Learning outcomes
- Use power as energy transferred or work done per unit time.
- Calculate efficiency from useful and total energy or power.
- Describe energy conversion in major generation methods.
- Compare sources by renewability, availability and environmental impact.
12.1 Power
Power is the rate of doing work or transferring energy: P = W/t = ΔE/t. The unit watt means joule per second. Two machines may perform the same work, but the one that completes it in less time has greater power.
Power rating does not directly state total energy use; energy = power × time. A lower-power device used for much longer can transfer more energy than a high-power device used briefly.
12.2 Efficiency
Efficiency = useful energy output / total energy input, or useful power output / total power input. Multiply by 100% when a percentage is required. Efficiency cannot exceed 100% because useful output cannot be greater than total input.
Improving efficiency reduces unwanted transfers. Lubrication reduces friction, insulation reduces heat transfer, streamlined shapes reduce drag and low-resistance conductors reduce electrical heating. The “wasted” energy is usually dissipated to internal stores of surroundings.
12.3 Thermal generation pathway
In fossil-fuel, biofuel and many nuclear power stations, an energy source heats water in a boiler or reactor. Steam turns a turbine, and the turbine drives a generator. The generator transfers mechanical energy into electrical energy. Cooling systems condense steam for reuse.
Nuclear fission releases nuclear energy without carbon dioxide at the reactor, but mining, construction, waste management and accident risk must be considered. Fossil fuels are reliable and dispatchable but non-renewable and release carbon dioxide and pollutants.
12.4 Renewable sources
Hydroelectric power uses falling or flowing water to turn turbines. It is renewable and can respond quickly, but dams flood land and alter ecosystems. Wind and solar have no fuel cost and low operational emissions but are intermittent and require suitable sites, storage or backup.
Geothermal energy uses thermal energy from Earth and can provide steady output in suitable regions. Tidal power is predictable but limited to certain coasts and can affect habitats. Wave power is renewable but devices face harsh marine conditions. Biofuels can be renewable if replanted, but land use may compete with food production and combustion still emits gases.
12.5 Comparing sources in examination answers
A strong comparison uses paired, relevant points rather than a list. State whether the resource is renewable, whether output is controllable or weather-dependent, and one specific environmental effect. Distinguish reliability from predictability: tides are intermittent but highly predictable, whereas wind is less predictable.
Costs may be mentioned when relevant, but vague claims such as “cheap” need context. Separate construction cost, fuel cost and operating cost. Avoid claiming renewable sources have no environmental impact.
Worked examples
Power climbing stairsA student gains 1800 J of GPE in 6.0 s. Power = 1800/6.0 = 300 W.
EfficiencyA motor takes 500 W electrical power and provides 360 W useful mechanical power. Efficiency = 360/500 ×100% = 72%.
Energy usedA 1.5 kW heater operates for 20 min = 1200 s. Energy transferred = 1500×1200 = 1.8×10⁶ J.
Practical focus
InvestigationEstimate personal power by timing a student climbing stairs. Measure mass and vertical height, calculate mgh and divide by time. Discuss why this estimates mechanical output power rather than total metabolic input power.
Examination guidance
- Efficiency ratio and percentage must use corresponding useful and total quantities.
- Do not write that turbines “make energy”; they transfer energy and drive generators.
- In comparisons, name the actual environmental effect rather than writing only “pollution”.
Check your understanding
- Define power.
- A device is 80% efficient with 200 J input. Useful output?
- Give one advantage and one disadvantage of wind power.
Answers
- Work done or energy transferred per unit time.
- 160 J.
- Renewable/low operational emissions; intermittent and site-dependent, among other valid answers.
