Energy, work, power and energy resources
Energy, work, power and energy resources tests energy transfers, calculations and evaluation of supply options.
Energy stores and transfers
- Energy is conserved; it is transferred between stores: kinetic, gravitational, elastic, thermal, chemical, nuclear, electrostatic.
- Work done W = Fs when force and displacement are in the same direction. Unit joule (J).
- Kinetic energy Ek = ½mv²; gravitational potential energy ΔEp = mgΔh.
- Power P = work/time = energy/time, unit watt (W). Efficiency = useful output / total input ×100%.
- Renewable resources include solar, wind, hydroelectric, wave, tidal, geothermal, biomass; non-renewable include coal, oil, gas, nuclear fuel.
- Evaluate: reliability, cost, environmental impact, power density, intermittency.
Worked example
A 2.0 kg ball is lifted 3.0 m. ΔEp = mgh = 2.0×10×3.0 = 60 J. If released and air resistance ignored, this becomes kinetic energy: 60 = ½×2.0×v², so v² = 60, v ≈ 7.7 m/s. If a motor lifts it in 2.0 s, useful power = 60/2.0 = 30 W. If the motor input is 40 W, efficiency = 30/40×100 = 75%.
Traps
Using mass instead of weight in Fs; forgetting to square velocity; mixing kW and W; efficiency over 100%; saying energy is 'used up' rather than transferred.