Energy, work and power
Energy stores and transfers, conservation of energy, kinetic and gravitational potential energy, work, power, efficiency and energy resources.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.7.1Identify energy stores and describe transfers between them using the principle of conservation of energy.
- 1.7.2Recall and use Ek = ½mv² and ΔEp = mgΔh.
- 1.7.3Recall and use W = Fd and P = E / t.
- 1.7.4Calculate efficiency as useful output energy divided by total input energy.
- 1.7.5Describe how energy is obtained from a range of resources and compare their advantages and disadvantages.
8 minute read
Energy stores, work and power
Energy is never created or destroyed — it is transferred between stores. The examinable stores are: kinetic, gravitational potential, elastic (strain), chemical, nuclear, internal (thermal), electrostatic.
Work
Work done = force × distance moved in the direction of the force: W = Fd, in joules.
Doing work on an object is one way of transferring energy to it. If you lift a box, you do work against gravity and its gravitational potential store increases.
The two equations you will use most
- Kinetic energy:
Ek = ½mv² - Change in gravitational potential energy:
ΔEp = mgΔh
Notice the squared in the kinetic energy equation. Doubling the speed multiplies the kinetic energy by four — this is why stopping distances grow so sharply with speed.
Power
Power is the rate of energy transfer: P = E / t, in watts. One watt is one joule per second. It is also true that P = W / t since work is energy transferred.
Efficiency
efficiency = useful output energy ÷ total input energy (× 100 for a percentage).
Efficiency can never exceed 100%. If you calculate more than that, you have swapped the numerator and denominator.
Energy resources
You should be able to compare: fossil fuels, nuclear, hydroelectric, solar, wind, geothermal, tidal and biofuel — on availability, reliability, cost, and environmental impact. Almost all of them ultimately trace back to the Sun; the exceptions are nuclear, geothermal and tidal.
Think of it like this
Energy is like money in different accounts. Spending does not destroy it — it moves it. Efficiency is how much of the transfer actually reached the account you wanted.
Worked examples
Method, step by step
A 0.20 kg ball is thrown at 15 m/s. Calculate its kinetic energy.
- 1Ek = ½mv²
- 2Ek = ½ × 0.20 × 15²
- 3Ek = ½ × 0.20 × 225
Ek = 22.5 J
A motor lifts a 45 kg load 8.0 m in 12 s. Take g = 10 N/kg. Calculate the useful power output.
- 1Energy transferred = ΔEp = mgΔh = 45 × 10 × 8.0 = 3600 J
- 2P = E / t = 3600 / 12
P = 300 W
Common misconceptions
- Saying energy is "used up". It is transferred, often to the internal store of the surroundings where it is no longer useful.
- Forgetting to square the speed in ½mv².
- Using the total distance moved rather than the distance moved in the direction of the force when calculating work.
In the exam
- For efficiency, write the fraction before substituting — a wrong-way-up answer loses the method mark too.
- Energy-resource questions expect balanced answers: one advantage and one disadvantage, both specific.
- "Describe the energy transfers" means name the store it comes from and the store it goes to, in order.