Conservation of EnergyEdexcel GCSE Physics: Revision notes

Section 1

What does 'conservation of energy' mean?

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another.

In a closed system, there is no net change to the total energy — the total amount of energy before and after any change is the same.
Energy is stored in different ways: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, electrostatic and magnetic stores.
When a system changes, energy is transferred between these stores, but the total quantity of energy stays constant.

Key termsconservation of energyclosed system

Section 2

How do we calculate gravitational potential energy and kinetic energy?

Two key equations describe changes in stored energy:

Change in gravitational potential energy: $\Delta GPE = m \times g \times \Delta h$ $Δ GP E = m \times g \times Δ h$
- $m$ = mass (kg), $g$ = gravitational field strength (N/kg), $\Delta h$ = change in height (m)
Kinetic energy: $KE = \frac{1}{2} \times m \times v^2$ $K E = \frac{1}{2} \times m \times v^{2}$
- $m$ = mass (kg), $v$ = speed (m/s)

Energy transfer diagrams (such as Sankey diagrams) can be drawn and interpreted to represent how energy is transferred between stores, including how much is usefully transferred and how much is wasted.

Key termsgravitational potential energykinetic energy

Example

A ball dropped from height h converts GPE into KE as it falls; assuming no air resistance, GPE lost = KE gained.

Section 3

How is energy stored differently as a system changes?

For each type of system change, energy is transferred between stores:

System change	Energy transfer
Object projected upwards	Kinetic store → gravitational potential store
Moving object hits an obstacle	Kinetic store → mainly thermal store (and sound) of the object and surroundings
Object accelerated by a constant force	Chemical/other store (of whatever supplies the force) → kinetic store
Vehicle slowing down	Kinetic store → thermal store (of brakes, tyres, road, surroundings)
Water brought to the boil in a kettle	Electrical store → thermal store (of the water)

Exam tip

Examiners want the store named specifically (e.g. 'thermal store of the brakes') rather than a vague 'heat' or 'lost'.

Section 4

Why do energy transfers become wasteful?

In every real system change, energy is dissipated, meaning it spreads out and ends up stored in less useful ways — usually in the thermal store of the surroundings.
Mechanical processes (e.g. friction between moving parts) become wasteful when they cause a rise in temperature, dissipating energy to the surroundings rather than doing useful work.
Once energy has dissipated to the surroundings, it becomes increasingly spread out and difficult to use for further useful work, even though the total amount of energy is unchanged.

Key termsdissipated

Common mistake

Never say energy is 'lost' — it is always conserved. Say instead that it is 'dissipated to the thermal store of the surroundings' and becomes less useful.

Section 5

How can we reduce unwanted energy transfers?

Unwanted energy transfers (mostly to the thermal store of the surroundings through friction) can be reduced by:

Lubrication — reduces friction between moving surfaces, so less energy is dissipated as heat
Thermal insulation — reduces the rate at which thermal energy is transferred out of a system (e.g. loft insulation, cavity wall insulation)

For buildings, the rate of cooling depends on:

The thickness of the walls — thicker walls generally reduce the rate of heat loss
The thermal conductivity of the wall material — materials with lower thermal conductivity reduce the rate of heat loss more effectively

(These effects are described qualitatively — no calculation is required.)

Key termslubricationthermal insulationthermal conductivity

Must Know

Energy cannot be created or destroyed — only transferred between stores; total energy in a closed system stays constant.
$\Delta GPE = m \times g \times \Delta h$ and $KE = \frac{1}{2} \times m \times v^2$ .
Learn the standard energy transfers: projectile upwards (KE→GPE), object hits obstacle (KE→thermal/sound), constant force accelerating an object (chemical→KE), vehicle slowing (KE→thermal), kettle boiling water (electrical→thermal).
Mechanical processes become wasteful when they raise temperature, dissipating energy to the surroundings.
Energy is always dissipated in system changes, ending up stored in less useful ways — never say it is 'lost'.
Lubrication and thermal insulation reduce unwanted energy transfers; wall thickness and thermal conductivity affect a building's rate of cooling.

That's the notes covered.

Carry on to the next subtopic.