KS3 Physics topics

Energy stores & transfers

Track where energy starts, moves and ends up. Read the full explainer →

Speed = distance ÷ time

Calculate speed and read distance–time graphs.

Builds on: Ratio notation (MATH)

Forces & balance

Identify forces and reason about balanced pairs.

Light & sound basics

How waves carry energy — reflection, pitch, loudness.

Energy in food

Food labels measure energy in kilojoules — the same energy that moves muscles and keeps you warm. Comparing labels turns lunch into physics.

Builds on: Energy stores & transfers — “Food is an energy store — the store idea has to come first.”

Renewable and non-renewable resources

Where the country's energy comes from: sources that top themselves up (wind, sun, waves) and sources that run down (coal, oil, gas). Read the full explainer →

Builds on: Energy stores & transfers — “Renewable or not, every resource is a store we've learned to tap.”

Temperature is not energy

A spark is hotter than a bath, but the bath holds far more energy. Temperature says how hot; energy says how much.

Builds on: Energy stores & transfers — “Temperature and energy only pull apart once you can track the energy.”

Contact and non-contact forces

Some forces need touching — pushes, pulls, friction. Others act across a gap: gravity, magnetism and static electricity. Read the full explainer →

Measuring forces in newtons

Force has a unit — the newton — and a tool, the newtonmeter, whose spring stretches by an amount you can read off.

Builds on: Contact and non-contact forces — “You measure forces once you know what counts as one.”

Drawing force diagrams

Each force becomes a labelled arrow: longer means stronger, and the direction shows which way it pushes or pulls.

Builds on: Contact and non-contact forces — “Every arrow you draw is a force you first learned to spot.”

Friction and drag

Surfaces and air push back against motion. Friction grips, drag slows — sometimes a nuisance, sometimes exactly what you want.

Builds on: Contact and non-contact forces — “Friction only makes sense as a force pushing back.” · Measuring forces in newtons

Stretching springs

Pull a spring with twice the force and it stretches twice as far — until you overdo it. That tidy pattern is Hooke's law.

Builds on: Contact and non-contact forces · Measuring forces in newtons — “Hooke's pattern comes out of measuring pull after pull.”

Vibrations make sound

Every sound starts with something shaking — a string, a drum skin, your voice box. Stop the vibration and the sound stops with it.

Pitch and frequency

Faster vibrations sound higher. Frequency counts the vibrations per second, in hertz — pitch is how our ears report it.

Builds on: Vibrations make sound — “Pitch is how fast the vibration repeats — no vibration, no pitch.” · Light & sound basics

Loudness and amplitude

Bigger vibrations sound louder. Amplitude measures the size of the vibration; decibels report the loudness we hear.

Builds on: Vibrations make sound — “Loudness is the size of the shake, so the shake comes first.” · Light & sound basics

Sound needs a medium

Sound travels as particles bumping their neighbours, so it needs something to travel through — no air, no sound, which is why space is silent.

Builds on: Vibrations make sound — “Only when sound is a travelling vibration does 'no air, no sound' follow.” · The particle model of matter

Magnets: poles push and pull

Every magnet has a north and a south pole. Unlike poles attract, like poles repel — and no amount of cutting removes a pole.

Plotting magnetic field lines

The invisible region where a magnet acts can be mapped: a small compass or iron filings trace field lines looping from north pole to south.

Builds on: Magnets: poles push and pull — “Field lines map pushes and pulls you've already felt.”

Earth's magnetic field

The Earth behaves like a giant bar magnet, which is why a free-swinging compass needle settles pointing north.

Builds on: Magnets: poles push and pull — “A compass needle is one magnet answering another — the other is Earth.” · Plotting magnetic field lines

Static electricity: charging by rubbing

Rubbing some materials moves electrons between them and leaves each one charged — enough to lift hair, crackle a jumper or stick a balloon to a wall.

