Efficiency (GCSE Physics)

Efficiency

Efficiency

We previously mentioned that not all the energy from a transfer is converted into useful energy. Instead, some of the energy will be converted into wasted energy, which is of no use to us.

To make energy transfers more efficient, we have to maximise the useful energy and minimise the wasted energy from a transfer.

Calculating Efficiency

We can calculate efficiency in one of two ways. In this tutorial we will focus on efficiency in terms of energy. In both cases, efficiency is measured as a ratio, so the units will cancel out.

Efficiency in Terms of Energy

Where:

• Energy is measured in joules, J
• Efficiency has no units

Question: Megan is using her hair dryer. It wastes 376J of every 1500J of electrical energy it uses up. How efficient is Megan’s hairdryer, to 2dp?

1. Write out the equation.
   In this case, we are dealing with energy so the appropriate equation is:Efficiency = useful output energy transfer / total input energy transfer
2. Work out the useful output energy.
   In the question, we have only been told how much energy the hairdryer wastes, not how much is converted into useful energy.

Percentage of Efficiency

The percentage of efficiency can be calculated by multiplying the efficiency by 100.

Question: A LED bulb is supplied a total of 135J. Out of the total energy supplied, 102J is usefully transferred. How efficient is the bulb, expressed as a percentage to 2 dp?

1. Write out the equation.
   Again, we are dealing with energy so the appropriate equation is:Efficiency = useful output energy transfer / total input energy transfer
2. Substitute in the numbers.
   We know the numbers from the question, so we can simply put them into the equation.

Drawing Diagrams to Show Efficiency

Sankey Diagrams

Seen we know that most devices are not 100% efficient, we can draw diagrams to visualise how much energy is actually useful.

This is good for comparing efficiency, for example in lightbulbs. We most commonly use Sankey diagrams, where the thickness of the arrows show how much energy is being transferred.

The diagrams comparing a filament light bulb and an energy saving light bulb, show us that more energy is wasted as heat in the filament light bulb.