Energy Transfer Examples Revision Notes | GCSE AQA Higher Physics
In an electric kettle, electrical energy from the mains transfers almost completely to thermal energy in the water. Free GCSE Physics notes and 15 FAQs.
Updated 20 Mar 2026 · Revise Right Team
Energy Transfer Examples is part of Energy changes in a system, and the ways energy is stored before and after such changes in GCSE AQA Higher Physics.
4 min
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GCSE AQA Higher
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What this lesson covers
In an electric kettle, electrical energy from the mains transfers almost completely to thermal energy in the water. Free GCSE Physics notes and 15 FAQs.
Describe energy transfers in four key scenarios: projectile motion, collisions, vehicle braking, and heating water
Explain how energy changes form but is conserved in each transfer process
Calculate energy changes using appropriate equations and identify energy stores before and after transfers
Apply conservation of energy principles to predict outcomes in unfamiliar situations
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Use this lesson to stay inside Energy changes in a system, and the ways energy is stored before and after such changes while connecting the detail back to the wider Energy topic.
Energy
Energy changes in a system, and the ways energy is stored before and after such changes
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Energy Transfer Examples before testing yourself from memory.
1. Energy Transfers Around Us
Energy never disappears - it just moves around and changes form. When you throw a ball upwards, brake a car, or boil water for tea, energy is constantly transferring between different stores.
Understanding these transfers helps us predict what happens next and explains why perpetual motion machines are impossible. Energy always spreads out, often ending up as heat in the surroundings.
We'll examine four classic scenarios that appear regularly in GCSE exams. Each one demonstrates the same fundamental principle: energy is conserved, but it changes form and location.
2. Projectile Motion - The Upward Journey
Picture throwing a tennis ball straight up into the air. At the moment it leaves your hand, all the energy is in the kinetic energy store - the ball is moving fast but hasn't gained height yet.
As the ball rises, it slows down because gravity pulls against its motion. The kinetic energy store decreases whilst the gravitational potential energy store increases. At the highest point, the ball stops momentarily - all energy is now gravitational potential.
During the fall, this process reverses. Gravitational potential energy transfers back to kinetic energy. The ball speeds up as it drops, reaching maximum speed just before it hits the ground.
3. Collision Dynamics - When Objects Meet Obstacles
When a moving object hits a stationary obstacle, the kinetic energy doesn't just vanish. It transfers rapidly into other energy stores, often with dramatic results.
Consider a car hitting a concrete barrier. The kinetic energy store empties in milliseconds, transferring mainly to thermal energy (heat) and sound energy. The metal deforms, particles vibrate faster (heating up), and sound waves carry energy away from the crash site.
Some energy also goes into elastic potential energy as materials compress and bend. However, most materials can't spring back perfectly after such violent deformation, so much of this becomes thermal energy too.
Worked example: Calculating Energy in a Collision
A 1200 kg car travelling at 15 m/s hits a wall and stops. Calculate the kinetic energy that must be transferred. [3 marks]
$KE = \frac{1}{2}mv^2$ — This gives energy in joules when m is in kg and v is in m/s [M1]
$KE = \frac{1}{2} \times 1200 \times 15^2$ — Mass = 1200 kg, velocity = 15 m/s [M1]
$KE = \frac{1}{2} \times 1200 \times 225 = 135,000$ J — Remember to include units in your final answer [A1]
Answer: 135,000 J (or 135 kJ)
4. Vehicle Braking - Controlled Energy Transfer
When you brake a car, you're deliberately transferring kinetic energy to thermal energy. The brake pads press against the discs, creating friction that converts the car's motion into heat.
This is why brake discs get extremely hot during heavy braking - they're absorbing all the kinetic energy that was stored in the moving vehicle. Racing cars often have glowing brake discs after hard cornering.
The energy transfer rate determines how quickly the car stops. More friction means faster energy transfer and shorter stopping distances. However, there's a limit - too much friction causes the tyres to skid, reducing control.
5. Heating Water - Electrical to Thermal Transfer
An electric kettle demonstrates one of the most efficient energy transfers in your home. Electrical energy from the mains supply transfers almost completely to thermal energy in the water.
The heating element converts electrical energy to thermal energy through resistance. As current flows through the element, it heats up and transfers this thermal energy to the surrounding water molecules.
The water molecules gain kinetic energy - they vibrate faster and move around more. When they gain enough energy, they change state from liquid to gas, forming steam bubbles that rise to the surface.