Forces between charges

Like charges push apart and unlike charges pull together, without touching — electricity's version of magnetic attraction and repulsion.

Builds on: Contact and non-contact forces · Static electricity: charging by rubbing — “First you charge things; then you study how they push and pull.”

The particle model of matter

Everything is made of tiny particles in constant motion. How tightly they hold together decides whether stuff is solid, liquid or gas. Read the full explainer →

Drawing particle diagrams

Circles in boxes, drawn honestly: touching and ordered for solids, touching and jumbled for liquids, far apart for gases.

Builds on: The particle model of matter — “You can't draw the particles until you believe in them.”

Melting and freezing

Warm a solid and its particles shake until the structure loosens into liquid; cool it and the order returns. Same particles, different grip.

Builds on: The particle model of matter — “Melting is particles loosening their grip — the model does the explaining.” · Drawing particle diagrams

Evaporating, boiling and condensing

Particles at a liquid's surface can escape into gas at any temperature; boiling is escape from everywhere at once; condensing is the return trip.

Builds on: The particle model of matter — “Evaporation is particles escaping — pure particle model.” · Melting and freezing

Diffusion: particles spreading out

Because particles never stop moving, smells cross rooms and squash spreads through water — no stirring required.

Builds on: The particle model of matter — “Diffusion is what restless particles can't help doing.” · Drawing particle diagrams

Day, night and Earth's spin

The Earth turns once a day, carrying you into and out of the Sun's light. Sunrise is not the Sun moving — it is you.

Seasons and the tilted Earth

The Earth orbits the Sun once a year, leaning at a constant tilt. Summer is when your half leans towards the Sun — higher sun, longer days — not when the Earth is closer.

Builds on: Day, night and Earth's spin — “Seasons ride on the spin-and-orbit picture that day and night set up.” · The solar system

The solar system

Eight planets orbit the Sun; moons orbit planets. Knowing the map — what circles what, and roughly how far — anchors everything else in space.

Current & simple circuits

Build circuits; measure current and voltage.

Builds on: Static electricity: charging by rubbing — “Current is charge on the move — static hands you the charge idea.”

Pressure & moments

Levers, turning forces and pressure in fluids.

Builds on: Forces & balance — “Moments are forces given a lever arm.”

Electromagnets

Coils, cores and fields you can switch off.

Builds on: Current & simple circuits — “No electromagnet without a working circuit.” · Plotting magnetic field lines

Power ratings on appliances

The watts printed on a kettle or charger say how fast it moves energy, not how much it holds.

Builds on: Energy stores & transfers — “A power rating is a transfer speed — transfers first, ratings second.”

The kilowatt-hour

Electricity bills count kilowatt-hours: run a one-kilowatt appliance for one hour and you have bought one unit.

Builds on: Power ratings on appliances — “A kilowatt-hour is a power rating times time; without the rating, the unit is noise.”

Conduction: heat through solids

Energy passes through a solid as jiggling particles nudge their neighbours — fast in metals, slow in wood or wool.

Builds on: Temperature is not energy — “Conduction is energy flowing from hot to cold — that difference has to mean something first.” · The particle model of matter

Convection: heat by flow

Warmed liquid or gas expands, becomes less dense and rises, carrying energy with it — the reason one radiator can heat a whole room.

Builds on: Temperature is not energy — “Convection moves energy down a temperature difference — no difference, no flow.” · Conduction: heat through solids · Density: how tightly matter packs — “Warm fluid rises because it's less dense — no density, no convection.”

Thermal radiation

Hot things give off energy as invisible radiation that crosses empty space — it is how the Sun warms the Earth with nothing in between.

Builds on: Temperature is not energy — “You can't see why hot things radiate until temperature means energy.” · Light travels in straight lines

Resultant force

Forces along a line combine into one overall force: add the pushes one way, subtract the other way, and see what is left.

