AQA GCSE Physics (8463) — Energy
If you trap a swinging pendulum inside a perfect vacuum, it would theoretically never stop swinging.
Within a closed system, energy can be transferred between different stores, but the net change in total energy is always zero. This is due to the Principle of Conservation of Energy, which states that energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.
We can verify that energy is conserved by calculating the energy transfers step-by-step. If we treat an electric kettle and its contents as a closed system:
Step 1: Identify the total initial energy. The kettle receives of electrical work.
Step 2: Identify the final energy stores. is transferred to the useful thermal store of the water. is transferred to the non-useful thermal store of the kettle body and trapped air.
Step 3: Calculate the net change. First, calculate the total final energy: . Then, subtract the initial energy from the final energy: .
The total energy before exactly equals the total energy after. Because no energy has entered or left the system boundary, the net change is precisely zero.
Every time you use a mobile phone, it eventually gets warm in your hands.
Energy is typically dissipated through specific transfer pathways:
Students often state that energy is 'lost', 'produced', or 'created'. You must never use these words in an exam; always write that energy is 'transferred' between stores or 'dissipated'.
When asked to describe wasted energy, AQA examiners specifically look for the exact phrase 'transferred to the thermal energy store of the surroundings' to award the mark.
If a question asks you to 'describe' dissipation, always clearly state three things: the initial store, the transfer pathway (e.g., mechanical work done against friction), and the final store.
System
An object or a group of objects being studied.
Open system
A system that can exchange both matter and energy with its surroundings.
Closed system
A system isolated from its environment, meaning it cannot exchange energy or matter with its surroundings; the net change in total energy is zero.
Net change
The difference between the total energy before and after a transfer. In a closed system, this is always zero.
Principle of Conservation of Energy
The rule that energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.
Dissipation
The process where energy is transferred to a non-useful store and becomes spread out into the surroundings, making it unavailable for useful work.
Wasted energy
Energy that is not transferred to a useful store for the intended purpose of the system.
Surroundings
Everything outside the boundary of the defined system, typically where wasted energy is dissipated.
Friction
A resistive force that opposes the motion of two surfaces in contact, causing energy to be transferred mechanically to thermal stores.
Electrical resistance
The opposition to the flow of electrical current, resulting in energy being transferred to the thermal store of the components.
Air resistance
A frictional force that opposes the motion of objects moving through the air.
Put your knowledge into practice — try past paper questions for Physics
System
An object or a group of objects being studied.
Open system
A system that can exchange both matter and energy with its surroundings.
Closed system
A system isolated from its environment, meaning it cannot exchange energy or matter with its surroundings; the net change in total energy is zero.
Net change
The difference between the total energy before and after a transfer. In a closed system, this is always zero.
Principle of Conservation of Energy
The rule that energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.
Dissipation
The process where energy is transferred to a non-useful store and becomes spread out into the surroundings, making it unavailable for useful work.
Wasted energy
Energy that is not transferred to a useful store for the intended purpose of the system.
Surroundings
Everything outside the boundary of the defined system, typically where wasted energy is dissipated.
Friction
A resistive force that opposes the motion of two surfaces in contact, causing energy to be transferred mechanically to thermal stores.
Electrical resistance
The opposition to the flow of electrical current, resulting in energy being transferred to the thermal store of the components.
Air resistance
A frictional force that opposes the motion of objects moving through the air.
