Energy Stores and TransfersAQA GCSE Physics: Revision notes

Section 1

What Is a System and How Does Energy Move?

A system is an object or group of objects. When a system changes, energy is transferred between different energy stores - for example, kinetic, thermal, gravitational potential, elastic potential, chemical, and nuclear.

For everyday situations you must be able to describe every energy store involved:

An object projected upwards: kinetic energy store decreases, gravitational potential energy store increases (until it stops, then reverses on the way down)
A moving object hitting an obstacle: kinetic energy store decreases, thermal energy stores of the object and surroundings increase (and sound)
A vehicle slowing down: kinetic energy store decreases, thermal energy stores of the brakes/surroundings increase
Bringing water to a boil: chemical energy store of the fuel decreases, thermal energy store of the water increases

Key termssystemenergy store

Exam tip

When asked to 'describe the energy changes', always name the store that decreases AND the store(s) that increase - marks are given for both.

Section 2

Calculating Energy Transferred

Energy can be transferred to or from a system by:

Heating
Work done by a force
Work done when a current flows

Calculations can be used to show, on a common scale (e.g. a Sankey-style bar), how the total energy in a system is redistributed when the system changes - the total energy before and after a change is always the same.

Key termswork done

Section 3

Kinetic and Elastic Potential Energy

Kinetic energy is the energy a moving object has: Ek = 1/2 m v^2 (kinetic energy in J, mass in kg, speed in m/s).

Elastic potential energy is stored in a stretched or compressed spring, provided the limit of proportionality has not been exceeded: Ee = 1/2 k e^2 (spring constant in N/m, extension in m)

Key termskinetic energyelastic potential energy

Example

A 2 kg trolley moving at 3 m/s has Ek = 0.5 × 2 × 3² = 9 J.

Section 4

Gravitational Potential Energy and Thermal Energy

Gravitational potential energy depends on height above a reference point: Ep = mgh (g will be given in the exam).

The change in thermal energy stored in an object depends on its mass, specific heat capacity, and temperature change: ΔE = mcΔθ.

Specific heat capacity is the energy required to raise the temperature of one kilogram of a substance by one degree Celsius.

Key termsgravitational potential energyspecific heat capacity

Common mistake

Don't confuse specific heat capacity (temperature change, no state change) with specific latent heat (state change, no temperature change) - they are easily mixed up.

Section 5

Power: The Rate of Energy Transfer

Power is defined as the rate of energy transfer, or the rate of doing work: P = E/t and P = W/t (power in W, energy/work in J, time in s).

A more powerful motor transfers the same amount of energy in less time - for example, two electric motors lifting the same weight through the same height, but one does it faster, so it is more powerful.

Key termspower

Think of it like this

Power is like how quickly you empty a bath, not how much water is in it - two baths with the same amount of water empty at different rates depending on how wide the plughole is.

Must Know

Every energy change must state which store decreases and which store(s) increase
Ek = ½mv², Ee = ½ke², Ep = mgh, ΔE = mcΔθ
Specific heat capacity = energy to raise 1 kg by 1°C
Power = energy transferred ÷ time = P = E/t = W/t, measured in watts
Total energy in a system is conserved - it is only redistributed, never lost

That's the notes covered.

Carry on to the next subtopic.