The Story of GCSE Physics: The History Behind Every AQA Topic

Every topic in GCSE Physics started with someone asking a question and doing an experiment to find the answer. This page tells you the story behind each AQA topic, so you can see where the ideas came from and how they fit together. It’s a good way to remember the facts and equations, because each one is linked to a person and a discovery. Click on any scientist’s name to find out more about them on Wikipedia.

If you see (Physics only), you only need it if you’re doing GCSE Physics, not Combined Science. If you see (Higher Tier), you only need it for the Higher paper.

Energy
Electricity
Particle Model of Matter
Atomic Structure
Forces
Waves
Magnetism and Electromagnetism
Space Physics (Physics only)

1. Energy

Where the idea came from

Back in the 1700s, scientists couldn’t agree on what happens when things move and crash into each other. In 1740, Émilie du Châtelet backed an idea that turned out to be right. When metal balls were dropped into soft clay, a ball going twice as fast made a dent four times as deep. So the effect of a moving object depends on its speed squared. That’s why kinetic energy has v² in it. In 1807, Thomas Young was one of the first people to use the word “energy” the way we do now.

Energy stores and systems

Today we think about energy as being kept in stores. A system is just the object, or group of objects, you’re looking at. When something happens to a system, energy moves from one store to another.

The stores you need to know are: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear. Energy moves between stores in four ways: by a force doing work, by an electric current, by heating, or by radiation (waves).

You can calculate three of the stores:

Kinetic energy (energy of moving things): E_k = ½mv²
Gravitational potential energy (energy from being high up): E_p = mgh
Elastic potential energy (energy in a stretched spring): E_e = ½ke²

Example: when you throw a ball up, energy moves from its kinetic store to its gravitational potential store. When a car brakes, energy moves from its kinetic store to the thermal store of the brakes, which get hot.

Specific heat capacity

In the 1760s, a Scottish scientist called Joseph Black noticed that some materials need much more energy than others to warm up by the same amount. We call this specific heat capacity. It’s the energy needed to heat 1 kg of something by 1 °C. You use ΔE = mcΔθ to work out the energy. Water has a really high specific heat capacity, which is why a full kettle takes ages to boil and the sea stays cold even in summer.

Conservation of energy

In the 1840s, James Joule did a clever experiment. He used a falling weight to spin a paddle wheel inside a tub of water. The water got slightly warmer, and the more work the weight did, the warmer it got. This proved that doing work and heating are just two different ways of moving energy around.

In 1847, Hermann von Helmholtz put this into one big rule: the conservation of energy. Energy can’t be made or destroyed. It can only be moved (transferred), stored or spread out (dissipated). The unit of energy, the joule (J), is named after Joule.

Power

In the 1760s and 1770s, James Watt made steam engines much better. To help sell them, he compared them to horses, which is where the word “horsepower” comes from. The unit of power, the watt (W), was later named after him.

Power is how fast energy is transferred. You can use P = E/t or P = W/t (where W is work done). 1 watt means 1 joule every second. If two motors lift the same box to the same height, the more powerful one does it faster.

Efficiency and wasted energy

Even Watt’s improved engines wasted lots of energy. In fact, every real machine wastes some. The wasted energy is usually dissipated, which means it spreads out into the surroundings as heat and isn’t useful anymore.

Efficiency = useful energy out ÷ total energy in. (You can use power instead of energy too.) You can cut down on wasted energy by:

Lubrication (like oil in a bike chain), which reduces friction.
Thermal insulation, which slows down heat escaping.

A material with a high thermal conductivity lets heat through quickly. So to stop a house losing heat, you want thick walls made from materials with a low thermal conductivity.

Energy resources

Watt’s steam engines ran on coal, and coal powered the Industrial Revolution. Today we get energy from lots of resources: fossil fuels (coal, oil and gas), nuclear fuel, bio-fuel, wind, hydro-electricity, geothermal, tides, the Sun and water waves.

A renewable resource is one that gets replaced as fast as we use it, like wind or sunlight. Fossil fuels are reliable, but burning them releases carbon dioxide, which adds to climate change, and sulfur dioxide, which causes acid rain. Renewables don’t release carbon dioxide when they make electricity, but some (like wind and solar) don’t work all the time because they depend on the weather. Deciding which resources to use isn’t just about science. Money, politics and people’s opinions matter too.

