Efficiency and energy resources

What you'll learn

How to describe useful and wasted energy transfers.
How to recall and use the efficiency equation.
How efficiency can be increased by reducing wasted energy transfers.
How to compare renewable and non-renewable energy resources and explain trends in their use.

Energy transfers: the starting point

In any device, energy is transferred from one store to another. For example, a motor transfers energy electrically from a power supply into the kinetic energy store of moving parts.

Energy is never destroyed. However, not all of the transferred energy usually goes where we want it to go. Some is spread out to the surroundings, often by heating or sound. This spreading out is called dissipation.

Definition

Useful and wasted energy

A useful energy transfer is the part of the total energy transfer that does the job we want. A wasted energy transfer is energy transferred in a way we do not want, often to the thermal energy store of the surroundings.

Key Idea

Energy is conserved

The total energy supplied to a device equals the useful energy transferred plus the wasted energy transferred. Efficiency is about the proportion that is useful.

Sankey diagrams

A Sankey diagram is a diagram showing energy transfers using arrows. The width of each arrow represents the amount of energy. A wider arrow means more energy.

The input arrow shows the total energy supplied. The output arrows show where the energy goes: useful transfers usually continue forwards, while wasted transfers often branch off.

Sankey diagram for an electric motor showing 100 J input, 70 J useful kinetic energy and 30 J wasted thermal and sound energy

Tip

Sankey diagram check

If a Sankey diagram is correct, the total input energy must equal the total output energy. For example, 100 J in can become 70 J useful plus 30 J wasted.

Efficiency

Efficiency tells you how much of the energy supplied is usefully transferred.

For Edexcel 1PH0, this is a recall equation, so you need to learn it:

device\text{efficiency}=\frac{\text{useful energy transferred by the device}}{\text{total energy supplied to the device}}

Efficiency can be written as:

a decimal, such as 0.64
a percentage, such as 64%

To convert a decimal efficiency into a percentage, multiply by 100%.

Definition

Efficiency

Efficiency is the fraction or percentage of the total energy supplied that is transferred usefully. It has no unit.

Example

Calculating efficiency from energy transfers

A motor is supplied with 500 J of electrical energy. It transfers 320 J usefully to the kinetic energy store of moving parts. Calculate its efficiency as a percentage.

Identify the useful energy and the total energy supplied: useful energy is 320 J, total energy supplied is 500 J.
Substitute into the efficiency equation:
$J=0.64\begin{aligned} \text{efficiency} &= \frac{320\,\text{J}}{500\,\text{J}} \\ &= 0.64 \end{aligned}$
Convert the decimal to a percentage:
$0.64 \times 100\% = 64\%$

So the motor is 64% efficient.

Rearranging the efficiency equation

Sometimes you are given the efficiency and asked to find the useful energy transferred.

You can rearrange the equation:

supplied\text{useful energy transferred}=\text{efficiency}\times\text{total energy supplied}

If the efficiency is given as a percentage, convert it to a decimal first.

Example

Finding useful energy from efficiency

A phone charger is 80% efficient. It is supplied with 1500 J of energy. Calculate the useful energy transferred to the phone battery.

Convert the percentage efficiency into a decimal:
$80%=0.8080\% = 0.80$
Use the rearranged equation:
$supplied\text{useful energy transferred}=\text{efficiency}\times\text{total energy supplied}$
Substitute the values:
$J\begin{aligned} \text{useful energy transferred} &= 0.80 \times 1500\,\text{J} \\ &= 1200\,\text{J} \end{aligned}$

So 1200 J is usefully transferred to the phone battery.

Common Mistake

Percent versus decimal

Do not use 80 directly in the equation for 80% efficiency. Use 0.80, or your answer will be 100 times too large.

Increasing efficiency

If you are sitting Higher Tier, you also need to explain how efficiency can be increased.

A device becomes more efficient when a greater proportion of the input energy is transferred usefully. This usually means reducing unwanted energy transfers.

