Power & Efficiency

Physics: AQA A Level, topic 4.1.22

Power

The "energy transferred per second" is equivalent to saying "the rate of energy transfer". Both are definitions for power.

Rate of energy transfer

Rate of energy transfer

The rate at which energy is transferred from one form to another is called power.
This is equivalent to saying the energy transferred per second.
The energy transferred is also known as the work done, W.
So, power can also be described as the work done per second (or the rate of doing work).

Equation

Equation

Power is the rate of doing work.
Power = work done ÷ time taken
$P = \frac{\Delta W}{\Delta t}$

Units

Units

Power is measured in Watts (W).
1 Watt is defined as 1 Joule transferred per second.

Force × velocity

Force × velocity

Power is also equal to the force applied times the velocity of the object the force is applied to.
$P = F v$
This equation can be more useful if you're given the speed and not the distance travelled.

Efficiency

When energy is transferred, there are some losses. For example, some energy may be dissipated as heat or sound.

Useful energy

Useful energy

Efficiency is the fraction of energy that is used for something useful.
The higher the efficiency, the better.
Efficiency is calculated by:
Efficiency = $\frac{useful\space output\space power}{input \space power}$
A kettle supplied with 50 W of power and converts 40 W to heat water has an efficiency of $\frac{40}{50} = 0.8$ .

Percentage

Percentage

Efficiency can also be expressed as a percentage:
Efficiency (%) = $\frac{useful\space output\space power}{input \space power} \times 100$
A kettle supplied with 50 W of power and converts 40 W to heat water has an efficiency of 80%.

Test yourself on Power & Efficiency

What is the equation for the power?
How are power and force related?
What is the equation for efficiency?

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