Equations to learn: E_k = ½mv² · E_p = mgh · E_e = ½ke² · ΔE = mcΔθ · P = E/t · P = W/t · efficiency = useful out ÷ total in

2. Electricity

Static electricity and electric fields (Physics only)

Around 600 BC, the Greek thinker Thales of Miletus is said to have noticed that if you rub amber (a kind of fossilised tree resin) with fur, it picks up small bits of straw. That’s static electricity. In the 1740s, Benjamin Franklin came up with the words “positive” and “negative” for charge, which we still use.

Here’s what’s happening. When you rub two insulators together, electrons get rubbed off one and onto the other. The one that gains electrons becomes negative. The one that loses electrons becomes positive. Remember: only electrons move, never positive charges. Like charges repel (push apart) and opposite charges attract (pull together).

Every charged object has an electric field around it. This is the area where it can push or pull on other charges. The field is strongest close to the object. If enough charge builds up, the field gets so strong that electrons jump through the air, and you get a spark.

Current and charge

In the 1780s, Luigi Galvani noticed that a dead frog’s leg twitched when two different metals touched it. He thought the frog was making “animal electricity”. Alessandro Volta thought the metals were causing it. To prove his point, in 1800 he stacked up discs of zinc and copper with salty card in between, and made the very first battery.

Volta’s battery made charge flow continuously for the first time. This flow of charge is called current. Current is measured in amps (A), named after André-Marie Ampère. The equation is Q = It: charge (coulombs) = current (amps) × time (seconds).

For a current to flow, you need a complete loop (a closed circuit) and something to push the charge, like a battery. In a single loop, the current is the same everywhere.

Potential difference and resistance

Potential difference (p.d.) tells you how much energy each bit of charge gives to a component as it passes through. It’s measured in volts (V), named after Volta. The equation is E = QV.

In 1827, a German teacher called Georg Ohm tested lots of wires. He found that if you double the p.d. across a wire, you double the current, as long as the temperature stays the same. How hard it is for current to flow is called resistance, measured in ohms (Ω). This gives us V = IR.

Things that follow this rule, like a resistor at a constant temperature, are called ohmic conductors. Their current–p.d. (I–V) graph is a straight line through the origin. But some components don’t follow the rule:

Filament lamp: the more current flows, the hotter it gets, and the hotter it gets, the higher its resistance. So the graph curves.
Diode: only lets current flow one way. It has a really high resistance the other way.
Thermistor: resistance goes down when it gets hotter. Used in thermostats.
LDR (light-dependent resistor): resistance goes down when it gets brighter. Used in lights that switch on at night.

Series and parallel circuits

In a series circuit, everything is in one loop:

The current is the same through every component.
The p.d. from the battery is shared out between the components.
You add the resistances to get the total: R_total = R₁ + R₂.

In a parallel circuit, the components are on separate branches:

The p.d. across each branch is the same.
The currents in the branches add up to the total current.
The total resistance is less than the smallest resistor. Adding a branch gives the charge another route, so it’s easier for current to flow.

Your house is wired in parallel. That way every appliance gets the full p.d., and you can turn each one on and off without affecting the others.

Electricity at home: AC and DC

In 1882, Thomas Edison opened one of the first power stations, in New York. It supplied direct current (DC), which only flows one way, like the current from a battery. Nikola Tesla and George Westinghouse argued for alternating current (AC), which keeps changing direction. AC won in the end, mainly because you can easily change its voltage using transformers. In 1893, Westinghouse used AC to light up the whole Chicago World’s Fair.

In the UK, mains electricity is AC with a frequency of 50 Hz (it changes direction 50 times a second) and a p.d. of about 230 V. Most appliances plug in with a three-core cable:

Live wire (brown): brings in the 230 V from the supply. This is the dangerous one.
Neutral wire (blue): completes the circuit. It’s at about 0 V.
Earth wire (green and yellow stripes): a safety wire. It only carries current if something goes wrong.

Touching the live wire can kill you, even if the switch is off. Your body is at 0 V, so there’s a big p.d. between you and the wire, and current can flow through you.

Power and energy in appliances

Joule (from the Energy topic) also found that when current flows through a wire, the wire heats up. That’s handy in a kettle or toaster, but it’s a waste in long cables. To work out power in a circuit, use P = VI or P = I²R. The energy an appliance uses depends on its power and how long it’s on: E = Pt.

Look at P = I²R. Because the current is squared, doubling the current makes the heating four times bigger. This is really important for the next bit.

The National Grid

After a law was passed in 1926, Britain started joining all its power stations into one big network called the National Grid. The problem was how to send electricity a long way without losing loads of energy as heat in the cables.

