The most common method of magnetizing iron is by stroking to align the domains.

• A moving magnetic field makes a current flow in a conductor. So does a changing magnetic field. This is the basis of electromagnetic induction.

• The Earth's magnetic field experiences polar reversals from time to time, as shown by 'frozen' magnetic particles in igneous rocks which remain as a record.

• Many animals have a magnetic sense. Birds seem to use the Earth's magnetic field to navigate, from experiments where they are fitted with magnets or weights.

• The most recent reversal of the Earth's magnetic field, known as the 'Jaramillo Event' is calculated to have happened somewhere around 900,000 years ago.

• In 1832, Karl Gauss, whose name is now attached to one of the basic units of magnetism, put together a consistent set of units for use with magnetic effects.

• In 1948, Alpher, Bethe, and Gamow considered a rapidly expanding and cooling universe and suggested the elements were produced by rapid neutron capture.

• In 1895, Pierre Curie described how magnetization is proportional to magnetic field strength, and how magnetism is lost at high temperature, the Curie point.

• In 1750 John Michell stated that the inverse square law applied also to magnetic fields, and described magnetic induction, extending the operation of the law.

• A paramagnetic molecule is attracted by a magnetic field, while a diamagnetic molecule is repelled by a magnetic field. 

 The principles of radiation

• To really understand the inner workings of electromagnetic radiation, we need to understand blackbody radiation, which is covered under 'quantum physics'.

• Many materials that appear opaque to us when we rely on the visible spectrum, are transparent at other wavelengths such as X-rays and ultraviolet radiation.

• In 1845, Michael Faraday found that light propagation in a material can be influenced by external magnetic fields (rotation of polarized light by magnetism).

• In 1850, Michael Faraday experimented to find the link between gravity and electromagnetism, but all his efforts failed, a situation that continues today.

• In 1861, James Clerk Maxwell set out his four laws of electromagnetic fields, proving mathematically that there was such a thing as electromagnetic radiation.

• In 1864, James Clerk Maxwell published on his dynamical theory of the electromagnetic field, and his equations of electromagnetic wave propagation in the ether.

• In 1873, James Clerk Maxwell published his Treatise on electricity and described the electromagnetic nature of light and predicted radio waves.

• In 1883, George FitzGerald developed a theory of radio transmission, and explained how to create electromagnetic waves such as radio waves, but did not do so.

• In 1894, Heinrich Hertz reported that radio waves travel at speed of light and can be both refracted and polarized. He had measured their wavelength in 1888.

• In 1879, Joseph Stefan pointed out that the total radiant flux from a black-body is always proportional to the fourth power of its temperature.

• In November 1895, Wilhelm Röntgen discovered some of the effects of X-rays, and spent almost two months identifying as many of their other effects as possible.

• Cherenkov radiation is produced as bright flashes when high speed particles enter a medium, travelling faster than the speed of light in that medium. 

The principles of heat

• All matter has a capacity to hold heat, measured as its specific heat or heat capacity. Heat capacity and temperature are not the same thing at all.

• Heat makes changes happen, including expansion and contraction. The varying expansion coefficients of materials can be measured and used in many ways.

• Heat generally increases the chemical rate of reaction, and it can also cause pyrolysis, the breakdown of compounds under the application of heat.

• Heat at the junction of two metals causes a potential difference. The thermocouple formed can be used to generate electricity or to measure temperature.

• Heat is a form of energy and travels mainly as infrared radiation, but also by convection and conduction. Heat may be converted to other forms of energy.

• In 1849, William Thomson ( Lord Kelvin ) coined the term 'thermodynamics' to describe the science of heat flow which is basic to the scientific study of energy.

• The movement of heat happens in accordance with the laws of thermodynamics, especially the second law, which means heat goes from warmer to cooler areas.

• Sufficient heat will make a change of state happen: boiling, condensing, melting, solidifying (freezing), sublimation, to form gases, liquids and solids.

• The rate at which bodies cool follows Newton's law of cooling. As the difference between an object and its environment gets less, the rate of cooling slows.

• When matter is heated, energy is gained and the molecules move or vibrate faster. As matter cools, the molecules lose energy and move or vibrate more slowly.

• Heat generally travels from hot to cold. Convection occurs in gases and liquids. Conduction occurs in solids, liquids and gases. Radiation can occur in a vacuum

• Isaac Newton showed that the rate of cooling in a hot body was proportional to the difference between it and its surroundings: that hot things cool faster.

