Efficiency, Power and Energy Resources

Power

Power = energy transferred ÷ time      P = E / t
Power = work done ÷ time               P = W / t

Measured in watts (W); 1 W = 1 joule per second.

Efficiency

              useful output energy
efficiency = ───────────────────────   (× 100 for %)
               total input energy

• Always less than 1 (100%) — energy is always wasted, usually as heat.
• Reduce waste: lubrication (less friction), insulation (less heat loss).

Worked example

A motor takes in 200 J and usefully transfers 150 J:

efficiency = 150 / 200 = 0.75 = 75%

Energy resources

• Renewable (won't run out): solar, wind, hydroelectric, tidal, wave, geothermal, biomass.
• Non-renewable (finite): coal, oil, gas, nuclear.

	Renewables	Fossil fuels
Supply	Unlimited	Will run out
CO₂ emissions	Low/none	High
Reliability	Often weather-dependent	Reliable, on demand

Exam tip

Efficiency can never exceed 100%. Always state where the wasted energy goes (usually thermal energy to the surroundings).