Energy stores, transfers and conservation

What you'll learn

How to describe energy using stores and transfer pathways.
How to calculate changes in gravitational potential energy and kinetic energy.
What conservation of energy means in a closed system.
Why energy can become dissipated, and how we reduce unwanted transfers.

Energy is stored, then transferred

Energy is measured in joules (J). In GCSE Physics, we often describe energy using a store model: energy is not “made” or “used up”; it is transferred between stores.

Definition

System, energy store and transfer

A system is the object or group of objects you choose to study. An energy store is a way energy is associated with a system, such as a kinetic, gravitational potential, thermal, chemical or elastic store. An energy transfer is energy moving from one store to another.

Energy can be transferred by different pathways, including mechanical working when a force moves an object, electrical working when a current transfers energy, and heating when energy is transferred because of a temperature difference.

An energy-transfer diagram uses boxes for stores and arrows for transfers. In this motor example, useful energy increases the gravitational potential energy store of the load, while unwanted energy is dissipated to thermal stores.

Energy transfer diagram for an electric motor lifting a load

Tip

Drawing energy transfer diagrams

Label the store that decreases, the transfer pathway, and the store that increases. If energy spreads into the surroundings, label it as energy transferred to the thermal energy store of the surroundings.

Gravitational potential energy

Gravitational potential energy, often shortened to GPE, is energy stored because an object is in a gravitational field and has height. When you lift an object upwards, its GPE store increases.

The symbol $Δ\Delta$ means “change in”. For Edexcel 1PH0, you need to recall and use this equation:

ΔGPE=m×g×Δh\Delta \text{GPE} = m \times g \times \Delta h

where:

$ΔGPE\Delta \text{GPE}$ is the change in gravitational potential energy in joules (J)
$m$ is mass in kilograms (kg)
$g$ is gravitational field strength in newtons per kilogram (N/kg)
$Δh\Delta h$ is change in vertical height in metres (m)

On Earth, $g$ is often taken as $N/kg9.8\,\text{N/kg}$ , unless the question tells you to use another value.

Example

Calculating a change in gravitational potential energy

A 12 kg box is lifted vertically by 2.5 m. Use $9.8\,\text{N/kg}$ . Find the increase in GPE.

Identify the values that match the equation: $12\,\text{kg}$ , $9.8\,\text{N/kg}$ and $m\Delta h = 2.5\,\text{m}$ .
Substitute the values into $ΔGPE=m×g×Δh\Delta \text{GPE} = m \times g \times \Delta h$ :
$m\Delta \text{GPE} = 12\,\text{kg} \times 9.8\,\text{N/kg} \times 2.5\,\text{m}$
Multiply the numbers and keep the energy unit:
$J\Delta \text{GPE} = 294\,\text{J}$

Common Mistake

Using the slope distance

For GPE, use the vertical height change, not the distance travelled along a ramp or slope.

Kinetic energy

Kinetic energy is energy in the store of a moving object. Anything moving has kinetic energy.

For Edexcel 1PH0, you also need to recall and use this equation:

Ek=12×m×v2E_k = \frac{1}{2} \times m \times v^2

where:

$E_k$ is kinetic energy in joules (J)
$m$ is mass in kilograms (kg)
$v$ is speed in metres per second (m/s)

The squared speed matters: if speed doubles, kinetic energy becomes four times bigger.

Example

Calculating kinetic energy

A 900 kg car travels at 20 m/s. Calculate its kinetic energy.

Match the values to the equation: $900\,\text{kg}$ and $20\,\text{m/s}$ .
Square the speed before multiplying:
$(m/s)2v^2 = \left(20\,\text{m/s}\right)^2 = 400\,\text{(m/s)}^2$
Substitute into the kinetic energy equation:
$JE_k = \frac{1}{2} \times 900\,\text{kg} \times 400\,\text{(m/s)}^2 = 180000\,\text{J}$

Common Mistake

Forgetting the square

Do not calculate $12×m×v\frac{1}{2} \times m \times v$ . The speed must be squared: $v^2$ .

Conservation of energy

Energy is conserved. This means the total energy does not change overall: energy cannot be created or destroyed, only transferred between stores.

Definition

Closed system

A closed system is a system where no energy enters or leaves. Energy can still be transferred between stores inside the system.

Key Idea

Conservation of energy

In a closed system, there is no net change in the total energy, even though the energy may be stored in different ways before and after a change.