• Metals are usually good conductors of heat, non-metals are usually poor conductors. Conduction is the transfer of energy from one atom or molecule to the next.

• A higher temperature means more energy has been stored in a body than when it was a lower temperature. Temperature can be measured with a thermometer.

• Refrigeration depends on adiabatic cooling to move heat from one place to another, pumping it out of the cold compartment. Cold is an absence of heat.

• There is a lowest possible temperature, called absolute zero. Matter which is at absolute zero on the Kelvin scale is completely motionless in all dimensions.

• In 1761 Joseph Black discovered that ice absorbs heat without changing temperature when melting, and outlined the effects of latent and specific heats.

• Latent heat is absorbed or released during a change of state. This is why steam at 100º C contains more heat than an equal mass of water at 100º C.

• In 1798, Count Rumford reported that mechanical energy could be converted to heat when cannon barrels were bored with drills, whether they were blunt or sharp.

• By careful measurement, Count Rumford was able to establish that if heat had any mass, then a single calorie had to be less than 0.000013 milligrams.

• William Herschel used a thermometer to detect heat falling outside the visible solar spectrum, and so became the first to observe infrared radiation.

• We cannot see infrared radiation, although we can detect it as heat, and we can also detect it with some photographic films and special cameras. 

The principles of light

• Light is a form of electromagnetic radiation, part of the electromagnetic spectrum. It has electrical and magnetic properties, just like radio waves and X-rays.

• Light is a form of energy, and it is usually released when other forms of energy are converted. Hot bodies radiate light, if they are sufficiently hot.

• Light can be converted to other forms of energy such as electricity in photovoltaic cells and it is converted into chemical energy in plants.

• Because light radiates out in all directions, we can treat light as straight-line rays. Rays of light do not exist, but they are a convenient 'fiction'.

• Light and other forms of radiation are reflected. Most surfaces are rough, and reflect light in many directions but a mirror has a comparatively smooth surface.

• Light can be 'piped' through a carefully designed optic fibre, and the light source can be modulated to carry signals from one place to another.

• As you get further from a light source, the apparent intensity drops according to the inverse square law, dropping to a quarter when the distance is doubled.

• As a general rule, light travels in a straight line, but it will bend when it travels through a transparent medium that is not a vacuum, such as water or glass.

• Projection systems and pinhole cameras rely on light travelling in straight lines, which can be assumed in a uniform medium with no massive bodies nearby.

• Light can be bent away from a straight line of travel by the force of gravity when it passes very close to a very large mass such as a star or a black hole.

• The way that light bends when it travels through a transparent medium is called refraction. Curved lenses and prisms work because of refraction.

• In 1666 Isaac Newton demonstrated the composite nature of white light while carrying out studies directed at minimizing chromatic dispersion in lenses.

• Rainbows are seen when sunlight shines on small spherical water droplets, and is reflected and refracted several times inside the drops before exiting again.

• Some colours are produced by dispersion, the effect where a prism bends different wavelengths to different extents, separating white light into components.

• The colours in white light may be separated by filtering light through coloured transparent material that absorbs some wavelengths while transmitting others.

• Some colours are produced by selective absorption and reflection of different wavelengths of white light giving objects the colour of the reflected light.

• A 'blue' object absorbs other colours and reflects blue light. A black object absorbs all colours equally. A white object reflects all colours equally

• The Tyndall effect, where dust causes scattering of light, is behind the blue colour of the sky, and the red colour of sunsets, and the moon in a lunar eclipse.

• White light is what we see when we look at all of the colours that form the visible spectrum in combination. These colours have different wavelengths.

• The different wavelengths (colours) that make up white light are separated into the standard colours of the spectrum by a prism in a process called dispersion.

• When light is refracted in a medium such as a lens, different colours are refracted to different extents, causing coloured fringes like a rainbow.

• Some forms of colour are produced by selective scattering, as in the Tyndall effect, where dust in the atmosphere scatters blue light as it lets red light pass.

• The colours in some animals arise from birefringence, which depends on anisotropy, a measurable difference in optical properties in different directions.

• In 1665 Robert Hooke and Christiaan Huygens pointed out that the colours of an oil film are explained by combining the wave theory of light with interference.

• Light travels through a vacuum at a speed of 300,000 kilometres per second. That means it takes 500 seconds for light from the Sun to reach Earth.

• Light has a constant velocity of 300,000 kilometres a second when it travels in a vacuum, but when light enters a more dense medium, it slows down.

• In the first century AD, Hero of Alexandria said the speed of light must be infinite. He thought light came from the eyes, and we see as soon as we open them.

