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Energy is the capacity or capability to do work and energy is used when work are done.
The unit for energy is joule J, where
1 J = 1 Nm
which is the same unit as for work.
There can be several forms of energy, including
Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as
μ = Eo / Ei (1)
where
μ = energy efficiency
Eo = useful energy output
Ei = energy input
It is common to state efficiency as a percentage by multiplying (1) with 100.
A lift moves a mass 10 m up with 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 %
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