The answer is transformers. A step-up transformer raises the p.d. to a really high value (up to 400,000 V today). For the same power, a higher p.d. means a lower current (because P = VI). A lower current means much less energy is wasted heating the cables. Then a step-down transformer lowers the p.d. to a safe 230 V before it gets to your house. (You’ll find out how transformers work in Magnetism and Electromagnetism.)

Equations to learn: Q = It · V = IR · R_total = R₁ + R₂ · E = QV · P = VI · P = I²R · E = Pt

3. Particle Model of Matter

Density and states of matter

Around 400 BC, the Greek thinker Democritus suggested that everything is made of tiny bits that can’t be cut any smaller. He called them atoms. About 150 years later, there’s a famous story about Archimedes. A king asked him to check if his crown was pure gold. Archimedes realised he could find the crown’s volume by seeing how much water it pushed out of the way. Then he could compare its mass and volume with real gold. This is the idea of density: ρ = m/V.

The particle model explains why different states have different densities:

Solid: particles packed tightly in a regular pattern. They vibrate but stay in place.
Liquid: particles still close together, but in a random arrangement. They can slide past each other.
Gas: particles far apart, moving fast in random directions. That’s why gases have a much lower density.

Changing state (like melting or boiling) is a physical change. The mass stays the same, and you can reverse it.

Internal energy and latent heat

The particles in a substance have energy. Internal energy is all the kinetic and potential energy of the particles added together. When you heat something, its internal energy goes up. This either makes it hotter (use ΔE = mcΔθ from the Energy topic) or makes it change state.

Around 1761, Joseph Black (the same scientist from the Energy topic) noticed something odd. Melting ice stays at 0 °C even though you keep heating it. He called this hidden energy latent heat. While a substance is changing state, the energy goes into breaking the bonds between particles, not into making it hotter. That’s why heating graphs have flat bits.

Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature: E = mL. There are two types:

Specific latent heat of fusion: for melting or freezing (solid ↔ liquid).
Specific latent heat of vaporisation: for boiling or condensing (liquid ↔ gas).

Particles in gases

In 1738, Daniel Bernoulli suggested that gas pressure is caused by gas particles hitting the walls of their container. In 1827, a scientist called Robert Brown looked at tiny bits from pollen grains in water under a microscope and saw them jiggling about randomly. Nobody knew why.

In the 1860s and 1870s, James Clerk Maxwell and Ludwig Boltzmann worked out the maths of how gas particles move. Then in 1905, Albert Einstein explained Brown’s jiggling: the bits were being knocked about by water molecules too small to see. This was strong proof that atoms and molecules are real.

So, what you need to know: gas particles are always moving randomly. The hotter the gas, the faster the particles move on average (temperature is linked to their average kinetic energy). If you heat a gas in a closed container, the particles hit the walls harder and more often, so the pressure goes up.

Pressure in gases (Physics only)

In 1662, Robert Boyle showed that if you squash a gas into a smaller space, its pressure goes up. For a fixed amount of gas at the same temperature, pV = constant. So if you halve the volume, the pressure doubles.

Gas pressure pushes at right angles (90°) on the walls of the container. If you do work on a gas, like pumping up a bike tyre, you increase its internal energy, so it gets warmer. That’s why a bike pump gets hot.

Equations to learn: ρ = m/V · ΔE = mcΔθ · E = mL · pV = constant (Physics only)

4. Atomic Structure

How our idea of the atom changed

The model of the atom has changed lots of times. Each time, it changed because someone found new evidence. You need to know this order:

Tiny solid spheres: In the early 1800s, John Dalton brought back Democritus’s idea. He said atoms were tiny solid balls that couldn’t be split.
Plum pudding model: In 1897, J. J. Thomson discovered the electron. So atoms could be split into smaller bits. He said the atom was a ball of positive charge with negative electrons stuck in it, like fruit in a pudding.
Nuclear model: In 1909, Hans Geiger and Ernest Marsden fired alpha particles at very thin gold foil. Most went straight through, but a few bounced off at angles, and a tiny number bounced straight back. In 1911, Ernest Rutherford explained this. The atom is mostly empty space, with almost all its mass in a tiny positive centre called the nucleus.
Electron shells: In 1913, Niels Bohr said the electrons go round the nucleus at fixed distances, in shells (energy levels).
Protons and neutrons: Later experiments showed the positive charge in the nucleus is made of protons. In 1932, James Chadwick discovered the neutron.

Atoms, isotopes and ions

Atoms are tiny. An atom’s radius is about 1 × 10^-10 m, and the nucleus is less than 1/10,000 of that. Atoms have no overall charge, because they have the same number of protons (+) and electrons (−).