Builds on: Drawing force diagrams — “You can't add arrows you haven't drawn.” · Friction and drag · Forces & balance — “Resultant force is 'balanced or not?' made precise.”

Pressure = force ÷ area

The same push hurts more through a drawing pin than a thumb — spreading force over area is what pressure measures.

Builds on: Measuring forces in newtons · Pressure & moments — “P = F ÷ A turns the pressure idea into a number.”

Pressure in liquids

Water pushes harder the deeper you go, and it pushes in every direction — which is why dams are built thicker at the bottom.

Builds on: Pressure = force ÷ area · Pressure & moments — “Pressure with depth builds on knowing what pressure is at all.”

Atmospheric pressure

We live at the bottom of an ocean of air, and its weight presses on everything. Climb higher and the press gets weaker.

Builds on: Pressure in liquids — “The atmosphere is the liquid-pressure argument run in air.”

Floating and upthrust

Water pushes up harder on the bottom of an object than down on its top — the difference is upthrust, and floating is upthrust matching weight.

Builds on: Pressure = force ÷ area · Pressure in liquids — “Upthrust is the pressure gap between an object's top and its bottom — depth does the lifting.” · Density: how tightly matter packs

Hearing and the ear

The ear turns air vibrations into signals: the eardrum shakes, tiny bones pass it on, and the range we can hear narrows as we age.

Builds on: Vibrations make sound — “The ear is a vibration detector — you need to know what it detects.” · Loudness and amplitude · Sound needs a medium

Light travels in straight lines

Light does not bend round corners on its own — it travels in straight lines, which is why shadows are sharp and we cannot see round walls.

Builds on: Light & sound basics — “From 'light bounces' to rules — straight lines are rule number one.”

Reflection: angle in, angle out

Light bounces off a mirror as neatly as a snooker ball off a cushion: the angle coming in equals the angle going out.

Builds on: Light travels in straight lines — “Angle-in-equals-angle-out only means something on straight-line paths.”

Drawing ray diagrams

Ray diagrams turn light into ruler lines and arrows, so you can work out where an image forms instead of guessing.

Builds on: Light travels in straight lines — “A ray diagram is the straight-line rule turned into a drawing.” · Reflection: angle in, angle out

Refraction: light changes direction

Light changes speed when it enters glass or water, and that change of speed bends its path — the reason a straw looks broken in a drink.

Builds on: Light travels in straight lines — “You only notice bending when you expected straight.” · Drawing ray diagrams

How the eye focuses light

The cornea and lens bend incoming light to a sharp point on the retina — a camera does the same job with glass and a sensor.

Builds on: Light travels in straight lines · Refraction: light changes direction — “The eye is a refraction machine — bending light to a point is its whole job.”

Circuit symbols and diagrams

Circuits are drawn in an international shorthand — neat symbols and straight lines — so anyone can read or build the same circuit.

Builds on: Current & simple circuits — “The symbols stand for components you've met in real circuits.”

Current around a series loop

Current is the flow of charge round the loop, and in a series circuit it is the same everywhere — nothing gets used up on the way.

Builds on: Current & simple circuits — “You trace current round a loop you can already build and measure.” · Circuit symbols and diagrams

Voltage: the electrical push

Physicists call it potential difference: the push a battery gives the charge. More volts means more energy per charge — and brighter bulbs.

Builds on: Current & simple circuits — “Voltage readings need a circuit to read them in.” · Current around a series loop

Series vs parallel circuits

Components in one loop share the push; components on separate branches each get the full push — the reason your lights do not all die together. Read the full explainer →

Builds on: Circuit symbols and diagrams · Current around a series loop — “Parallel rules are the series rules with a junction to negotiate.”

Heating and expansion

Heated particles jiggle harder and take up more room, so solids, liquids and gases all swell slightly when warm — bridges are built with gaps for exactly this.

Builds on: Temperature is not energy · The particle model of matter — “Expansion is particles claiming more room as they jiggle harder.”