Ways to increase efficiency include:

Lubrication: reduces friction, so less energy is wasted by heating.
Thermal insulation: reduces energy transfer by heating to the surroundings.
Streamlining: reduces air resistance or water resistance.
Using low-resistance electrical components: reduces unwanted heating in wires.
Using more efficient designs, such as LED lamps instead of filament lamps.

Key Idea

Improving efficiency

You cannot create extra energy. To improve efficiency, you reduce wasted energy transfers so a larger fraction of the same input energy is useful.

Example

Comparing efficiency before and after reducing waste

A machine is supplied with 1000 J of energy. Before lubrication, it usefully transfers 600 J. After lubrication, wasted energy transfers are reduced to 250 J. Calculate the new efficiency.

Use conservation of energy to find the new useful energy:
$J\begin{aligned} \text{useful energy} &= \text{total energy supplied} - \text{wasted energy} \\ &= 1000\,\text{J} - 250\,\text{J} \\ &= 750\,\text{J} \end{aligned}$
Substitute into the efficiency equation:
$J=0.75\begin{aligned} \text{efficiency} &= \frac{750\,\text{J}}{1000\,\text{J}} \\ &= 0.75 \end{aligned}$
Convert to a percentage:
$0.75 \times 100\% = 75\%$

The new efficiency is 75%.

Energy resources on Earth

An energy resource is a source of energy humans can use for electricity, heating or transport.

Energy resources are often grouped as renewable or non-renewable.

Definition

Renewable and non-renewable resources

A renewable energy resource is naturally replaced as it is used. A non-renewable energy resource will run out because it is used much faster than it is replaced.

Concept map grouping energy resources into renewable and non-renewable, with main uses

Main resources you need to know

Energy resource	Renewable?	Main uses	Key points
Fossil fuels: coal, oil and natural gas	Non-renewable	Electricity, heating, transport	Reliable and energy-dense, but release carbon dioxide and other pollutants when burned.
Nuclear fuel	Non-renewable	Electricity generation	Very high energy output and no carbon dioxide during generation, but produces radioactive waste.
Bio-fuel	Renewable if replanted sustainably	Heating, electricity, transport fuel	Made from living or recently living material; burning releases carbon dioxide, but growing plants absorb carbon dioxide.
Wind	Renewable	Electricity generation	No fuel needed, but output changes with weather.
Hydro-electricity	Renewable	Electricity generation	Uses falling water; reliable in suitable locations, but dams can damage habitats and flood land.
Tides	Renewable	Electricity generation	Very predictable, but only useful in certain coastal locations.
The Sun, including solar cells	Renewable	Electricity generation and heating	No fuel needed, but output depends on daylight, weather and season.

Common Mistake

Renewable does not mean perfect

Renewable resources are naturally replaced, but they can still have disadvantages. For example, hydro-electric dams can damage ecosystems, and bio-fuels still release carbon dioxide when burned.

Comparing how resources are used

Different resources are useful in different situations.

Fossil fuels are still widely used because they are reliable, can generate large amounts of energy when needed, and existing power stations and transport systems already use them. However, burning fossil fuels releases carbon dioxide, a greenhouse gas that contributes to climate change.

Nuclear fuel is useful for generating steady electricity, but nuclear power stations are expensive to build and radioactive waste must be stored safely.

Some renewable resources are intermittent, meaning their output is not constant. Wind turbines need wind. Solar cells need daylight. This means storage or backup generation may be needed when demand is high but renewable output is low.

Example

Choosing a suitable energy resource

A coastal town has a very large tidal range and wants a renewable source of electricity. Explain why tidal power could be suitable, but also give one limitation.

Link the location to the resource: a large tidal range means there is a large change in sea level, so moving water can be used to generate electricity.
Explain a strength: tides are predictable, so engineers can forecast when electricity generation will be high or low.
Give a limitation: tidal schemes are expensive to build and may affect marine habitats or shipping routes.

So tidal power could be suitable, but the decision must consider cost and environmental impact.

Patterns and trends in energy resource use

A trend is an overall change in data over time, such as coal use decreasing. A pattern is a repeated or noticeable relationship in data, such as solar generation being higher in summer than in winter.

