What is energy? Stores, transfers and the bar chart
By the end of this lesson you will be able to
- Define energy as the capacity to make something move or change, and state its unit, the joule.
- Identify the main energy stores: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear — and the ways energy is transferred: by forces, by heating, by electricity, by light and sound.
- Describe an energy transformation naming two forms — the one it starts in and the one it ends in — with the useful and the wasted parts.
- Draw and interpret an energy bar chart for the start, middle and end of a process.
- Estimate everyday energies in joules and compare them sensibly.
The puzzle
A bowl of pap and a litre of petrol both contain “energy”. A stretched catapult, a car on a hill, a hot cup of tea and a charged phone all contain “energy” too. Nobody has ever seen energy, weighed it or held it in their hand — yet it is the most important idea in the whole of science, and every electricity bill, diet plan and climate treaty is written in its unit. So what is it, exactly?
Write your own definition of energy in one sentence. Then list three things on your desk right now and say what kind of energy each one has stored in it, if any.
Compare your answer
Most first attempts say “energy is what makes things work” or “energy is power” — close, but circular. The scientific definition: energy is the capacity to make something move or change: to lift, push, heat, light or drive a current. On your desk: a pen (no store worth naming unless it is up high — then a little gravitational potential energy), a phone (chemical energy in its battery), an elastic band (elastic potential energy if stretched), a cup of warm tea (thermal energy). If you said the phone has “electrical energy”, almost: the battery stores chemical energy and transfers it electrically when you use it.
The science — structured notes
1.1 Energy is the capacity to cause change
The IB’s definition for this unit is short: energy is the capacity to move an object — and, more widely, to make anything happen: heat it, light it, stretch it, make it react. Whenever something changes, energy has been transferred from one place or form to another. The unit of energy is the joule (J): one joule is roughly the energy needed to lift an apple through one metre. A kilojoule (kJ) is 1000 J; a megajoule (MJ) is a million.
| Roughly how much energy? | Joules |
|---|---|
| Lifting an apple 1 m | 1 J |
| A sprinter at top speed | 4 000 J (4 kJ) |
| Boiling a kettle of water | 300 000 J (300 kJ) |
| A bowl of pap | 1 200 000 J (1.2 MJ) |
| A day’s food for a teenager | 10 000 000 J (10 MJ) |
| A litre of petrol | 35 000 000 J (35 MJ) |
| Lightning strike | 1 000 000 000 J (1 GJ) |
1.2 The stores: where energy sits
Energy is described by the store it sits in. There are only a handful of stores, and you should be able to name the right one at a glance:
| Store | What it depends on | Examples |
|---|---|---|
| Kinetic | Mass and speed of a moving object | A running cheetah, wind, a thrown ball, moving water |
| Gravitational potential | Mass and height above the ground | Water in a dam, a raised hammer, a diver on the board |
| Elastic potential | How far something is stretched or squashed | A drawn bow, a compressed spring, a bouncing ball at the floor |
| Chemical | The bonds between atoms in a substance | Food, petrol, wood, a battery, the ATP in your muscles |
| Thermal (internal) | The temperature and mass of an object — how fast its particles jiggle | Hot tea, a warm engine, the Sun’s surface |
| Nuclear | The nucleus of atoms | Uranium in a reactor, hydrogen in the Sun |
| Magnetic / electrostatic | Position of magnets or charges | Two magnets pulled apart, a charged balloon |
1.3 The transfers: how energy moves between stores
Energy moves from one store to another by a transfer pathway. There are four:
- Mechanically — a force does work: your hand pushes a trolley, gravity pulls a diver down, a spring pushes a ball.
- By heating — energy flows from a hotter object to a cooler one: a stove heats a pot.
- Electrically — a current carries energy: the battery to the phone screen.
- By radiation — light, sound and other waves carry energy: the Sun to a solar panel, a loudspeaker to your ear.
An energy transformation is what happens when a transfer changes the store: chemical (petrol) → kinetic (car) → thermal (brakes, air). Describing one is an exam skill with a rule the 2026 physics examiners spelled out: “two energy forms are required to describe an energy transfer”. “The hearing aid uses electrical energy” scored nothing; “chemical energy in the battery → electrical → sound energy” scored.
Describe the energy transformations when a torch is switched on.
Start: chemical energy stored in the battery.
Transfer: electrically, by the current through the bulb.
End: light radiated from the bulb (useful) and thermal energy in the bulb and the air (wasted).
Sentence: “Chemical energy in the battery is transferred electrically and transformed into light energy, with some transformed into thermal energy that warms the bulb.”
Write the transformation for each, naming the starting store, the transfer pathway, the useful end store and the wasted one.
(a) A girl eats breakfast and then runs up a hill.
(b) A solar panel charges a phone.