• In the early 1600s, Galileo Galilei tried measuring the speed of light by flashing lanterns from two hilltops, and decided the speed of light was infinite.

• The distance of the hilltops Galileo Galilei used were only about one forty-thousandth of a light-second apart, making it hard to reach a reliable estimate.

• In 1676, Ole Rømer used variations in the eclipses of Jupiter's moons to estimate the speed of light at around 227 million kilometres a second, about 25% out.

• About 1690, Christiaan Huygens estimated that the speed of light was might be as high as 35 million kilometres a second, which he thought extreme but possible.

• Armand Fizeau developed an earth-bound variation on the method developed by Galileo Galilei to measure the speed of light, using a toothed wheel and reflection.

• Fizeau's speed of light was about 5% higher than the value we accept to day, but this was adjusted the following year, when Jean Foucault refined the method.

• Light is produced when fuels burn because energy is released, and some of this is used to energize some of the electrons in atoms in the flame.

• In 1782, Aimé Argand invented the highly efficient fuel lamp which is still known as the Argand lamp. It had a hollow flame or wick and was much brighter.

• The combination of the Fresnel lens and the Argand lamp allowed much better light-houses to be set up, visible from a much greater distance in bad weather.

• Light of a suitable wavelength, shining on certain metals, can generate a charge by the photoelectric effect by energizing electrons in the metal.

• We cannot see ultraviolet radiation: black light is another name for ultraviolet light, which makes certain objects fluoresce at visible wavelengths. 

The principles of optics

• Mirror images can be explained by geometrical optics, assuming that light is made up of rays. It is not made up of rays, but the method works.

• Some mirrors magnify their images, and reflecting telescopes use surface-silvered magnifying mirrors as well as lenses, to produce clear images.

• Images from flat mirrors reverse left and right, but curved mirrors reflect differently, depending on the curvature and the distances of object and viewer.

• Around 1000, Alhazen studied lenses and their operation in Cairo, tried to puzzle out where the colours of the rainbow come from, and used a camera obscura.

• Around 1250, Roger Bacon studied the use of lenses to assist the vision, and he may even have found the principle of combining different lenses in a telescope.

• In 1621, Willebrod Snell published his law of refraction, which related the sines of the angles of incidence and refraction, known today simply as Snell's law.

• In 1637, René Descartes used Snell's law about the bending of rays passing in and out of glass to explain fully the operations of concave and convex lenses.

• One effect of light slowing down is that it bends in a mathematically predictable way. This is called refraction, and refracted light obeys Snell's law.

• Any transparent material has a refractive index that can be measured. This value can then be used to predict how that material will refract light.

• Lenses and prisms depend on the bending of light in the process of refraction. Prisms can bend or reflect light internally, depending on the angle of incidence.

• Imperfect images from lenses may be caused by spherical aberration. Spherical aberration can be reduced by using a smaller aperture for viewing an object.

• We see more clearly with a bright light, because the iris of our eye closes to give a smaller aperture, as a smaller aperture gives a greater depth of field.

• Imperfect images with coloured fringes may be caused by chromatic aberration in lenses. Chromatic aberration may be kept under control with an achromatic lens.

• Lenses can be used to focus rays so that they converge at what is called the focal point. This is why lenses can be used to form and project an image. 

The principles of wave properties

• Electromagnetic radiation involves the oscillation of electric and magnetic fields at the same time. This principle applies to all forms of radiation.

• When waves travel back and forth in a medium, they form a standing wave as a result of interference effects between the two halves of the wave.

• Waves interact through interference, and this interference can result in waves either cancelling each other out or joining to make a bigger wave.

• Diffraction is an interference effect seen when waves encounter a regular array. Diffraction demonstrates the wave-like nature of whatever is diffracted.

• When light passes through a grating, it behaves as a wave. Longer wavelength light diffracts through a greater angle than shorter wavelength light.

• In 1912, Max von Laue began investigating the use of a crystal of zinc sulfide to diffract X-rays, thus revealing any regular, repeated structure it might have.

• X-ray crystallography depends on the analysis of diffraction effects from arrays of atoms in a crystal acting like the lines in a diffraction grating.

• Clinton Davisson demonstrated electron diffraction, showing that electrons can sometimes be treated as waves. This property is used in the electron microscope.

• A wave may be represented as a longitudinal wave or as a transverse wave, depending on which is most convenient for understanding it or making predictions.

• Light is most easily considered as a wave, but it arrives in small packets known as photons. There is no single simple view fitting all the observed facts.