Atomic number = number of protons.
Mass number = number of protons + neutrons.
Isotopes are atoms of the same element with different numbers of neutrons. Frederick Soddy started using the word “isotope” in 1913.
If an atom loses one or more outer electrons, it becomes a positive ion.

Electrons can jump up to a higher energy level by absorbing electromagnetic radiation, and drop back down by giving it out.

Radioactive decay

In 1896, Henri Becquerel left some uranium in a drawer with photographic plates. The plates got fogged up, even in the dark. Something invisible was coming out of the uranium. Marie Curie and Pierre Curie then found two new radioactive elements, polonium and radium, in 1898. Marie Curie came up with the word “radioactivity”. In 1899, Rutherford named alpha and beta radiation, and in 1900 Paul Villard discovered gamma radiation.

Radioactive decay happens when an unstable nucleus gives out radiation to become more stable. It’s completely random: you can’t predict which nucleus will decay next. Activity is how many nuclei decay each second, measured in becquerels (Bq). Count-rate is how many decays a detector, like a Geiger-Müller tube, picks up each second.

Alpha (α): 2 protons and 2 neutrons (a helium nucleus). Very ionising. Only travels a few cm in air and is stopped by paper. Mass number goes down by 4, atomic number goes down by 2.
Beta (β): a fast electron that comes out when a neutron in the nucleus turns into a proton. Stopped by a few mm of aluminium. Mass number stays the same, atomic number goes up by 1.
Gamma (γ): an electromagnetic wave from the nucleus. Not very ionising but very penetrating. Reduced by thick lead or concrete. Doesn’t change the mass number or atomic number.
Some nuclei can also give out a neutron.

Half-life, contamination and irradiation

In the early 1900s, Rutherford and Soddy noticed that a radioactive sample always takes the same amount of time to lose half its activity, no matter how much you start with. This is the half-life: the time it takes for the number of unstable nuclei to halve, or for the count-rate to drop to half.

The early scientists didn’t know how dangerous radiation was. Marie Curie died in 1934 from an illness that was probably caused by years of working with radioactive materials. Now we know the difference between two types of risk:

Irradiation: you’re exposed to radiation from a source outside your body. You don’t become radioactive.
Contamination: radioactive atoms actually get onto or into something (or someone).

Research about radiation is checked by other scientists before it’s published. This is called peer review.

Background radiation and uses in medicine (Physics only)

Background radiation is around us all the time. Some is natural, from rocks and from cosmic rays from space. Some is man-made, from things like nuclear weapons tests and nuclear accidents. The radiation dose you get is measured in sieverts (Sv), named after Rolf Sievert.

Doctors use radioactive isotopes as tracers to see inside the body, and to kill cancer cells. If they put an isotope inside someone, they choose one with a short half-life so it doesn’t stay radioactive for long.

Nuclear fission and fusion (Physics only)

In 1938, Otto Hahn and Fritz Strassmann fired neutrons at uranium and found barium, a much lighter element, in what was left. Lise Meitner and Otto Frisch worked out what had happened: the uranium nucleus had split in two. They called it fission.

In fission, a big nucleus like uranium-235 absorbs a neutron and splits into two smaller nuclei. It also gives out two or three neutrons, some gamma rays and lots of energy. Those new neutrons can hit other nuclei and make them split too. This is a chain reaction. In 1942, Enrico Fermi led the team that built the first nuclear reactor with a controlled chain reaction. In a nuclear power station the chain reaction is controlled. In a nuclear bomb it isn’t.

Fusion is the opposite: two small nuclei join together to make a bigger one. Some of the mass turns into energy. Fusion is what powers the Sun and other stars (see Space Physics). Scientists are still trying to build a fusion power station that works properly.

5. Forces

From Aristotle to Newton

The Greek thinker Aristotle believed two things that sound sensible but are wrong. He thought things need a force pushing them all the time to keep moving, and that heavy things fall faster than light things. People believed him for nearly 2,000 years.

In the early 1600s, Galileo Galilei actually tested these ideas by rolling balls down slopes. He worked out that, without friction, a moving object would just keep going. He also showed that objects fall at the same rate if there’s no air resistance. Then in 1687, Isaac Newton published his three laws of motion and his law of gravity. Most of this topic comes from Newton. The unit of force, the newton (N), is named after him.

