What Is Energy?
In 1845, English physicist James Prescott Joule proved that 4,184 joules of energy can heat exactly 1 litre of water by 1°C — making energy measurable for the first time in history.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · Before you get out of bed, your alarm clock, the light, and your heater have already used energy. Where did all that energy come from originally?
Q2 · A stretched rubber band just sits there — but it can launch a paper plane. Where is the energy before and after you launch it?
● Know
- Energy = the capacity to cause change
- Energy is measured in Joules (J)
- The seven main forms of energy
● Understand
- Why energy cannot be created or destroyed — only transformed
- The difference between kinetic and potential energy
- How to trace an energy transformation chain
● Can do
- Identify the main form of energy in everyday situations
- Match situations to energy forms
- Trace an energy chain (e.g. Sun → plant → you running)
- Kinetic energy
- Gravitational PE
- Chemical energy
- Thermal energy
- Elastic PE
- Stored in chemical bonds — food, fuel, batteries
- Energy of motion — moving car, flowing water
- Stored in a stretched or compressed object
- Stored by height — water in a dam, you at top of stairs
- Energy from the motion of particles — heat
Right now, without moving a muscle, your heart is squeezing 70 times per minute, your lungs are expanding, and your brain is processing these words — all of it powered by a stream of energy that came from your breakfast. Everything that happens requires energy. You can't do ANYTHING without it.
Definition: Energy is the capacity to cause change. It is measured in Joules (J), named after the English scientist James Prescott Joule.
To put the Joule in perspective:
- 1 J — lifts a small apple about 10 cm.
- ~100 J — the energy in one heartbeat.
- ~2,000,000 J (2 MJ) — a cheeseburger.
- ~3.6 × 10⁶ J (1 kWh) — running a light bulb for one hour.
- 3.8 × 10²⁶ J — the energy the Sun releases every single second.
Energy comes in many forms — but all of them can cause change, and all of them can be measured in Joules.
| Form | What it is | Examples |
|---|---|---|
| Kinetic | Energy of motion | Moving car, flowing water, wind, running person |
| Gravitational PE | Stored by height | Water behind a dam, you at top of stairs, a dropped ball before it falls |
| Elastic PE | Stored by stretch or compression | Spring, rubber band, bow drawn back, trampolining |
| Chemical | Stored in chemical bonds | Food, petrol, coal, batteries, explosives |
| Thermal (heat) | From particle motion — faster particles = more thermal energy | Hot cup of tea, warm engine, geothermal steam |
| Light (radiant) | Energy carried by electromagnetic waves | Sunlight, torch, laser, X-rays, microwaves |
| Electrical | Energy of moving charges (current) | Lightning, batteries, power lines, electric motors |
Two more forms for completeness: Sound — energy of vibrating particles (a thunderclap, music); Nuclear — energy stored in the atomic nucleus (very dense — tiny amounts equal enormous energy, as in nuclear power plants or the Sun).
One of the most important laws in all of science: Energy cannot be created or destroyed. It can only be transformed from one form to another. The total amount of energy in a closed system stays constant.
When energy seems to "disappear", it's actually being converted to heat, sound or light — forms that are harder to see but still real.
A real-world energy chain:
Chemical energy (food) → Chemical energy (muscle cell reaction) → Kinetic energy (movement) + Thermal energy (body warmth) + Sound energy (footsteps)
Nothing was created, nothing was destroyed — it all transformed.
Australian example — Snowy Hydro scheme: Water held in alpine reservoirs (gravitational PE) is released, flows downhill (kinetic energy), spins turbines (kinetic → electrical), and flows into homes as electricity (electrical → light + heat). At every step, energy transforms but the total never changes.
Wrong: "Burning petrol creates energy." Burning petrol transforms chemical energy (stored in bonds) into thermal energy and kinetic energy. You can't create energy — you can only change its form.
Right: Burning petrol TRANSFORMS chemical energy into thermal and kinetic energy. The total energy doesn't change — it just changes form.
