Work, Power & Energy ResourcesEdexcel IGCSE Physics: Revision notes
Section 1
What is work done?
Work done is the energy transferred when a force moves an object. The relationship is:
W = F × d
where W is work done (J), F is force (N), and d is distance moved in the direction of the force (m).
Work done equals energy transferred — doing work on an object is exactly how energy is transferred to or from it (e.g. by lifting, pushing or stretching).
A force of 20 N pushes a box 3 m. Work done = F × d = 20 × 3 = 60 J, so 60 J of energy is transferred.
Section 2
How are gravitational potential energy and kinetic energy calculated?
Gravitational potential energy (GPE) is the energy an object has due to its height above a reference point:
GPE = m × g × h
where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).
Kinetic energy (KE) is the energy an object has due to its motion:
KE = ½ × m × v²
where m is mass (kg) and v is speed (m/s).
A 2 kg object is 5 m above the ground (g = 10 N/kg). GPE = 2 × 10 × 5 = 100 J. If it falls freely, this converts to KE, so at ground level ½ × 2 × v² = 100 J, giving v ≈ 10 m/s.
Section 3
How does conservation of energy link GPE, KE and work?
By the conservation of energy, when an object falls freely, the gravitational potential energy it loses is transferred into kinetic energy gained (ignoring air resistance):
loss in GPE = gain in KE
More generally, work done against a force (e.g. gravity, friction) transfers energy between stores — for example, work done lifting an object against gravity increases its GPE by the same amount of energy.
This simple GPE-to-KE conversion only applies exactly when air resistance/friction is ignored — in reality some energy is always transferred to thermal stores.
Section 4
What is power?
Power is the rate of transfer of energy, or equivalently the rate of doing work:
P = W / t
where P is power (W), W is work done/energy transferred (J), and t is time (s).
A higher power means energy is transferred (or work is done) more quickly.
A motor does 1200 J of work in 4 s. Power = W/t = 1200/4 = 300 W.
Section 5
What energy resources are used to generate electricity?
Electricity is generated from a range of resources, each with a different energy transfer pathway:
| Resource | Energy transfer to generate electricity |
|---|---|
| Wind | Kinetic energy of moving air turns turbines connected to generators |
| Water (hydroelectric/tidal) | Gravitational/kinetic energy of moving water turns turbines connected to generators |
| Geothermal | Thermal energy from hot rocks underground heats water to steam, which turns turbines |
| Solar heating systems | Radiation from the Sun directly heats water for domestic use |
| Solar cells | Radiation from the Sun is converted directly into electrical energy |
| Fossil fuels | Chemical energy is released by burning, heating water to steam, which turns turbines |
| Nuclear power | Nuclear energy is released by fission, heating water to steam, which turns turbines |
Renewable resources (wind, water, geothermal, solar) will not run out and generally have lower pollution during operation, but can be unreliable (e.g. wind/solar depend on weather) and have high initial set-up costs or land-use impact.
Non-renewable resources (fossil fuels, nuclear) provide reliable, large-scale continuous power, but fossil fuels release greenhouse gases and pollutants, and nuclear power produces radioactive waste and has high decommissioning costs, though neither runs out quickly in supply terms compared to renewables' availability limits.
When comparing resources, always balance reliability, cost, pollution and scale — a strong answer weighs both advantages and disadvantages rather than only listing one.
Must Know
- W = F × d; work done equals energy transferred
- GPE = mgh and KE = ½mv²
- Conservation of energy links GPE, KE and work: loss in GPE = gain in KE for a freely falling object
- P = W/t; power is the rate of energy transfer/doing work
- Wind, water and geothermal all ultimately turn turbines connected to generators; solar cells convert radiation directly to electricity
- Fossil fuels and nuclear power heat water to steam to turn turbines
- Renewables don't run out but can be unreliable; non-renewables are reliable but cause pollution/waste and will eventually run out
That's the notes covered.
Carry on to the next subtopic.