• Light can be thought of as a wave or as a particle, depending on what we look for: this is called wave-particle duality. Sometimes we speak of 'wavicles'.

• Discussing 'Newton's rings', Thomas Young pointed out that light appeared to be capable of destructively interfering with itself, clearly a wave phenomenon. 

The principles of particle physics

• Particles exert attractive and repulsive forces on each other, mostly from their electrical charges, in part from other forces which control atomic behaviour.

• There is a limited number of fundamental particles over and above the standard electron, neutron and proton, which set is all that most people know.

• Most of chemistry can be explained with no more than the proton, neutron and electron. When it comes to physics, the behaviour of atoms needs more structure.

• When particles collide at high speed, we can learn a great deal from the careful study of the fragments that are thrown off, and their energies.

• Mesons are of medium mass, between the size of an electron and a proton, and they are very unstable, medium-mass elementary particles with short life spans.

• Matter exists also in the form of antimatter, and when it comes in contact with ordinary matter, the two will annihilate each other, becoming energy.

• In 1873, Johannes van der Waals wrote about intermolecular forces in fluids, and introduced the idea of weak attractive forces between molecules.

• In 1930, Fritz London explained van der Waals forces in terms of their being caused by the interacting fluctuating dipole moments between molecules.

• In 1911, Victor Hess discovered high altitude radiation from space after ascending in a balloon. At this time, cosmic rays were referred to as 'Hess rays'.

• In 1912, Victor Hess used more ascents to show that the ionization of air increases with altitude indicating the existence of some form of cosmic radiation.

• In 1927, Eugene Wigner concluded that parity is conserved in a nuclear reaction, that the laws of physics should not distinguish between right and left.

• In 1958, Yang and Lee showed that, contrary to Wigner, certain types of reaction involving the weak nuclear force, such as beta decay, do not conserve parity.

• The Standard Model says that there are hundreds of particles, but that these are all made up of various combinations of six quarks and six leptons.

• In 1924, Edward Appleton demonstrated the presence of the ionosphere when he used radio ranging to measure the distance to the Heaviside layer.

• The F-layer or Appleton layer (after Sir Edward Appleton) is a layer of ions about 200 km above the Earth by day, and 300 km above the Earth at night.

• The Appleton layer reflects radio waves at frequencies up to about 50 MHz, and so allows radio signals below that frequency to travel around the world.

• Ernst Rutherford predicted that there must be a neutron as early as 1920, but finding it was harder. Chadwick did not detect one experimentally until 1932.

• In 1931, Wolfgang Pauli suggested that the neutrino could explain both the missing energy and spin in weak nuclear decay, starting the search for neutrinos.

• In 1932, Werner Heisenberg suggested that nuclei are made of protons and neutrons, which would explain why there are isotopes, when the neutron number varies.

• In 1923, Arthur Compton discovered the Compton effect which confirmed photons as particles. Compton and Debye provided the theory of Compton effect.

• A lepton is a light-weight charged or uncharged particle: each with an anti-particle. They are the electron, the muon, the tau, and their associated neutrinos.

• Within the nucleus, two main forces operate: the repulsion of the positively charged protons, and the strong nuclear force which pulls them together.

• In 1934, Pavel Cherenkov discovered that what is now Cherenkov radiation was caused when very fast particles entered an optically dense medium. 

The principles of fluid flow

• There are two sorts of fluid that we meet in daily life: gases and liquids. Each has the property that the particles are not closely bonded, so they can flow.

• An object moving through a fluid experiences drag. Engineers design the shapes of aircraft, ships and vehicles to reduce drag and improve efficiency.

• Laminar flow is more efficient than turbulent flow because turbulence absorbs energy, and so slows the fluid or the object passing through the fluid.

• The pressure exerted by a moving fluid is described by Bernoulli's principle, which is rooted in the assumption that a fluid is made of separate particles.

• Animals which rely on swimming or flying fast to catch food or to avoid being food, have evolved streamlined bodies that produce a laminar flow and reduce drag.

• The relative speed of a fluid may be measured with a Pitot tube, which uses the pressure detected to deduce the velocity of the fluid (or the tube). 

The principles of sound

• Sound is made of vibrations. A musical note is a uniform vibration. Waves with a greater amplitude have more energy, which makes them sound louder.

• Every musical note or tone has in it harmonics or overtones which add to the richness of the sound, and producing the characteristics of different instruments.

• Sound vibrations may be seen with an oscilloscope. You can also make the vibrations of a string visible by using a very long, thick string or wire.

