Energy - 1.2.2 Efficiency (GCSE Physics AQA)

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 both cases, efficiency is measured as a ratio, so the units will cancel out.

1. 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.

1500 – 376 = 1124J

3. Substitute in the numbers.
Now that we know the numbers, we can simply put them into the equation.

Efficiency = 1124 / 1500
Efficiency = 0.7493
Efficiency = 0.75

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2. In Terms of Power

Where:

• Power is measured in watts, W
• Efficiency has not units

Question: Harry is making a cup of tea. His kettle has an input power of 750W, but is only 60% efficient. Calculate the useful output power.

1. Write out the equation.
In this equation, the appropriate equation would be the one for power.

Efficiency = useful power output / total power input

2. Rearrange the equation.
In this situation, we have been given the efficiency and need to calculate the output power.

useful power output = efficiency x total power input

3. Substitute in the numbers.
Now we need to put in the numbers.

useful power output = 0.6 x 750
useful power output = 450 W

Increasing Efficiency

• We can increase efficiency in several ways. We can increase the efficiency of energy transfers in a few different ways, which we previously discussed in 4.1.2.2, Increasing Efficiency. Just to recap, there were three different ways: thermal insulation, lubrication and streamlining.
• 100% efficiency is usually impossible. It is normally impossible for an energy transfer to be 100% efficient. There is usually always some wasted heat energy in the form of heat / thermal energy. Sometimes the definition of ‘useful’ energy can change – e.g. some lights are designed to provide heat and light, so the heat energy is also useful!