Efficiency
Simple Explanation
Efficiency measures what fraction of the energy supplied to a machine is actually converted into useful output, rather than being wasted (usually as heat due to friction or resistance). No real machine is 100% efficient.
Why Do We Need It?
Efficiency is a key measure of how well a machine performs β higher efficiency means less energy wasted, which matters both for cost and environmental impact.
Formula
Efficiency
Ξ· = (useful energy output / total energy input) Γ 100%
Efficiency measures what fraction of the energy put into a machine or process is converted into useful output, rather than being wasted (usually as heat).
- Ξ·
- β Efficiency, expressed as a percentage
- useful energy output
- β The energy converted into the intended, useful form, in joules
- total energy input
- β The total energy supplied to the machine or process, in joules
When to use it: Use to evaluate how well a machine or energy-conversion process performs β no real machine achieves 100% efficiency, since some energy is always lost, usually to heat or friction.
Worked Example
Calculating efficiency
An electric motor is supplied with 500 J of electrical energy and produces 350 J of useful mechanical work. Find its efficiency.
Why Does This Work?
By the law of conservation of energy, no energy is truly destroyed β but not all of it emerges in the intended, useful form; some is always converted to unwanted forms (usually heat from friction) that cannot be used for the machine's intended purpose. Efficiency simply measures the useful fraction of the total.
Real-Life Example
Why incandescent bulbs are inefficient
A traditional incandescent light bulb converts most of its electrical energy into heat, not light.
Incandescent bulbs are typically only about 5% efficient at producing light β the other 95% is wasted as heat, which is exactly why they have largely been replaced by far more efficient LED bulbs.
Practice
A machine receives 800 J of input energy and produces 600 J of useful output. Find its efficiency.
MediumCommon mistake
Assuming a machine could be 100% (or more) efficient β by the law of conservation of energy, output can never exceed input, and in practice some energy is always lost to unwanted forms, so real efficiency is always below 100%.
Quick Review
- Ξ· = (useful output/total input) Γ 100% measures energy conversion performance.
- No real machine achieves 100% efficiency β some energy is always wasted, usually as heat.
- Higher efficiency means less energy wasted for the same useful output.