• Every object has a natural frequency at which it vibrates, which is known as its resonant frequency. When struck, it will vibrate at this frequency.

• Vibrations can be made in a variety of ways. Plucked strings and struck objects vibrate to make a tone, and resonance in a tube can make a tone.

• Tone and pitch are both aspects of the frequency of the note being heard. The tone is a single frequency, the pitch is a subjective perceived frequency.

• Sound can be observed and/or visualized in Chladni figures, made when a violin bow is rubbed on the edge of a steel plate scattered with fine sand.

• Sound can be reflected and refracted. Acoustics is basically the study of how sounds are changed in an environment as they reflect and refract in a space.

• Sound from a moving source that is moving towards or away, relative to the listener, appears to change frequency, due to the principles of the Doppler effect.

• Thunder is caused by air being heated along the lightning flash, causing an increase in pressure. The bang from more distant parts takes longer to arrive.

• Sonic booms are caused when an aircraft or other object travels faster than the speed of sound in the atmosphere at the level at which it is flying.

• Beats occur when two very similar waves move in and out of synchronization, either reinforcing each other or cancelling each other at different times.

• The bang of a gun, a firework or a handclap are all caused by the sudden release of gas under pressure. The bang is the pressure wave reaching our ears.

• Sound travels through all materials as compressions and rarefactions, although it travels through some materials better (and sometimes faster) than others.

• Sound is most easily considered as a wave, but may also be thought of as a cyclic variation in pressure. The model we use does not change the sound's nature.

• In 1640, Marin Mersenne established a reasonable estimate of the speed of sound in air, which he set at 320 metres per second. The usual value today is 330 m/s.

• The velocity of sound can be measured and shown to vary with the transmission medium and also travelling faster when the temperature of the medium increases.

• The intensity of sound can be measured in decibels. Sound above a certain intensity can cause temporary damage to the delicate parts of the ears, or deafness. 

The principles of relativity

• Our daily experience is quite adequately explained by Galilean relativity and Newtonian physics, without any need to refer to Einsteinian relativity.

• In 1845, Urbain Leverrier observed a 35" per century excess precession of Mercury's orbit, a discrepancy that could not be explained until Einstein did so.

• In 1882, Simon Newcomb observed a 43" per century excess precession of Mercury's orbit, a discrepancy which lacked an explanation in normal physics.

• At the start of the 1900s, it was clear that there were a number of unsolved problems which needed a new theory or theories to explain them adequately.

• The first problem was that the speed of light did not behave like the speeds of ordinary objects made of matter, because it seemed to be constant.

• The next problem was that some forms of matter, in particular the radioactive elements, showed a powerful and seemingly unpredictable instability.

• The next problem was that there was no way to account for the way atoms emitted light and other forms of radiation when they were heated or excited.

• The next problem was that Newton's laws, which generally worked perfectly well, could not explain oddities in the precession of Mercury's orbit.

• The last problem was that the heat capacity of molecular gases was not what it should have been, using calculations based on Newtonian theory.

• In 1907, Albert Einstein offered his equivalence principle (gravitation and inertia) and predicted the existence of gravitational redshift from this.

• In 1915, Albert Einstein completed his theory of general relativity, predicted light bending and offered an explanation for perihelion shift of Mercury.

• Einstein's theory of special relativity indicated that time was relative, that the speed of light was constant, and that mass and energy were equivalent.

• Physicists say if Albert Einstein had not proposed special relativity, the time was right and Hendrik Lorentz or Jean Perrin would have done so soon after.

• Einstein's general theory of relativity offered the disturbing view that gravitation, rather than being a force, is a curved field in the space-time continuum.

• The key effect of the two theories of relativity was to make us aware that space and time are not separate: they are an intertwined space-time.

• In 1861 and 1865, James Clerk Maxwell showed from theory alone, that there should be electromagnetic radiation, and that light was part of it.

• James Clerk Maxwell showed that his mathematically derived electromagnetic radiation would always travel at what we now call the speed of light.

• If we see a ruler going past us at close to the speed of light, or if we pass one at that sort of speed, it will appear to be considerably shorter than it is.

• The shortening effect comes about because the ruler's length relates to the average separation of the atoms, and in the direction of travel, this is shorter.

• A person standing beside (or travelling with) the apparently shortened ruler, will see it at its normal length as they are in the same frame of reference. 

The principles of quantum physics

• The photoelectric effect was dependent on wavelength but not on intensity of the light, a puzzle that led to the discovery of what we now call quantum physics.