(c) A gas stove boils water.
Reveal answer
(a) Chemical (food) → mechanically, by her muscles → kinetic (running) and gravitational potential (height gained), with much wasted as thermal (she gets hot).
(b) Light (from the Sun) → by radiation to the panel → transferred electrically → chemical store in the phone battery; some wasted as thermal in the panel and wires.
(c) Chemical (gas) → by heating (the flame) → thermal store of the water (useful) and of the pan, air and kitchen (wasted).
1.4 The energy bar chart
The clearest way to show a transformation is a bar chart of the stores at each stage: one bar per store, one chart per moment. The 2026 examiners listed “interpreting energy bar charts representing intermediate stages of motion” as a specific weakness, so learn to draw them:
Chart 1: all the energy is kinetic — the ball is moving fast, at ground level.
Chart 2: KE has fallen to 48 %, GPE has risen to 48 % (the ball is slower and higher), and 4 % has become thermal (air resistance). The total is still 100 %.
Chart 3: KE is zero (the ball stops for an instant at the top), GPE is at its maximum, and thermal has grown to 8 %. If there were no air resistance, GPE would be exactly 100 % here.
The rule the charts show: the bars move; the total does not. That is the law of conservation of energy, Lesson 3.3.
A pendulum bob is pulled to the side and released. Draw (describe) energy bar charts for KE, GPE and thermal at:
(a) the moment of release;
(b) the bottom of the swing;
(c) the far side of the first swing, which is slightly lower than the release point.
Reveal answer
(a) GPE bar full (100 %), KE zero, thermal zero — it is high and not yet moving.
(b) KE bar almost full (say 98 %), GPE zero (lowest point), thermal small (2 %, from air resistance and the string’s friction).
(c) KE zero (it stops for an instant), GPE about 96 % (slightly lower than the start), thermal 4 %. Each swing transfers a little more to thermal, which is why a pendulum eventually stops.
1.5 Useful and wasted: where the rest goes
Every real transformation has a useful part (what you wanted) and a wasted part (what you did not) — and the wasted part is almost always thermal: friction, air resistance and electrical resistance all warm things up. The energy is not destroyed; it has spread out into the surroundings where it is no longer useful. A lamp that turns 10 % of its electrical energy into light and 90 % into heat is a heater that happens to glow. Later in the unit this becomes a number — efficiency — and, in Lesson 3.5, a reason why the world’s energy choices matter.
An electric drill: electrical energy → kinetic (the bit turning: useful), thermal (motor and bit warming: wasted), sound (the whine: wasted). A hairdryer: electrical → thermal in the air (useful!) and kinetic of the air (useful) and sound (wasted). The same store can be useful in one device and wasted in another; it depends on what the device is for.
“When a car brakes to a stop, its kinetic energy is used up.”
Reveal answer
False. Energy is never used up. The car’s kinetic energy is transformed, by friction in the brakes and the tyres, into thermal energy — the brake discs of a heavily braked car glow red — and some into sound. The energy still exists, spread through the discs, the air and the road; it is simply no longer in a store that can move the car. “Used up” is the phrase examiners cross out; “transferred to the thermal store of the surroundings” is the phrase they tick.
Try it — the stretched spring
First, a real one: stretch an elastic band between your fingers, hold it against your lip and let it snap back. It feels warm — elastic potential energy became kinetic, then thermal. Now the lab: a spring stores elastic potential energy when a mass stretches it. Hang five different masses, three trials each, and record the extension. Then think about energy: which mass has stored the most energy in the spring, and what happens to that energy if the mass is released? Where did the energy come from in the first place — what store, and what pathway?
A 500 g mass is hung on the spring and lowered gently until it hangs still.
(a) Name the store the energy came from and the store it went into.
(b) Instead, the mass is released suddenly from the unstretched position. Describe what happens and name the stores involved as it bounces.
(c) The bouncing dies away after a minute. Explain where the energy has gone.
Reveal answer
(a) From the gravitational potential store of the mass (it went down) into the elastic potential store of the spring (it stretched) — transferred mechanically, by the force of the mass on the spring. Your hand took away a little as well.
(b) It falls, overshoots the rest position and bounces up and down: GPE → KE → elastic potential (at the bottom, spring most stretched, mass momentarily still) → KE → GPE (at the top) and round again.
(c) Air resistance on the mass and internal friction in the spring’s metal transform a little energy into thermal energy each bounce; the total is conserved, but the mechanical stores drain into the surroundings until the mass hangs still.
Common mistakes to avoid
- Naming only one form — a transformation needs the start store and the end store. “It uses electrical energy” is not a description.
- “Energy is used up / lost / runs out” — it is transferred, usually to the thermal store of the surroundings.
- Calling light or electricity a store — they are transfer pathways between stores.