Wrong: "A ball at rest has no energy." A ball on a high shelf is not moving, but it has gravitational potential energy — stored by its height. Drop it and that PE transforms into kinetic energy (and finally sound + heat when it hits the floor).
Right: Objects at rest can still have potential energy (gravitational, elastic, or chemical) even when they're not moving.
Wrong: "Energy is 'lost' when a bouncing ball doesn't return to its original height." Energy isn't lost — it's transformed into sound (the bounce sound) and thermal energy (tiny warming of ball and floor). The total is the same; it's just in less useful forms.
Right: "Lost" energy becomes heat and sound. Conservation of energy: the total is always the same.
A coal-fired power station burns coal to generate electricity. Trace the energy transformations from coal in the ground to a light bulb turning on in a Sydney home. Name each energy form at each step.
Show model answer
Model chain: Chemical energy (coal) → Thermal energy (furnace burns coal, heats water) → Kinetic energy (steam spins turbine) → Electrical energy (generator converts kinetic → electrical) → Electrical energy (transmission lines to Sydney) → Light energy + Thermal energy (light bulb converts electrical → light + heat). Note: at every step some energy "escapes" as heat — conservation of energy means the total is always constant.
Before turning on a torch (flashlight), where is all the energy stored? Write down every form of energy you predict will exist: (a) while the torch is on, and (b) one minute after the battery has gone completely flat.
How close was your prediction?
The hook at the start of this lesson asked: where did the energy in your breakfast originally come from? The answer traces an amazing chain — all the way back to the Sun!
Trace the complete energy transformation chain from the Sun to you running in a PE lesson. Then explain the rubber band launching a paper plane using the same conservation idea. Use the words elastic potential energy, kinetic energy, and conservation of energy at least once each.
Q1. Identify the main form of energy stored in each of the following: (a) a fully charged phone battery (b) a ball at the top of a ramp (c) a hot cup of tea (d) a compressed spring. (4 marks)
Q2. Trace the energy transformations from the Sun to a plant to you running. Name each energy form at each step. (3 marks)
Q3. Explain the law of conservation of energy and use it to explain what happens to the "lost" energy when a ball bounces and doesn't return to its original height. (4 marks)
Answers
▾MCQ 1
B — Energy is the capacity to cause change. It's not force × distance (that's work, covered later), not speed of a particle (that's about temperature), and not heat alone (that's one form of energy).
MCQ 2
C — A book at height has gravitational potential energy. It's not moving (no kinetic), has no compressed structure (no elastic PE), and we're not concerned with its chemical bonds here.
MCQ 3
C — Conservation of energy: energy cannot be created or destroyed. It always transforms from one form to another. The total never changes in a closed system.
MCQ 4
C — The SI unit of energy is the Joule (J). Newtons measure force, Watts measure power (rate of energy use), and kilograms measure mass.
MCQ 5
B — A wound-up (compressed or twisted) spring stores elastic potential energy — energy stored in its deformed shape, ready to release when it unwinds.
Short Answer 1
Model answer: (a) Phone battery → chemical energy (stored in chemical reactions inside the battery). (b) Ball at top of ramp → gravitational potential energy (stored by height). (c) Hot cup of tea → thermal energy (energy of fast-moving water particles). (d) Compressed spring → elastic potential energy (stored in the compressed/stretched shape). One mark each.
Short Answer 2
Model answer: Sun → light energy (radiant). Plant absorbs sunlight → chemical energy (stored in glucose via photosynthesis) — 1 mark. You eat the plant → chemical energy (in food). Your muscles use chemical energy → kinetic energy (you run) — 1 mark. Some energy also transforms to thermal energy (body heat) and sound (footsteps) — 1 mark.
Short Answer 3
Model answer: The law of conservation of energy states that energy cannot be created or destroyed — it can only be transformed from one form to another, and the total stays constant (1 mark). When a ball bounces and doesn't return to its original height, its gravitational PE at the top is less than before (1 mark). The "lost" energy has been transformed into thermal energy (the ball and floor warm up slightly from the impact) and sound energy (the "thud" of the bounce) — 1 mark. Nothing was destroyed — it simply changed into less visible forms (1 mark).