Power
- Power is the rate at which energy is transferred, or the rate at which work is done.
- P = E ÷ t and P = W ÷ t, where P is in watts (W), E and W in joules (J) and t in seconds (s).
- An energy transfer of 1 joule per second is a power of 1 watt. 1 kW = 1000 W.
- Two motors lifting the same load the same height: the one that does it faster has the higher power.
Efficiency
- Efficiency = useful output energy transfer ÷ total input energy transfer.
- Efficiency = useful power output ÷ total power input.
- Efficiency can be given as a decimal (such as 0.75) or a percentage (75%). It can never be more than 1 (100%), because energy cannot be created.
Wasted energy
- In any energy transfer, some energy is dissipated, usually as thermal energy to the surroundings. This is wasted energy.
- Wasted energy = total input − useful output.
- Ways to reduce waste: lubricating moving parts to reduce friction, using thermal insulation, and streamlining vehicles to reduce air resistance.
Sankey diagrams
- A Sankey diagram shows energy transfers. The width of each arrow is proportional to the amount of energy.
- The input arrow splits into useful output and wasted energy arrows, which add up to the input.
Key terms
- Power
- The rate at which energy is transferred or work is done.
- Watt
- The unit of power: 1 W = 1 J/s.
- Efficiency
- The proportion of the input energy that is transferred usefully.
- Useful energy
- Energy transferred to the store where it is wanted.
- Wasted energy
- Energy that is dissipated and not useful, usually as thermal energy.
- Sankey diagram
- A diagram with arrows whose widths show amounts of energy transferred.
- Kilowatt
- 1000 watts.