Common patterns and trends include:

Fossil fuel use has been high for a long time because fossil fuels are reliable and easy to store and transport.
Renewable electricity generation has increased in many countries because of climate concerns, improved technology and government policies.
Coal use has decreased in the UK because it produces large amounts of carbon dioxide and pollution.
Energy demand can increase due to population growth and industrial development.
Energy demand can decrease due to better insulation, more efficient devices and changes in behaviour.

Tip

Explaining trends

When a question gives you a graph or table, do two things: describe the trend using data, then explain a sensible reason for it.

Example

Explaining a change in electricity generation

A graph shows that coal’s share of electricity generation fell from 40% to 5% over several years, while wind and solar increased.

Describe the trend with values: coal generation decreased greatly, from 40% to 5%, while wind and solar increased over the same period.
Link the decrease in coal to reasons: coal releases carbon dioxide and pollutants, so governments and energy companies have moved away from it.
Link the increase in wind and solar to reasons: renewable technology has improved, costs have fallen, and there is pressure to reduce greenhouse gas emissions.

This explains both the pattern in the data and the reasons behind it.

Exam technique

In the exam

For efficiency calculations, identify the useful energy and the total energy supplied before substituting into the equation.
If efficiency is given as a percentage, convert it to a decimal before using it in rearranged calculations.
For energy resource comparisons, give a balanced answer: include reliability, cost, environmental impact, location and whether the resource is renewable.
For trend questions, quote data from the graph or table and then explain the likely reason for the change.

Self review

Check yourself

A device is supplied with 800 J and usefully transfers 520 J. How would you calculate its efficiency?
Why does lubrication usually increase the efficiency of a machine?
Give one advantage and one disadvantage of using wind power to generate electricity.

Efficiency and energy resources

What you'll learn

How to describe useful and wasted energy transfers.
How to recall and use the efficiency equation.
How efficiency can be increased by reducing wasted energy transfers.
How to compare renewable and non-renewable energy resources and explain trends in their use.

Energy transfers: the starting point

In any device, energy is transferred from one store to another. For example, a motor transfers energy electrically from a power supply into the kinetic energy store of moving parts.

Definition

Useful and wasted energy

Key Idea

Energy is conserved

The total energy supplied to a device equals the useful energy transferred plus the wasted energy transferred. Efficiency is about the proportion that is useful.

Sankey diagrams

A Sankey diagram is a diagram showing energy transfers using arrows. The width of each arrow represents the amount of energy. A wider arrow means more energy.

The input arrow shows the total energy supplied. The output arrows show where the energy goes: useful transfers usually continue forwards, while wasted transfers often branch off.

Sankey diagram for an electric motor showing 100 J input, 70 J useful kinetic energy and 30 J wasted thermal and sound energy

Tip

Sankey diagram check

If a Sankey diagram is correct, the total input energy must equal the total output energy. For example, 100 J in can become 70 J useful plus 30 J wasted.

Efficiency

Efficiency tells you how much of the energy supplied is usefully transferred.

For Edexcel 1PH0, this is a recall equation, so you need to learn it:

device\text{efficiency}=\frac{\text{useful energy transferred by the device}}{\text{total energy supplied to the device}}

Efficiency can be written as:

a decimal, such as 0.64
a percentage, such as 64%

To convert a decimal efficiency into a percentage, multiply by 100%.

Definition

Efficiency

Efficiency is the fraction or percentage of the total energy supplied that is transferred usefully. It has no unit.

Example

Calculating efficiency from energy transfers

A motor is supplied with 500 J of electrical energy. It transfers 320 J usefully to the kinetic energy store of moving parts. Calculate its efficiency as a percentage.

Identify the useful energy and the total energy supplied: useful energy is 320 J, total energy supplied is 500 J.
Substitute into the efficiency equation:
$J=0.64\begin{aligned} \text{efficiency} &= \frac{320\,\text{J}}{500\,\text{J}} \\ &= 0.64 \end{aligned}$
Convert the decimal to a percentage:
$0.64 \times 100\% = 64\%$

So the motor is 64% efficient.