Types of forces

First, some key words:

A scalar just has a size (e.g. speed, distance, mass).
A vector has a size and a direction (e.g. velocity, displacement, force).
Contact forces need objects to touch: friction, air resistance, tension, normal contact force.
Non-contact forces work from a distance: gravity, electrostatic force, magnetic force.

Newton showed that every object with mass pulls on every other object. Your weight is the force of gravity pulling on you: W = mg. On Earth, g is about 9.8 N/kg. Weight acts from an object’s centre of mass.

When lots of forces act on something, you can replace them with one force that has the same effect. This is the resultant force. You can show the forces on a free body diagram.

Work done and springs

When a force makes something move, work is done: W = Fs (force × distance moved). Pushing with 1 N for 1 m does 1 J of work. Work done against friction makes things warm up, which links back to the Energy topic.

In 1678, Robert Hooke found that if you double the force on a spring, it stretches twice as far. This is F = ke (k is the spring constant, e is the extension). It only works up to the limit of proportionality. After that, the spring stretches more than it should.

If a spring goes back to its normal shape when you let go, it’s been elastically deformed. If it stays stretched, it’s been inelastically deformed. The work you do stretching a spring is stored as elastic potential energy (E_e = ½ke²).

Moments, levers and gears (Physics only)

Archimedes studied levers around 250 BC. He supposedly said that with a long enough lever and somewhere to stand, he could move the Earth. The turning effect of a force is called a moment: M = Fd, where d is the distance from the pivot (measured at right angles to the force).

If something is balanced, the clockwise moments equal the anticlockwise moments. Levers and gears let a small force have a big turning effect. That’s why a long spanner makes it easier to undo a tight nut.

Pressure in liquids and air (Physics only)

Archimedes also discovered upthrust. When something is in water, it gets pushed up by a force equal to the weight of the water it pushes out of the way. This happens because the pressure in a liquid gets bigger the deeper you go: p = hρg. So the bottom of an object feels more pressure than the top. If an object’s weight equals the upthrust, it floats. Things float if they’re less dense than the liquid. Pressure on a surface is p = F/A.

In 1643, Evangelista Torricelli made the first barometer and showed that air presses down on us. In 1648, Blaise Pascal had a barometer carried up a mountain and showed the pressure drops as you go higher. That’s because there’s less air above you, and the air gets thinner the higher you go. The unit of pressure, the pascal (Pa), is named after him.

Describing motion

Distance is how far something travels. Displacement is how far it ends up from where it started, in a straight line, with a direction. Speed is worked out with s = vt. Typical speeds you should know:

Walking: about 1.5 m/s
Running: about 3 m/s
Cycling: about 6 m/s

Velocity is speed in a particular direction. So if something goes round in a circle at a steady speed, its velocity is still changing, because its direction keeps changing.

Galileo was the first to study acceleration properly. Acceleration is how quickly velocity changes: a = Δv/t. For graphs:

Distance–time graph: the gradient (steepness) is the speed.
Velocity–time graph: the gradient is the acceleration, and the area under the line is the distance travelled.

For steady acceleration, you can also use v² − u² = 2as. Anything falling freely near Earth speeds up at about 9.8 m/s².

When something falls, air resistance gets bigger as it speeds up. Eventually the air resistance equals the weight. Now the resultant force is zero, so it stops speeding up and falls at a steady speed. This is its terminal velocity.

Newton’s three laws

First law: if the resultant force is zero, a still object stays still, and a moving object keeps moving at the same speed in the same direction. (This is Galileo’s idea.) Objects tending to keep doing what they’re doing is called inertia.
Second law: a bigger force gives a bigger acceleration, and a bigger mass gives a smaller acceleration: F = ma.
Third law: when two objects push or pull on each other, the forces are equal in size and opposite in direction.

Stopping distances

Newton’s laws are now used to keep people safe on roads. Stopping distance = thinking distance + braking distance.

Thinking distance is how far the car goes while the driver reacts. Reaction times are usually 0.2 to 0.9 s. They get longer if the driver is tired, has taken drugs or alcohol, or is distracted (like looking at a phone).
Braking distance is how far the car goes once the brakes are on. It gets longer if the road is wet or icy, or the brakes or tyres are worn.

When you brake, friction does work, moving energy from the car’s kinetic store to the thermal store of the brakes. Braking really hard can make the brakes overheat or make the driver lose control.

Momentum (Higher Tier)

In 1644, René Descartes wrote about a “quantity of motion” that stays the same when things crash together. Newton improved the idea, and we now call it momentum. Momentum is p = mv. In a closed system, the total momentum before something happens (like a crash) equals the total momentum after. This is the conservation of momentum.