- Bars that do not add up — if the end bars are shorter than the start, a store is missing (usually thermal).
- Confusing temperature with thermal energy — a bath at 40 °C stores far more thermal energy than a cup at 90 °C, because it has far more particles (Lesson 3.4).
How we found out — the brewer who weighed heat
Until the 1840s “heat” and “motion” were thought to be different things: heat was a weightless fluid called caloric that flowed from hot to cold, and motion was something else. James Prescott Joule, the son of a Manchester brewer with no university post, spent years on an experiment that most physicists thought pointless. A falling weight turned a paddle wheel inside an insulated tub of water; the paddles stirred the water and, with a thermometer he had made to read a two-hundredth of a degree, he measured that the water got very slightly warmer. Same weight, same fall, same rise in temperature, every time: a fixed amount of mechanical energy always produced a fixed amount of heat. He even took a thermometer on his honeymoon to measure the temperature of water at the top and bottom of a waterfall.
His result — that heat and motion are the same quantity in different stores, and that the amount is conserved when one becomes the other — was rejected by the Royal Society in 1843 and mocked at a scientific meeting in 1847, until a young William Thomson (later Lord Kelvin) stood up and said the experiment deserved attention. Within a few years the law of conservation of energy was accepted as the deepest rule in physics, and the unit of energy was named after the brewer. Joule’s method is the model for this whole unit: put a number on every store, and check that the numbers add up.
Why did Joule need a thermometer far more precise than any then available, and what does that tell you about designing an experiment?
Reveal answer
Stirring water warms it by only a few hundredths of a degree for a reasonable fall of a weight, so an ordinary thermometer (reading to 1 °C) would show nothing — the effect would be buried in the instrument’s error. Joule had to build an instrument whose resolution was smaller than the effect he expected. The design lesson from Units 1 and 2 again: know the size of the change you are looking for, and choose an instrument whose error is smaller than it — or, like Galileo, find a way to make the change bigger.
Where it is used now
- Food labels give energy in kJ (and kcal). A 500 g bag of maize meal is about 7 MJ; a day’s requirement for a teenager is 9–12 MJ, and the balance between the two is a matter of stores: what is not used is stored as chemical energy (fat).
- Botswana Power Corporation sells energy in kilowatt-hours; 1 kWh = 3.6 MJ. A bill is a count of joules that flowed through the meter.
- Sports science measures the energy stored in a stretched tendon; the Achilles tendon returns about 35 % of the energy of each stride, which is why running is cheaper than it “should” be.
- Car crash engineering is about where kinetic energy goes: into crumpling metal (useful — it is not going into the passengers).
Science now — and the scientist you need to be
What is happening in 2026
The biggest energy story of the decade is the one every household can see: energy is being moved between stores on a scale never attempted. Solar farms transform light into electrical energy by day; grid-scale batteries hold it as chemical energy for the evening; in the Kalahari, Botswana’s first large solar plants now feed the grid that used to run on coal alone. AI is at both ends of the story. Data centres that run AI models are now among the largest new users of electricity on Earth, and AI is also what runs the grid: programs that predict tomorrow’s sunshine and demand to the hour and decide, second by second, which store to draw from. Meanwhile laboratories chasing nuclear fusion — the store that powers the Sun — have achieved reactions that release more energy than the lasers put in, though not yet more than the whole machine consumes.
Every one of these decisions is an energy bar chart. An engineer choosing between a battery and a pumped-water reservoir is comparing stores; a government deciding whether a data centre may be built is asking where the thermal store of all that waste heat will go. The person who can draw the chart, and notice when the bars do not add up, is the person the room listens to.
The mindshift
What you need to be able to do
Energy literacy — reading a bill, a food label, a battery specification or a climate report in joules and asking where they came from and where they went — is a life skill and an employable one. Every engineer, farmer, doctor, architect and policy-maker accounts for energy, and the account is always the same three questions: which store did it start in, which did it end in, and how much was wasted on the way?
See an example
Kettle, 2000 W, boils 1 litre in 3 minutes: 2000 J every second × 180 s = 360 kJ from the grid (electrical transfer). Charts: before — none in the water; during — thermal in the water rising, thermal in the kettle body and steam growing; after — about 330 kJ in the water (useful), 30 kJ in the kettle, steam and kitchen (wasted). 360 kJ ≈ 12 apples lifted to the top of Kgale Hill. The AI said “electrical energy heats the water” — one form named, no waste; I rewrote it with both.
