GCSE Combined Science revision · Physics: energy, electricity, particles and atoms

Energy stores and power

The idea

Energy is stored in kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear stores. Kinetic energy Eₖ = ½mv²; gravitational potential energy Eₚ = mgh (g = 9.8 N/kg); elastic potential energy Eₑ = ½ke² (within the limit of proportionality). Power is the rate of energy transfer: P = E ÷ t, measured in watts (1 W = 1 J/s).

Write the equation.
Substitute in SI units (kg, m, s).
Calculate and give the unit.

Watch out In Eₖ = ½mv², only the speed is squared, not the mass.

See it worked

Worked example

A 60 kg runner moves at 5 m/s. Calculate her kinetic energy.

Eₖ = ½ × 60 × 5².
= 0.5 × 60 × 25 = 750 J.

Answer 750 J

Your turn

Question 1Calculator allowed2 marks

A 2.0 kg book is lifted 1.5 m. Calculate the gain in gravitational potential energy (g = 9.8 N/kg).

Question 2Calculator allowed2 marks

A motor transfers 1200 J of energy in 20 s. What is its power in watts?

Question 3Calculator allowed2 marks

A spring with spring constant 200 N/m is stretched by 0.10 m. How much elastic potential energy does it store, in joules?

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