For example, when an object is thrown upwards, its kinetic energy store decreases and its gravitational potential energy store increases. If air resistance is included, some energy is dissipated to thermal stores of the surroundings.

Energy stores for an object thrown vertically upwards

Example

Finding the height reached from kinetic energy

A ball is thrown upwards at 14 m/s. Ignore air resistance. Calculate the maximum height gained using $9.8\,\text{N/kg}$ .

If air resistance is ignored, the kinetic energy lost becomes gravitational potential energy gained:
$Ek=ΔGPEE_k = \Delta \text{GPE}$
Substitute the two equations:
$12×m×v2=m×g×Δh\frac{1}{2} \times m \times v^2 = m \times g \times \Delta h$
The mass $m$ appears on both sides, so it cancels.
Rearrange and calculate the height:
$m\Delta h = \frac{v^2}{2g} = \frac{\left(14\,\text{m/s}\right)^2}{2 \times 9.8\,\text{N/kg}} = 10\,\text{m}$

Common system changes

You should be able to describe how energy stores change in different situations:

Object projected upwards or up a slope: kinetic energy decreases; gravitational potential energy increases; some energy may be dissipated by air resistance or friction.
Moving object hitting an obstacle: kinetic energy decreases; energy is transferred to thermal stores, elastic stores, and by sound.
Object accelerated by a constant force: energy is transferred mechanically to the object; its kinetic energy store increases.
Vehicle slowing down: kinetic energy decreases; brakes, tyres and surroundings warm up.
Electric kettle boiling water: energy is transferred electrically; the thermal energy store of the water increases, but some energy heats the kettle and surroundings.

Example

Identifying energy changes in a vehicle slowing down

A car brakes and comes to rest.

At the start, the car has a large kinetic energy store because it is moving.
Friction in the brakes and tyres transfers energy by heating, so the thermal energy stores of the brakes, tyres, road and air increase.
At the end, the car’s kinetic energy store is zero, but the energy has not vanished; it has been dissipated into less useful thermal stores.

Dissipated energy and wasteful processes

Dissipated energy is energy that has been transferred to stores where it is less useful, usually because it has spread out into the surroundings. Mechanical processes often become wasteful when friction causes a rise in temperature.

Key Idea

Dissipated does not mean destroyed

When energy is dissipated, it is still conserved, but it is harder to transfer back into a useful store.

Reducing unwanted energy transfers

A lubricant is a slippery substance, such as oil or grease, placed between moving surfaces. Lubrication reduces friction, so less energy is transferred to thermal stores.

Thermal insulation reduces unwanted energy transfer by heating. This is useful in homes, kettles, flasks and warm clothing.

Definition

Thermal conductivity

Thermal conductivity describes how easily energy is transferred through a material by conduction. A material with low thermal conductivity is a better insulator.

For a building wall:

A thicker wall reduces the rate of cooling because energy has further to transfer through the wall.
A wall with lower thermal conductivity reduces the rate of cooling because energy is transferred through it less easily.

Example

Choosing a wall to reduce cooling

A house can be built with either a thin concrete wall or a thick wall containing insulating foam. Which reduces cooling more?

To reduce cooling, you want to reduce the rate of energy transfer from the warm inside to the colder outside.
The thicker wall helps because energy must transfer through a greater thickness of material.
The insulating foam helps because it has low thermal conductivity, so the thick foam wall reduces cooling more than the thin concrete wall.

Exam technique

In the exam

Start by naming the system and the energy stores at the beginning and end.
For GPE, use the vertical height change; for kinetic energy, square the speed first.
Never say energy is “lost” or “used up”; say it is dissipated to thermal stores of the surroundings.
In insulation questions, compare thickness and thermal conductivity qualitatively.

Self review

Check yourself

A 5 kg object is lifted by 2 m. Which energy store increases, and which equation would you use?
If a cyclist’s speed doubles, what happens to their kinetic energy?
Why does adding oil to a machine reduce wasted energy transfer?

Energy stores, transfers and conservation

What you'll learn

How to describe energy using stores and transfer pathways.
How to calculate changes in gravitational potential energy and kinetic energy.
What conservation of energy means in a closed system.
Why energy can become dissipated, and how we reduce unwanted transfers.

Energy is stored, then transferred

Energy is measured in joules (J). In GCSE Physics, we often describe energy using a store model: energy is not “made” or “used up”; it is transferred between stores.