(Physics only, Higher Tier) Force is how fast momentum changes: F = mΔv/Δt. Seat belts, air bags, crumple zones, cycle helmets and gym mats all make a collision take longer. Because the change in momentum happens over a longer time, the force is smaller, so you’re less likely to get hurt.

Equations to learn: W = mg · W = Fs · F = ke · M = Fd (Physics only) · p = F/A and p = hρg (Physics only) · s = vt · a = Δv/t · v² − u² = 2as · F = ma · p = mv (Higher Tier) · F = mΔv/Δt (Physics only, Higher Tier)

6. Waves

Early ideas about light

In the early 1000s, the Arab scientist Ibn al-Haytham wrote the Book of Optics. He used experiments to show that light travels in straight lines, and that we see things because light from them goes into our eyes (not the other way round, as people thought before).

In 1621, Willebrord Snellius worked out the maths of how light bends (refraction). In the 1660s, Newton shone white light through a prism and showed it’s actually made of all the colours of the rainbow. In 1678, Christiaan Huygens said light is a wave, and around 1801 Thomas Young (from the Energy topic) did an experiment that gave strong evidence he was right.

How waves work

Waves carry energy from one place to another, but they don’t carry the material itself. Think of a Mexican wave in a stadium: the wave goes round, but the people stay in their seats. There are two types:

Transverse: the vibrations are at right angles to the direction the wave travels. Examples: ripples on water, light.
Longitudinal: the vibrations are in the same direction the wave travels, making squashed-up bits (compressions) and spread-out bits (rarefactions). Example: sound.

Words you need to know:

Amplitude: the biggest distance a point moves from its rest position.
Wavelength: the distance from one point on a wave to the same point on the next wave.
Frequency: how many waves pass a point each second, measured in hertz (Hz).
Period: the time for one wave: T = 1/f.

All waves follow the wave equation: v = fλ (speed = frequency × wavelength).

When a wave hits the boundary between two materials, it can be reflected (bounce back), absorbed (taken in) or transmitted (go through). Waves change speed when they go into a different material. If they hit the boundary at an angle, they change direction too. This is refraction.

Sound, ultrasound and earthquakes (Physics only)

Sound travels through solids by making the particles vibrate. When sound reaches your ear, it makes your eardrum vibrate. Humans can hear from about 20 Hz to 20 kHz.

Ultrasound is sound that’s too high for us to hear (above 20 kHz). When it hits the boundary between two materials, some of it bounces back. By timing how long the echo takes, you can work out how far away the boundary is. In the 1950s, a Glasgow doctor called Ian Donald started using ultrasound to scan babies before they’re born. Ultrasound is also used to find cracks inside metal, and echo sounding uses sound to measure how deep the sea is.

Earthquakes make seismic waves:

P-waves are longitudinal. They can go through solids and liquids.
S-waves are transverse. They can only go through solids.

Nobody can dig to the centre of the Earth, so scientists used these waves to work out what’s inside. In 1906, Richard Dixon Oldham used them to show the Earth has a core. In 1913, Beno Gutenberg worked out how deep the core is. In 1936, Inge Lehmann discovered the solid inner core. S-waves don’t reach the other side of the Earth, which tells us the outer core must be liquid.

Discovering the electromagnetic spectrum

Most of the electromagnetic spectrum is invisible, so it had to be discovered bit by bit:

1800: William Herschel put a thermometer just past the red end of a rainbow made by a prism. It still warmed up. He’d found infrared.
1801: Johann Ritter found invisible rays past the violet end. He’d found ultraviolet.
1865: Maxwell (from the Particle Model topic) showed light is an electromagnetic wave and predicted there must be others.
1887: Heinrich Hertz made and detected radio waves. The unit of frequency, the hertz, is named after him.
1895: Wilhelm Röntgen discovered X-rays.
1900: Villard (from Atomic Structure) discovered gamma rays.

All electromagnetic waves are transverse and travel at the same speed in a vacuum (3 × 10⁸ m/s). In order from lowest to highest frequency (longest to shortest wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Radio waves can be made by electrical circuits, and when they’re absorbed, they can make an alternating current (that’s how an aerial works). Changes inside atoms and nuclei can give out or absorb electromagnetic waves. Gamma rays come from changes in the nucleus.

