sciencegcse.co.uk← All lessons in this topic
Physics Paper 1 / Energy
Lesson 1.2 · AQA Combined Science

Conservation and dissipation of energy

Energy is conserved

Energy cannot be created or destroyed. It can be transferred between stores.

In a closed system, no energy enters or leaves, so total energy stays constant.

Useful and dissipated

A useful transfer does the intended job. Dissipated energy spreads into the surroundings and becomes less useful, often increasing thermal stores.

“Wasted” does not mean destroyed. The energy still exists but is harder to use for the intended purpose.

Example lamp: 100 J input; 30 J useful light transfer and 70 J dissipated.

Account for every joule

Input energy = useful output + dissipated energy

A motor receives 500 J and transfers 350 J usefully.

Dissipated = 500 − 350 = 150 J

Choose the system boundary carefully. A cooling drink loses energy, but the drink and its surroundings together conserve energy.

Describe collisions and braking

When a moving object hits an obstacle, its kinetic store decreases. Energy can increase thermal stores and deform objects; some is transferred by sound.

Friction transfers energy from moving parts into thermal stores. This explains why brakes can become hot.

Remember

Energy is conserved. Dissipated energy spreads into less useful stores; it is not destroyed.