Brownian motion

Smoke grains under a microscope jitter as if alive — knocked about by air particles too small to see. It was the first direct evidence that particles exist.

Builds on: The particle model of matter — “Brownian motion is the particle model caught on camera.” · Diffusion: particles spreading out

Density: how tightly matter packs

Density compares how much mass fits into a space. A kilogram of feathers and a kilogram of iron weigh the same — they just take up very different rooms.

Builds on: The particle model of matter — “Density is how tightly the particles pack — the model supplies the picture.”

Phases of the Moon

Half the Moon is always sunlit; the phase is simply how much of that lit half faces us as the Moon circles Earth each month.

Builds on: Light travels in straight lines · Day, night and Earth's spin · The solar system — “You need the Sun–Earth–Moon layout before you can light it correctly.”

Solar and lunar eclipses

When Sun, Moon and Earth line up exactly, one body shadows another: the Moon blocks the Sun, or the Earth shades the Moon. Rare, because the line-up must be perfect.

Builds on: Light travels in straight lines · Phases of the Moon — “Eclipses are the phase story's special alignments.”

Mass vs weight

Mass is how much stuff you are made of, in kilograms; weight is gravity's pull on that stuff, in newtons. Leave Earth and your weight changes — your mass does not.

Builds on: Contact and non-contact forces — “Weight is a non-contact pull — that's what splits it from mass.” · Forces & balance

Calculating weight (W = m × g)

Weight = mass × gravitational field strength. On Earth every kilogram is pulled with about 10 newtons — so your weight is your mass with a g attached.

Builds on: Mass vs weight — “You can only calculate weight once it stops meaning mass.”

Energy is always conserved

Energy is never created or destroyed — it only moves between stores. The total before always equals the total after.

Builds on: Energy stores & transfers — “Conservation is a claim about stores: nothing appears, nothing vanishes.”

Dissipation: energy spreading out

Energy tends to spread into warm, thin, hard-to-use forms — nothing is lost, but not all of it stays useful.

Builds on: Temperature is not energy · Energy is always conserved — “Dissipation is conservation's fine print — the total survives, the usefulness doesn't.”

Sankey diagrams

An arrow diagram where width means amount of energy: the useful share flows on, the wasted share branches off.

Builds on: Percentages of amounts (MATH) · Energy stores & transfers · Dissipation: energy spreading out — “A Sankey diagram is a picture of dissipation — you draw what thins out.”

Work = force × distance

Pushing something along transfers energy: the bigger the force and the further it moves, the more energy is transferred. That transfer is called work.

Builds on: Energy is always conserved · Measuring forces in newtons · Forces & balance — “Work is force times distance — no feel for forces, no feel for work.”

Calculating moments

A moment is a force times its distance from the pivot — the physics of why a long spanner beats a short one.

Builds on: Measuring forces in newtons · Drawing force diagrams · Pressure & moments — “You calculate moments once turning forces are in your hands.”

Relative motion

Speed depends on who is watching: walk up a moving train and the platform sees you fly. Motion is always measured against something.

Builds on: Speed = distance ÷ time — “Relative motion is speed with an honest observer attached.”

Acceleration: speeding up, slowing down

Acceleration measures how quickly speed changes — a sports car and a pushed trolley differ not in whether they speed up, but in how fast.

Builds on: Inequalities on a number line (MATH) · Speed = distance ÷ time — “Acceleration is how speed changes — speed has to come first.” · Friction and drag

Splitting white light

White light is every colour travelling together. A prism bends each colour by a slightly different amount, fanning them into a spectrum.

Builds on: Refraction: light changes direction — “A prism splits white light by bending each colour differently — refraction does the splitting.”

Seeing colour: filters and surfaces

A red jumper reflects red light and absorbs the rest; a filter passes one colour and blocks the others. Colour is what is left after the subtracting.

Builds on: Reflection: angle in, angle out · Splitting white light — “You can't reason about filters until white light is a mixture.”