Rearranging the efficiency equation

Sometimes you are given the efficiency and asked to find the useful energy transferred.

You can rearrange the equation:

supplied\text{useful energy transferred}=\text{efficiency}\times\text{total energy supplied}

If the efficiency is given as a percentage, convert it to a decimal first.

Example

Finding useful energy from efficiency

A phone charger is 80% efficient. It is supplied with 1500 J of energy. Calculate the useful energy transferred to the phone battery.

Convert the percentage efficiency into a decimal:
$80%=0.8080\% = 0.80$
Use the rearranged equation:
$supplied\text{useful energy transferred}=\text{efficiency}\times\text{total energy supplied}$
Substitute the values:
$J\begin{aligned} \text{useful energy transferred} &= 0.80 \times 1500\,\text{J} \\ &= 1200\,\text{J} \end{aligned}$

So 1200 J is usefully transferred to the phone battery.

Common Mistake

Percent versus decimal

Do not use 80 directly in the equation for 80% efficiency. Use 0.80, or your answer will be 100 times too large.

Increasing efficiency

If you are sitting Higher Tier, you also need to explain how efficiency can be increased.

A device becomes more efficient when a greater proportion of the input energy is transferred usefully. This usually means reducing unwanted energy transfers.

Ways to increase efficiency include:

Lubrication: reduces friction, so less energy is wasted by heating.
Thermal insulation: reduces energy transfer by heating to the surroundings.
Streamlining: reduces air resistance or water resistance.
Using low-resistance electrical components: reduces unwanted heating in wires.
Using more efficient designs, such as LED lamps instead of filament lamps.

Key Idea

Improving efficiency

You cannot create extra energy. To improve efficiency, you reduce wasted energy transfers so a larger fraction of the same input energy is useful.

Example

Comparing efficiency before and after reducing waste

A machine is supplied with 1000 J of energy. Before lubrication, it usefully transfers 600 J. After lubrication, wasted energy transfers are reduced to 250 J. Calculate the new efficiency.

Use conservation of energy to find the new useful energy:
$J\begin{aligned} \text{useful energy} &= \text{total energy supplied} - \text{wasted energy} \\ &= 1000\,\text{J} - 250\,\text{J} \\ &= 750\,\text{J} \end{aligned}$
Substitute into the efficiency equation:
$J=0.75\begin{aligned} \text{efficiency} &= \frac{750\,\text{J}}{1000\,\text{J}} \\ &= 0.75 \end{aligned}$
Convert to a percentage:
$0.75 \times 100\% = 75\%$

The new efficiency is 75%.

Energy resources on Earth

An energy resource is a source of energy humans can use for electricity, heating or transport.

Energy resources are often grouped as renewable or non-renewable.

Definition

Renewable and non-renewable resources

A renewable energy resource is naturally replaced as it is used. A non-renewable energy resource will run out because it is used much faster than it is replaced.

Concept map grouping energy resources into renewable and non-renewable, with main uses

Main resources you need to know

Energy resource	Renewable?	Main uses	Key points
Fossil fuels: coal, oil and natural gas	Non-renewable	Electricity, heating, transport	Reliable and energy-dense, but release carbon dioxide and other pollutants when burned.
Nuclear fuel	Non-renewable	Electricity generation	Very high energy output and no carbon dioxide during generation, but produces radioactive waste.
Bio-fuel	Renewable if replanted sustainably	Heating, electricity, transport fuel	Made from living or recently living material; burning releases carbon dioxide, but growing plants absorb carbon dioxide.
Wind	Renewable	Electricity generation	No fuel needed, but output changes with weather.
Hydro-electricity	Renewable	Electricity generation	Uses falling water; reliable in suitable locations, but dams can damage habitats and flood land.
Tides	Renewable	Electricity generation	Very predictable, but only useful in certain coastal locations.
The Sun, including solar cells	Renewable	Electricity generation and heating	No fuel needed, but output depends on daylight, weather and season.