Key terms
| Energy | The capacity to move an object or cause a change; measured in joules (J). |
| Joule (J) | The unit of energy; about the energy needed to lift an apple through one metre. 1 kJ = 1000 J; 1 MJ = 1 000 000 J. |
| Energy store | A way energy is held: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic. |
| Transfer pathway | How energy moves between stores: mechanically (forces), by heating, electrically, by radiation (light, sound). |
| Transformation | A change of energy from one store to another; described by naming at least the start and end stores. |
| Energy bar chart | A chart with one bar per store showing how much energy each holds at one moment; the total is the same at every stage. |
| Useful / wasted energy | The part of a transformation that does the intended job, and the part (usually thermal) that does not. |
| Dissipated | Spread out into the surroundings as thermal energy, where it can no longer be used. |
Think like a scientist
Criterion A A hydro-electric dam: water stored high in a reservoir flows down a pipe, spins a turbine connected to a generator, and electricity is sent to a town where it lights a lamp.
(a) List the energy stores and pathways in order from the reservoir to the light from the lamp.
(b) Draw (describe) energy bar charts for the water at the top of the pipe and at the turbine.
(c) Identify two places where energy is wasted and the store it goes into.
(d) Explain why the total energy of the light from the lamp is less than the energy the water had at the top, even though energy is conserved.
Model answer
(a) Gravitational potential store of the water → (mechanically, gravity pulling the water down the pipe) → kinetic store of the moving water → (mechanically, the water pushing the turbine blades) → kinetic store of the turbine and generator → (electrically, through the cables) → in the lamp, light radiated to the room (useful) and thermal store of the lamp (wasted).
(b) At the top: GPE bar full, KE zero, thermal zero. At the turbine: GPE zero, KE bar most of the height (say 90 %), thermal 10 % (friction in the pipe warms the water and pipe slightly). Same total.
(c) Friction in the pipe and turbine bearings → thermal store of the water, metal and air; electrical resistance in the cables → thermal store of the cables and air; the lamp itself → thermal (most of a filament lamp’s energy).
(d) Energy is conserved in total, but at every stage some is transformed into thermal energy that spreads into the surroundings and is not passed on. The light energy plus all the thermal energy dissipated along the way equals the original GPE; only the light was useful.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Define energy and state its unit.
Reveal answer
The capacity to move an object / to cause change [1]; the joule, J [1].
Describe the energy transformations in a wind-up toy car from winding it to it stopping.
Reveal answer
Chemical energy in the child’s muscles → (mechanically) elastic potential energy in the spring [1]; on release, elastic potential → kinetic energy of the car [1]; friction and air resistance transform the kinetic energy into thermal energy in the axles, floor and air (and a little sound) until the car stops [1].
A student writes: “A hearing aid uses electrical energy to make sound louder.” Explain why this would not score as a description of an energy transformation, and rewrite it.
Reveal answer
Only one energy form is named and the starting store is missing — a transfer needs at least two forms [1]. “Chemical energy in the battery is transferred electrically and transformed into sound energy, with some wasted as thermal energy” [1].
A ball is dropped and bounces to half its original height. Describe energy bar charts (KE, GPE, elastic, thermal) at the moment of release, at the instant the ball is fully squashed on the floor, and at the top of the bounce.
Reveal answer
Release: GPE 100 %, others 0 [1]. Fully squashed: KE 0, GPE 0, elastic potential large (about 60 %), thermal about 40 % (the squashing has already warmed the ball and floor) [1]. Top of bounce: GPE 50 %, KE 0, elastic 0, thermal 50 % — the total is still 100 % [1].
True or false, with a reason: “Light is an energy store.”
Reveal answer
False [1]. Light is a way energy is transferred (by radiation) from one store to another — from the thermal store of a bulb’s filament to the thermal store of your skin, for example; energy is not held “in” light in the way it sits in a stretched spring or a hot object [1].
A litre of petrol stores 35 MJ. A car engine transforms about 25 % of this into kinetic energy; the rest becomes thermal. Calculate the useful energy from one litre in joules, state where the other energy goes, and explain why an electric motor at 90 % changes the picture.
Reveal answer
0.25 × 35 000 000 = 8 750 000 J (8.75 MJ) useful [1]; the remaining 26.25 MJ becomes thermal energy in the engine, exhaust gases and air [1]. An electric motor wastes only 10 %, so for the same useful energy far less is dissipated as heat — which is why electric vehicles need less energy per kilometre, though the electricity must still come from some store [1].
Summary
- Energy is the capacity to move an object or cause change; unit the joule. One unit for food, fuel, motion and heat, because they are the same quantity.
- Energy sits in stores (kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic) and moves between them mechanically, by heating, electrically or by radiation.
- Describe a transformation with at least two forms: start store → pathway → useful end store + wasted (usually thermal).
- An energy bar chart shows the stores at one moment; the bars change, the total does not.
- “Used up” is wrong: energy is transferred and dissipated, never destroyed.
Next time → Kinetic and gravitational potential energy: putting numbers on the two stores that run the ramp, the pendulum and the dam.