Echoes and the speed of sound

Sound takes time to travel — about 340 metres each second in air. Time an echo's return and you can measure a distance you cannot reach.

Builds on: Substituting into formulae (MATH) · Speed = distance ÷ time — “An echo is a speed–distance–time question with a bounce in it.” · Reflection: angle in, angle out

Resistance: what opposes current

Everything a current passes through resists it a little — long and narrow wires more so. More resistance means less current for the same push.

Builds on: Current around a series loop — “Less current for the same push — you must be able to read the current.” · Voltage: the electrical push — “Resistance is what the push has to overcome.” · Series vs parallel circuits

Calculating resistance (R = V ÷ I)

One measurement of voltage and one of current gives a component's resistance, in ohms — the circuit world's most useful ratio.

Builds on: Solving two-step linear equations (MATH) · Resistance: what opposes current — “The formula only summarises a relationship you've already seen.”

Making an electromagnet stronger

More turns of wire, more current or an iron core all boost an electromagnet — and a fair test shows exactly how much each one helps.

Builds on: Current around a series loop · Electromagnets — “You can't strengthen an electromagnet you haven't built.”

Fault-finding a circuit

When a circuit will not work, guessing wastes time. Predict what each part should do, test with a meter, and let every reading rule places out.

Builds on: Circuit symbols and diagrams · Current around a series loop — “To find the break, you must know what an unbroken loop does.” · Series vs parallel circuits

Calculating density

Density = mass ÷ volume. Weigh it, measure its space — a ruler for blocks, displaced water for awkward shapes — then divide.

Builds on: Substituting into formulae (MATH) · Density: how tightly matter packs — “The formula puts a number on packing you can already picture.”

Gas pressure: particles hitting walls

A gas pushes on its container because billions of particles drum against the walls every second — squash the gas or heat it, and the drumming rises.

Builds on: Temperature is not energy · Atmospheric pressure · The particle model of matter — “Gas pressure is particles drumming on the walls.”

Why ice floats

Water is odd: freezing opens its particles into a roomier structure, so ice is less dense than the water it came from. That is why ponds freeze from the top.

Builds on: Floating and upthrust · Melting and freezing · Density: how tightly matter packs — “Floating ice is a density puzzle.”

Gravity on other worlds

Every planet and moon has its own g. Jupiter pulls harder than Earth, the Moon far more gently — same you, different weight everywhere you land.

Builds on: The solar system · Calculating weight (W = m × g) — “Other worlds just change g — the formula shows you where.”

The Sun is a star

Our Sun is an ordinary star seen close up; every night-sky star is a sun seen from unimaginably far away. That one swap reorganises the whole sky.

Builds on: Renewable and non-renewable resources · The solar system — “The Sun joins the stars only once the solar system's layout is clear.”

Galaxies and the universe

Stars gather in galaxies of billions; our Sun sits in one arm of the Milky Way, and the universe holds billions of galaxies more. The scale is the lesson.

Builds on: The Sun is a star — “Galaxies are collections of suns — the Sun-as-star step comes first.”

The light year

Space is so vast we measure it in time: a light year is how far light travels in a year. The next star over is four of them away.

Builds on: Speed = distance ÷ time · The Sun is a star — “Light-year distances start to bite at star scale.”

Absolute zero

A first look at the coldest possible temperature: cool anything and its particles slow; at −273 °C the motion runs out and no colder exists.

Builds on: Temperature is not energy · The particle model of matter — “Absolute zero is the particle model's full stop — the jiggling runs out.” · Heating and expansion

Why satellites stay up

A first look at orbits: a satellite is falling towards Earth constantly, but moving sideways so fast it keeps missing. Gravity supplies the endless turn.

Builds on: Acceleration: speeding up, slowing down · The solar system · Gravity on other worlds — “Orbits are gravity's endless fall — you need gravity beyond Earth first.”