Worked example: Energy Required to Heat Water
Calculate the energy needed to heat 2 kg of water from 20°C to 100°C. Specific heat capacity of water = 4200 J/kg°C. [4 marks]
$E = mc\Delta T$ — Where E is energy (J), m is mass (kg), c is specific heat capacity (J/kg°C), ΔT is temperature change (°C) [M1]
$\Delta T = 100 - 20 = 80°C$ — Final temperature minus initial temperature [M1]
$E = 2 \times 4200 \times 80$ — m = 2 kg, c = 4200 J/kg°C, ΔT = 80°C [M1]
$E = 672,000$ J = 672 kJ — Always include units and consider converting to more convenient units [A1]
Answer: 672,000 J (or 672 kJ)
6. Exam Success - Energy Transfer Mastery
Energy transfer questions are exam favourites because they test your understanding of conservation principles. Examiners often ask you to identify energy stores before and after a change, then explain the transfer process.
The key skill is recognising that energy never disappears - it always goes somewhere. When kinetic energy seems to 'vanish' during braking, it's actually transferred to thermal energy stores in the brakes and surroundings.
Watch out for questions asking about efficiency. Not all energy transfers are useful - some energy always spreads to the surroundings as heat. This is why no machine is 100% efficient.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
What is the energy transfer in an electric kettle?
In an electric kettle, electrical energy from the mains supply transfers almost completely to thermal energy in the water through the heating element's resistance. Heating 2 kg of water from 20°C to 100°C needs 2 × 4200 × 80 = 672,000 J.
Why are perpetual motion machines impossible?
Perpetual motion machines are impossible because energy always spreads out, often ending up as heat in the surroundings. Energy is conserved, but in every transfer some energy goes to the surroundings as heat rather than being useful.
How does friction affect a car's stopping distance?
More friction between the brake pads and discs means faster energy transfer from kinetic to thermal energy and a shorter stopping distance. There is a limit, though: too much friction causes the tyres to skid, reducing control.
What are the energy transfers when a ball is thrown up?
When a ball is thrown upwards, its kinetic energy store decreases while its gravitational potential energy store increases. At the highest point it stops momentarily and all energy is gravitational potential; during the fall this reverses and it reaches maximum speed just before hitting the ground.
What energy transfer happens when a car brakes?
When a car brakes, kinetic energy is deliberately transferred to thermal energy. The brake pads press against the discs, creating friction, so the brake discs get extremely hot because they absorb the kinetic energy stored in the moving vehicle.
Where does the energy go in a car crash?
In a car crash, the kinetic energy store empties in milliseconds, transferring mainly to thermal energy and sound energy. The metal deforms, particles vibrate faster, and some energy goes into elastic potential energy, though most of this also becomes thermal energy. A 1200 kg car at 15 m/s transfers 135,000 J.
Why is no machine 100% efficient?
Machines cannot be 100% efficient because not all energy transfers are useful: some energy always spreads to the surroundings as heat. Energy never disappears, but it often ends up as heat in the surroundings, which is also why perpetual motion machines are impossible.
What energy does a ball have at its highest point?
At its highest point, a ball thrown straight up stops momentarily, so all its energy is in the gravitational potential energy store. As it falls, this transfers back to kinetic energy, and the ball reaches maximum speed just before it hits the ground.
Why do brake discs get hot?
Brake discs get hot because braking transfers the car's kinetic energy to thermal energy. The brake pads press against the discs, and the friction converts the car's motion into heat, which is why racing cars often have glowing brake discs after hard cornering.
How does a kettle heating element work?
A kettle heating element converts electrical energy to thermal energy through resistance. As current flows through the element it heats up and transfers thermal energy to the water, whose molecules gain kinetic energy, vibrate faster and, with enough energy, change state to form steam bubbles.
How much kinetic energy is transferred when a 1200 kg car at 15 m/s hits a wall?
A 1200 kg car travelling at 15 m/s that hits a wall and stops must transfer 135,000 J, or 135 kJ, of kinetic energy. Using KE = ½mv², KE = ½ × 1200 × 15² = ½ × 1200 × 225 = 135,000 J.
How much energy is needed to boil 2 kg of water from 20°C?
Heating 2 kg of water from 20°C to 100°C needs 672,000 J, or 672 kJ. Using E = mcΔT with c = 4200 J/kg°C and ΔT = 100 − 20 = 80°C, E = 2 × 4200 × 80 = 672,000 J.
Why does a ball slow down as it rises?
A ball thrown upwards slows down as it rises because gravity pulls against its motion. Its kinetic energy store decreases whilst its gravitational potential energy store increases, until it stops momentarily at the highest point.
Why does water take a long time to heat up?
Water takes a long time to heat up because its specific heat capacity is high, 4200 J/kg°C, meaning 4200 J is needed to raise the temperature of 1 kg of water by 1°C.
What happens to elastic potential energy in a crash?
In a crash, some kinetic energy goes into elastic potential energy as materials compress and bend. Most materials cannot spring back perfectly after such violent deformation, so much of this also becomes thermal energy.