• Blackbody radiation was explained when quantum physics was developed, because it solved a number of apparent paradoxes about the energy of radiation.

• The 'violet catastrophe' said that if all frequencies are equally likely from a hot body, the many wavelengths beyond violet should swamp any emission spectrum.

• The 'violet catastrophe' does not occur, and unless the body is very hot, we do not even see any red light emitted from it, let alone violet or ultraviolet.

• Kirchhoff wanted to know why this was so. Because the higher frequencies are unlikely unless the body has very high energy, answered Planck's little equation.

• Wien produced a formula which explained the distribution of energy in a radiation spectrum as a function of both wavelength and the temperature of the body.

• Wilhelm Wien's displacement law explains why the sun's radiation peaks in the region we see best, because the sun has a surface temperature of around 6000 K.

• Wien's formula worked well at short wave-lengths, but failed at longer wave-lengths. Rayleigh's formula accurate at longer wavelengths but not at shorter ones.

• Rayleigh came up with a theory of black-body radiation, later modified by James Jeans, and often known as the Rayleigh-Jeans Law or the Rayleigh-Jeans theory.

• Sir James Jeans demonstrated the classical formula for the partition of radiant energy in an enclosure, which we now call the Rayleigh-Jeans Law.

• Linked to the displacement theory of Wilhelm Wien, the Rayleigh-Jeans Law more or less explained black-body radiation, until Max Planck found a better answer.

• In 1900, Max Planck proposed basic quantum theory, involving light quanta in black body radiation, Planck's black body law and Planck's constant.

• Planck saw that if the radiation was emitted only in 'packets' of a minimum energy, he could calculate a radiation law which was good for all wavelengths.

• In 1901, Max Planck made determinations of Planck's constant, Boltzmann's constant, Avogadro's number and the charge on the electron, all in one year.

• Later, Albert Einstein explained the photo-electric effect from Planck's work. A shorter wave-length photon had more energy, and so could dislodge an electron.

• In 1925, Walter Elsasser explained electron diffraction by regarding it as wave property of matter, further smearing the wave/particle distinction.

• Quantum mechanics is a mathematical description of quantum effects, relating to the way in which, on a small scale, variables cease to be continuous.

• Quantum physics has given us the interesting paradox of Schrödinger's Cat, which is a sort of thought experiment which appears to show a contradiction.

• When electrons move from one shell to another, they absorb or emit a specific amount of energy related to that shift, producing lines in a spectrum.

• The energy absorbed when electrons move to higher levels make part of the absorption spectrum, each transition contributing a single line to the spectrum.

• The energy emitted when excited electrons move to a lower level makes the emission spectrum of that atom: each shift contributes one line to the spectrum.

• So far, gravitation has not been incorporated into quantum theory, but that remains a hope and a goal for physicists researching in that area.

• The Heisenberg uncertainty principle indicates that not all measurements may be made simultaneously. It is widely misunderstood and misquoted.

• Wigner's friend is a variant on Schrödinger's Cat. The 'friend' is a human observer who replaces the cat in one of the thought experiments on quantum reality.

• In 1926, Erwin Schrödinger derived the spectrum of hydrogen atom using the wave equation, reinforcing the notion that waves and particles are interchangeable.

• In 1926, Schrödinger also showed the wave and matrix formulations of quantum theory were mathematically equivalent, combining the two sides of quantum physics. 

The principles of the nature of radioactivity

Needs more material

• In 1899, Ernest Rutherford discovered that uranium radiation is composed of positively charged alpha particles and negatively charged beta particles.

• In 1913, Niels Bohr identified radioactivity as a specific property of the nucleus of the atom, rather than being a general property of the atom as a whole.

• In 1917, Ernest Rutherford and Marsden described artificial transmutation of elements, after having produced hydrogen and oxygen from nitrogen.

• In 1929, John Cockcroft and Ernest Walton succeeded in 'atom-smashing' as it was called, using equipment based on four glass cylinders taken from petrol pumps.

• In 1932, John Cockcroft and Ernest Walton took linear proton accelerators to an energy of 700 keV and verified the mass/energy equivalence.

• Cockcroft and Walton used accelerated protons, hydrogen ions to split lithium and boron nuclei, and also to make unstable nuclei that were radioactive.

• In 1934, Irène Joliot-Curie and Frédéric Joliot-Curie bombarded aluminium atoms with alpha particles to create artificially radioactive phosphorus-30. 

There are other SPLATS to be found, but you will need to go back to the main SPLATS page to find the links.

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