# Energy

## 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)*

### Energy forms

There can be several forms of energy, including

- mechanical energy
- heat or thermal energy
- electrical energy
- chemical energy
- nuclear energy
- light energy

### Energy Efficiency

Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as

μ = E_{o }/ E_{i }(1)

where

μ = energy efficiency

E_{o }= useful energy output

E_{i }= 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 %