GCSE Physics · Physics
Energy: a clear study guide
Energy is transferred between stores rather than used up. Describe the initial and final stores, identify the transfer pathway and use the equation that matches the quantities in the question.
Stores and pathways
Common energy stores include kinetic, gravitational potential, elastic, thermal, chemical and magnetic stores. Energy can be transferred mechanically, electrically, by heating or by radiation such as light or sound.
In a real device some energy is dissipated to the thermal store of the surroundings. Dissipated energy has not disappeared; it has spread to less useful places.
- Kinetic energy depends on mass and speed.
- Gravitational potential energy depends on mass, gravitational field strength and height.
- Power is the rate of energy transfer.
Useful equations
Use kinetic energy = 0.5 x mass x speed^2 and gravitational potential energy = mass x gravitational field strength x height. For power, use power = energy transferred / time or power = work done / time.
Efficiency compares useful output to total input. Write it as a decimal or multiply by 100 for a percentage, following the form requested by the question.
Explain efficiency clearly
When a question asks why a device is not 100 percent efficient, identify the unwanted transfer, such as heating due to friction or electrical resistance, and say where the energy goes. Do not describe energy as being destroyed.
Quick check
What does power measure?
Answer: The rate of energy transfer or work done.
What happens to dissipated energy?
Answer: It is transferred to the surroundings, usually increasing their thermal store.
Common questions
Is energy used up?
No. Energy is conserved, but it can be transferred to stores that are less useful for the intended task.
How do I calculate efficiency as a percentage?
Divide useful energy output by total energy input, then multiply by 100.