Energy is the capacity to do work
Energy is the capacity (or capability) to do work.
Energy is used or consumed when work is done.
The SI unit for energy is joule - J, where
1 J = 1 Nm
which is the same unit as for work.
Imperial Units of Energy
- 1 J = 0.738 ft lb
- 1 ft lb = 1.356 Nm (J)
There can be several forms of energy, including
- mechanical energy
- heat or thermal energy
- electrical energy
- chemical energy
- nuclear energy
- light energy
Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as
μ = Eo / Ei (1)
μ = energy efficiency
Eo = useful energy output
Ei = energy input
It is common to state efficiency as a percentage by multiplying (1) with 100.
Example - Energy Efficiency
A lift moves a mass 10 m up by exerting a force of 100 N. The input energy to the lift is 1500 J. The energy efficiency of the lift can be calculated as
μ = 100 (N) 10 (m) / 1500 (J)
= 0.67 or
= 67 %