2.2.2Energy

Conservation and Dissipation of Energy — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 12

📚 Oxford AQA International GCSE Combined Science - Physics Hub / Topical Revision

0%read

Part 1 of 7The principle

The principle of conservation of energy says that energy cannot be created or destroyed; it can only be transferred from one store to another or dissipated. In a closed system the total amount of energy stays the same. This is one of the most reliable ideas in physics, and no experiment has ever shown it to fail.

The phrase "closed system" matters. If energy enters or leaves, for example by heating the surroundings, the system we chose is not closed, but the total energy of system plus surroundings is still constant.

Part 2 of 7Dissipation

When energy spreads into the surroundings as thermal energy, we say it is dissipated.

Dissipation

Dissipated energy is not lost from the universe; it is shared among so many particles that it is no longer practical to use.

A moving car brakes to rest: its kinetic energy is transferred to the thermal store of the brake discs, tyres and air.

You cannot easily collect it back to speed the car up again.

This is why perpetual motion machines do not work. Every real machine dissipates some energy through friction, electrical resistance or air resistance, so it needs an energy supply to keep going.

Part 3 of 7Using conservation in calculations

For an object falling freely with negligible air resistance, the kinetic energy gained equals the gravitational potential energy lost:

½mv² = mgh

where m is mass in kg, v speed in m s⁻¹, g gravitational field strength in N kg⁻¹ (9.8 N kg⁻¹ here) and h the height fallen in metres. The mass cancels, so all objects dropped from the same height reach the same speed in the absence of air resistance: v = √(2gh).

For a pendulum or a roller-coaster, you can write: energy at the top (all GPE) = energy at the bottom (all KE) + any energy dissipated. If dissipation is small it can be ignored; if it is large you can find it by difference.

Part 4 of 7Energy accounting with diagrams

Sankey diagrams show energy flow as arrows whose width is proportional to the energy in each stream.

Energy accounting with diagrams

The input arrow splits into a useful output arrow and one or more wasted arrows,

and the widths add up: the arrow widths leaving must equal the arrow width entering.

You can also draw a simple energy transfer chart or bar chart for each point of a motion, with bars for KE, GPE and thermal energy.

The total bar height stays the same at each stage.

Part 5 of 7Why it matters

Conservation of energy is a check on every answer.

Why it matters

If your calculation gives a pendulum bob more energy at the bottom than it had at the top, something is wrong.

It is also the basis for efficiency (useful output compared with total input) and for designing devices that waste less energy.

Many exam questions simply ask you to account for where energy has gone: if a ball bounces to a lower height, the missing energy appears as thermal energy in the ball and floor and as sound.

Part 6 of 7Worked reasoning strategy

A reliable method for energy questions is to write a short energy ledger.

Step 1: list every store at the start and at the end.

Step 2: say which stores should change, for example GPE falls and KE rises.

Step 3: write an equation that says the total at the start equals the total at the end, adding a dissipated term if the question mentions friction, air resistance or a rebound that is lower than the drop height.

Step 4: solve and check that the answer is sensible, for instance that a ball cannot be faster than the speed it would have in a vacuum.

Roller coasters are a good test case.

Worked reasoning strategy

The first hill must be the highest because the cars start with no kinetic energy and must have enough GPE to climb every later hill and pay for the energy dissipated by friction.

If a later hill were higher than the first, the cars could not reach its top, since that would require energy to appear from nowhere.

Similarly a pendulum released at 10 cm above its lowest point will never swing higher than 10 cm on the other side,

and in practice always peaks a little lower.

Part 7 of 7Worked examples

A 0.20 kg ball is dropped from rest from 5.0 m. Ignoring air resistance, find its speed just before it hits the ground.
A 60 kg skier starts from rest at the top of a slope 40 m above the bottom. She reaches the bottom at 20 m s⁻¹. How much energy is dissipated?

Key terms

Term	Meaning
Conservation of energy	Energy cannot be created or destroyed, only transferred between stores.
Dissipated energy	Energy spread out as thermal energy in the surroundings and not easily recovered.
Closed system	A system where no energy enters or leaves.
Sankey diagram	A diagram with arrows whose width shows the amount of energy in each stream.
Friction	A force opposing motion that transfers kinetic energy to thermal energy.

Avoid these mistakes

Thinking energy disappears when a car stops. It is transferred to thermal energy (mainly), so the total is unchanged.

Using the wrong form of the equation, for example mgh = mv² without the ½.

Forgetting that dissipated energy is still conserved; "wasted" does not mean "destroyed".

Using the mass of the object in the speed calculation as if heavier objects must fall faster, when the mass cancels.

Last updated: 20 October 2026