Common Mistake

Renewable does not mean perfect

Renewable resources are naturally replaced, but they can still have disadvantages. For example, hydro-electric dams can damage ecosystems, and bio-fuels still release carbon dioxide when burned.

Comparing how resources are used

Different resources are useful in different situations.

Nuclear fuel is useful for generating steady electricity, but nuclear power stations are expensive to build and radioactive waste must be stored safely.

Example

Choosing a suitable energy resource

A coastal town has a very large tidal range and wants a renewable source of electricity. Explain why tidal power could be suitable, but also give one limitation.

Link the location to the resource: a large tidal range means there is a large change in sea level, so moving water can be used to generate electricity.
Explain a strength: tides are predictable, so engineers can forecast when electricity generation will be high or low.
Give a limitation: tidal schemes are expensive to build and may affect marine habitats or shipping routes.

So tidal power could be suitable, but the decision must consider cost and environmental impact.

Patterns and trends in energy resource use

Common patterns and trends include:

Fossil fuel use has been high for a long time because fossil fuels are reliable and easy to store and transport.
Renewable electricity generation has increased in many countries because of climate concerns, improved technology and government policies.
Coal use has decreased in the UK because it produces large amounts of carbon dioxide and pollution.
Energy demand can increase due to population growth and industrial development.
Energy demand can decrease due to better insulation, more efficient devices and changes in behaviour.

Tip

Explaining trends

When a question gives you a graph or table, do two things: describe the trend using data, then explain a sensible reason for it.

Example

Explaining a change in electricity generation

A graph shows that coal’s share of electricity generation fell from 40% to 5% over several years, while wind and solar increased.

Describe the trend with values: coal generation decreased greatly, from 40% to 5%, while wind and solar increased over the same period.
Link the decrease in coal to reasons: coal releases carbon dioxide and pollutants, so governments and energy companies have moved away from it.
Link the increase in wind and solar to reasons: renewable technology has improved, costs have fallen, and there is pressure to reduce greenhouse gas emissions.

This explains both the pattern in the data and the reasons behind it.

Exam technique

In the exam

For efficiency calculations, identify the useful energy and the total energy supplied before substituting into the equation.
If efficiency is given as a percentage, convert it to a decimal before using it in rearranged calculations.
For energy resource comparisons, give a balanced answer: include reliability, cost, environmental impact, location and whether the resource is renewable.
For trend questions, quote data from the graph or table and then explain the likely reason for the change.

Self review

Check yourself

A device is supplied with 800 J and usefully transfers 520 J. How would you calculate its efficiency?
Why does lubrication usually increase the efficiency of a machine?
Give one advantage and one disadvantage of using wind power to generate electricity.

Efficiency and energy resources

What you'll learn

Energy transfers: the starting point

Sankey diagrams

Efficiency

Rearranging the efficiency equation

Increasing efficiency

Energy resources on Earth

Main resources you need to know

Comparing how resources are used

Patterns and trends in energy resource use

Recap questions

Efficiency and energy resources

What you'll learn

Energy transfers: the starting point

Sankey diagrams

Efficiency

Rearranging the efficiency equation

Increasing efficiency

Energy resources on Earth

Main resources you need to know

Comparing how resources are used

Patterns and trends in energy resource use

Recap questions

Efficiency and energy resources

What you'll learn

Energy transfers: the starting point

Sankey diagrams

Efficiency

Rearranging the efficiency equation

Increasing efficiency

Energy resources on Earth

Main resources you need to know

Comparing how resources are used

Patterns and trends in energy resource use

Recap questions

Efficiency and energy resources Revision Guide

Efficiency and energy resources

What you'll learn

Energy transfers: the starting point

Sankey diagrams

Efficiency

Rearranging the efficiency equation

Increasing efficiency

Energy resources on Earth

Main resources you need to know

Comparing how resources are used

Patterns and trends in energy resource use

Recap questions

Efficiency and energy resources Revision Guide