GCSE Physics revision

Efficiency

Conservation and dissipation of energy

AQA 4.1.2.2

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Describe ways to increase the efficiency of an intended energy transfer.

MOST 🎯🎯

Apply the scientific explanation of efficiency to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving efficiency and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is Efficiency?

Efficiency describes the proportion of input energy that is converted into useful energy output by a device.
Most devices are not 100% efficient because some energy is always transferred into an unwanted form, most commonly thermal energy.
The energy that is not transferred into the intended useful form is called wasted energy or non-useful energy output.

1. What is Efficiency?AQA

Notes

Efficiency describes the proportion of input energy that is converted into useful energy output by a device.
Most devices are not 100% efficient because some energy is always transferred into an unwanted form, most commonly thermal energy.
The energy that is not transferred into the intended useful form is called wasted energy or non-useful energy output.

Useful vs. Wasted Energy

For a lamp, the input is electrical energy, which is split into light energy (useful) and heat energy (wasted).
The purpose of the device determines which output is classed as useful — for a lamp, only light energy counts as useful output.
An electric heater is a special case where thermal energy is the intended output, so it is classed as useful rather than wasted.

2. Useful vs. Wasted EnergyAQA

Notes

For a lamp, the input is electrical energy, which is split into light energy (useful) and heat energy (wasted).
The purpose of the device determines which output is classed as useful — for a lamp, only light energy counts as useful output.
An electric heater is a special case where thermal energy is the intended output, so it is classed as useful rather than wasted.

The Efficiency Equation (Energy)

useful energy output
Efficiency can be calculated using the equation: efficiency = total energy input
This equation gives efficiency as a decimal, which can be converted to a percentage by multiplying by 100.
Efficiency values must always be between 0 and 1 (or 0% and 100%), as a value greater than 1 would imply more energy out than in, violating conservation of energy.

3. The Efficiency Equation (Energy)AQA

Notes

useful energy output
Efficiency can be calculated using the equation: efficiency = total energy input
This equation gives efficiency as a decimal, which can be converted to a percentage by multiplying by 100.
Efficiency values must always be between 0 and 1 (or 0% and 100%), as a value greater than 1 would imply more energy out than in, violating conservation of energy.

The Efficiency Equation (Power)

When working with power rather than energy, efficiency is calculated as:
efficiency = useful power output total power input
Power-based efficiency calculations use the same decimal or percentage format as energy-based ones.
To find useful power output, rearrange the equation to:
useful power output = efficiency × total power input

4. The Efficiency Equation (Power)AQA

Notes

When working with power rather than energy, efficiency is calculated as:
efficiency = useful power output total power input
Power-based efficiency calculations use the same decimal or percentage format as energy-based ones.
To find useful power output, rearrange the equation to:
useful power output = efficiency × total power input

Worked Example: Comparing Lamps

An incandescent bulb receiving 300 J and producing 45 J of light has an efficiency of 45
300
= 0.15 (15%).
225
An LED bulb receiving 300 J and producing 225 J of light has an efficiency of 300 = 0.75 (75%).
0.75
The LED lamp is 0.15 = 5 times more efficient than the incandescent bulb.

Worked Example: Microwave Oven

A microwave that is 70% efficient must first have its efficiency converted to a decimal:
70
100
= 0.7
With a total power input of 800 W, the useful power output is 0.7 × 800 = 560 W.

Common Mistakes to Avoid

A common error is inverting the efficiency fraction, dividing total input by useful output instead of the other way around.
If your calculated efficiency is greater than 1 (or greater than 100%), this is a clear sign the division has been performed the wrong way around.
The conservation of energy principle confirms that no device can have an efficiency above 100%, as energy cannot be created, only transferred.

7. Common Mistakes to AvoidAQA

Notes

A common error is inverting the efficiency fraction, dividing total input by useful output instead of the other way around.
If your calculated efficiency is greater than 1 (or greater than 100%), this is a clear sign the division has been performed the wrong way around.
The conservation of energy principle confirms that no device can have an efficiency above 100%, as energy cannot be created, only transferred.

Key Principles to Remember

No device is perfectly 100% efficient, as all real devices lose some energy, most often as thermal energy.
Efficiency is a ratio with no units, expressed either as a decimal between 0 and 1 or as a percentage between 0% and 100%.
Always identify which energy or power output is classed as useful before substituting values into the efficiency equation.

8. Key Principles to RememberAQA

Notes

No device is perfectly 100% efficient, as all real devices lose some energy, most often as thermal energy.
Efficiency is a ratio with no units, expressed either as a decimal between 0 and 1 or as a percentage between 0% and 100%.
Always identify which energy or power output is classed as useful before substituting values into the efficiency equation.