Uses you need to know:

Radio waves: TV and radio
Microwaves: satellite communications, cooking
Infrared: electric heaters, cooking, thermal cameras
Visible light: fibre optic cables
Ultraviolet: energy-efficient lamps, sun tanning
X-rays and gamma rays: medical imaging and treating cancer

Some of these can be harmful. Ultraviolet can make your skin age faster and increases the risk of skin cancer. X-rays and gamma rays are ionising, which means they can damage the genes in your cells and cause cancer. How much harm they do depends on the type of radiation and the dose (measured in sieverts).

Lenses, colour and black body radiation (Physics only)

Lenses: A convex lens bends light inwards to a point called the principal focus. It can make real or virtual images. A concave lens spreads light out and always makes virtual images. Magnification = image height ÷ object height.

Colour: Each colour of light has its own range of wavelengths. Smooth surfaces like mirrors give specular reflection (in one direction). Rough surfaces give diffuse reflection (scattered). An object looks a certain colour because of which wavelengths it reflects. A coloured filter lets some colours through and absorbs the rest.

Black body radiation: Everything gives out and takes in infrared radiation. The hotter something is, the more radiation it gives out, and the shorter the wavelengths. A perfect black body absorbs all the radiation that hits it, and it’s also the best at giving radiation out. In 1900, Max Planck explained this pattern, and his idea later led to a whole new area called quantum physics.

An object stays at the same temperature if it takes in radiation at the same rate it gives it out. The Earth’s temperature depends on this balance: the radiation it absorbs from the Sun and the radiation it gives out into space.

Equations to learn: T = 1/f · v = fλ · magnification = image height ÷ object height (Physics only)

7. Magnetism and Electromagnetism

Magnets

People have known about naturally magnetic rocks, called lodestones, for thousands of years. By the 11th century, sailors in China were using compasses to find their way. In 1600, an English doctor called William Gilbert worked out that the Earth itself acts like a giant magnet. That’s why a compass works: the needle is a tiny magnet that lines up with the Earth’s magnetic field.

Things you need to know about magnets:

The poles are where the magnetic force is strongest. Like poles repel, opposite poles attract.
A permanent magnet makes its own magnetic field.
An induced magnet only becomes magnetic when it’s in a magnetic field. It always attracts, and loses most or all of its magnetism when you take it away.
The magnetic materials are iron, steel, cobalt and nickel.
A magnetic field is the area around a magnet where it can push or pull on other magnets or magnetic materials. Field lines go from north to south, and the field is strongest near the magnet.

Electromagnetism

In 1820, Hans Christian Ørsted was giving a lecture when he noticed a compass needle move whenever current flowed through a wire nearby. This was the first proof that electricity and magnetism are connected. Ampère (from the Electricity topic) then started studying how wires with currents push and pull on each other.

A wire with a current flowing through it has a circular magnetic field around it. The field is stronger if the current is bigger, and stronger closer to the wire.

If you coil the wire up into a solenoid, the field gets stronger. Inside the solenoid, the field is strong and the same everywhere. Outside, it looks like the field of a bar magnet. Put an iron core inside and it gets even stronger. Now you’ve got an electromagnet. In 1825, William Sturgeon made the first one. Electromagnets are useful because you can turn them on and off.

The motor effect

In 1821, Michael Faraday made a wire with a current in it spin round and round a magnet. It was the first electric motor. When a wire carrying a current is in a magnetic field, the wire and the magnet push on each other. This is the motor effect.

To work out which way the wire moves, use Fleming’s left-hand rule, invented by John Ambrose Fleming. To work out how big the force is, use F = BIl. B is the magnetic flux density (how strong the field is), measured in tesla (T), named after Tesla from the Electricity topic. In an electric motor, a coil of wire with a current flowing through it spins in a magnetic field.

(Physics only) Loudspeakers and headphones use the motor effect. Changes in the current make the speaker move back and forth, which makes pressure changes in the air. That’s sound.

Generators and transformers (Physics only)

In 1831, Faraday made his biggest discovery. When he moved a magnet in or out of a coil of wire, a p.d. was created across the coil. If the circuit was complete, a current flowed. This is the generator effect. The current it makes creates its own magnetic field that works against the change that caused it. You get a bigger effect if you move the magnet faster or use a stronger magnet.

The generator effect is used in:

Alternators: make AC.
Dynamos: use a split-ring commutator to make DC.
Microphones: sound waves move a coil in a magnetic field, which makes a changing current. This is the opposite of a loudspeaker.

Almost all the electricity in the National Grid is made this way, in power station generators.

Also in 1831, Faraday wrapped two separate coils of wire around an iron ring. When he switched the current on or off in one coil, a current appeared in the other. This was basically the first transformer.

A transformer has a primary coil and a secondary coil wrapped around an iron core. Here’s how it works: AC in the primary coil makes a changing magnetic field in the iron core. This changing field creates an alternating p.d. in the secondary coil. The equations are:

V_p/V_s = n_p/n_s (the p.d.s are in the same ratio as the number of turns on each coil)
V_pI_p = V_sI_s (if the transformer is 100% efficient)

This is exactly how the National Grid steps the p.d. up and down, which takes us right back to the Electricity topic.

Equations to learn: F = BIl · V_p/V_s = n_p/n_s (Physics only) · V_pI_p = V_sI_s (Physics only)

8. Space Physics (Physics only)

Our Solar System

For a long time, people thought the Earth was at the centre of everything. Around AD 150, Ptolemy wrote down this idea in detail. In 1543, Nicolaus Copernicus suggested that actually the Earth and the other planets go round the Sun. In 1610, Galileo pointed a telescope at Jupiter and saw moons going round it, which showed not everything orbits the Earth. Johannes Kepler showed that planets move in slightly squashed circles (ellipses), and Newton explained that it’s gravity keeping them in orbit.

Our Solar System has one star (the Sun), eight planets, dwarf planets like Pluto, and moons (natural satellites) going round the planets. It’s part of a galaxy called the Milky Way. The Solar System formed from a cloud of dust and gas (a nebula) that was pulled together by gravity.

The life of a star

For a long time, nobody knew what made stars shine. In 1920, Arthur Eddington suggested they get their energy by fusing hydrogen into helium. In 1925, Cecilia Payne showed that stars are mostly made of hydrogen and helium. In 1939, Hans Bethe worked out exactly which fusion reactions happen inside them. (This links back to fusion in Atomic Structure.)

A star forms when gravity pulls dust and gas together into a protostar. When it gets hot and dense enough, fusion starts. For most of its life, the star is on the main sequence. It’s stable because gravity pulling in is balanced by the pressure from fusion pushing out. What happens next depends on how big the star is:

About the size of the Sun: main sequence → red giant → white dwarf → black dwarf
Much bigger than the Sun: main sequence → red super giant → supernova → neutron star or black hole

Around 1930, Subrahmanyan Chandrasekhar worked out the biggest mass a white dwarf can have, which helps explain why the biggest stars end up so differently. Fusion in stars makes all the elements up to iron. Elements heavier than iron are made in supernovae, and the explosion spreads all the elements out across the universe. So the atoms in your body were made inside stars.

Orbits and satellites

Gravity is the force that keeps planets and satellites moving in circular orbits. A satellite in orbit can be moving at a steady speed, but its velocity keeps changing because its direction keeps changing (remember this from Forces). For an orbit to stay stable, the closer something is to the planet, the faster it has to go.

The first artificial satellite, Sputnik 1, was launched in 1957. Now there are thousands, used for things like phone signals, weather forecasts and GPS.

Red-shift and the Big Bang

From 1912, Vesto Slipher looked at light from distant galaxies and found it was shifted towards the red end of the spectrum. This is red-shift: the light from galaxies moving away from us gets stretched to longer wavelengths (this links back to Waves).

In 1927, Georges Lemaître suggested the universe is getting bigger. In 1929, Edwin Hubble showed that the further away a galaxy is, the faster it’s moving away from us. If everything is moving apart now, it must have all been squashed together in the past.

That’s the Big Bang theory: the universe started as a tiny, extremely hot and dense region and has been expanding ever since. In 1965, Arno Penzias and Robert Wilson picked up microwave radiation coming from every direction in space. This cosmic microwave background radiation is left over from the Big Bang, and it’s strong evidence the theory is right.

There’s still a lot we don’t know. In the 1930s, Fritz Zwicky noticed galaxies moving as if there’s way more mass there than we can see. In the 1970s, Vera Rubin found the same thing from how galaxies spin. This invisible mass is called dark matter. In 1998, teams including Saul Perlmutter, Brian Schmidt and Adam Riess found that the universe’s expansion is actually speeding up. Scientists call whatever is causing this dark energy. Nobody knows yet what dark matter or dark energy actually are.

How it all links together

The topics in GCSE Physics aren’t separate. They’re all connected:

Joule’s work links Energy and Electricity.
Faraday’s discoveries in Electromagnetism make the National Grid in Electricity possible.
Rutherford’s nuclear model in Atomic Structure explains how stars shine in Space.
Newton’s laws in Forces explain both a car braking and a satellite orbiting the Earth.

If you understand the story, the facts make more sense, and they’re much easier to remember in the exam.

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