Definition

System, energy store and transfer

Energy transfer diagram for an electric motor lifting a load

Tip

Drawing energy transfer diagrams

Gravitational potential energy

The symbol $Δ\Delta$ means “change in”. For Edexcel 1PH0, you need to recall and use this equation:

ΔGPE=m×g×Δh\Delta \text{GPE} = m \times g \times \Delta h

where:

$ΔGPE\Delta \text{GPE}$ is the change in gravitational potential energy in joules (J)
$m$ is mass in kilograms (kg)
$g$ is gravitational field strength in newtons per kilogram (N/kg)
$Δh\Delta h$ is change in vertical height in metres (m)

On Earth, $g$ is often taken as $N/kg9.8\,\text{N/kg}$ , unless the question tells you to use another value.

Example

Calculating a change in gravitational potential energy

A 12 kg box is lifted vertically by 2.5 m. Use $9.8\,\text{N/kg}$ . Find the increase in GPE.

Identify the values that match the equation: $12\,\text{kg}$ , $9.8\,\text{N/kg}$ and $m\Delta h = 2.5\,\text{m}$ .
Substitute the values into $ΔGPE=m×g×Δh\Delta \text{GPE} = m \times g \times \Delta h$ :
$m\Delta \text{GPE} = 12\,\text{kg} \times 9.8\,\text{N/kg} \times 2.5\,\text{m}$
Multiply the numbers and keep the energy unit:
$J\Delta \text{GPE} = 294\,\text{J}$

Common Mistake

Using the slope distance

For GPE, use the vertical height change, not the distance travelled along a ramp or slope.

Kinetic energy

Kinetic energy is energy in the store of a moving object. Anything moving has kinetic energy.

For Edexcel 1PH0, you also need to recall and use this equation:

Ek=12×m×v2E_k = \frac{1}{2} \times m \times v^2

where:

$E_k$ is kinetic energy in joules (J)
$m$ is mass in kilograms (kg)
$v$ is speed in metres per second (m/s)

The squared speed matters: if speed doubles, kinetic energy becomes four times bigger.

Example

Calculating kinetic energy

A 900 kg car travels at 20 m/s. Calculate its kinetic energy.

Match the values to the equation: $900\,\text{kg}$ and $20\,\text{m/s}$ .
Square the speed before multiplying:
$(m/s)2v^2 = \left(20\,\text{m/s}\right)^2 = 400\,\text{(m/s)}^2$
Substitute into the kinetic energy equation:
$JE_k = \frac{1}{2} \times 900\,\text{kg} \times 400\,\text{(m/s)}^2 = 180000\,\text{J}$

Common Mistake

Forgetting the square

Do not calculate $12×m×v\frac{1}{2} \times m \times v$ . The speed must be squared: $v^2$ .

Conservation of energy

Energy is conserved. This means the total energy does not change overall: energy cannot be created or destroyed, only transferred between stores.

Definition

Closed system

A closed system is a system where no energy enters or leaves. Energy can still be transferred between stores inside the system.

Key Idea

Conservation of energy

In a closed system, there is no net change in the total energy, even though the energy may be stored in different ways before and after a change.

Energy stores for an object thrown vertically upwards

Example

Finding the height reached from kinetic energy

A ball is thrown upwards at 14 m/s. Ignore air resistance. Calculate the maximum height gained using $9.8\,\text{N/kg}$ .

If air resistance is ignored, the kinetic energy lost becomes gravitational potential energy gained:
$Ek=ΔGPEE_k = \Delta \text{GPE}$
Substitute the two equations:
$12×m×v2=m×g×Δh\frac{1}{2} \times m \times v^2 = m \times g \times \Delta h$
The mass $m$ appears on both sides, so it cancels.
Rearrange and calculate the height:
$m\Delta h = \frac{v^2}{2g} = \frac{\left(14\,\text{m/s}\right)^2}{2 \times 9.8\,\text{N/kg}} = 10\,\text{m}$

Common system changes

You should be able to describe how energy stores change in different situations:

Object projected upwards or up a slope: kinetic energy decreases; gravitational potential energy increases; some energy may be dissipated by air resistance or friction.
Moving object hitting an obstacle: kinetic energy decreases; energy is transferred to thermal stores, elastic stores, and by sound.
Object accelerated by a constant force: energy is transferred mechanically to the object; its kinetic energy store increases.
Vehicle slowing down: kinetic energy decreases; brakes, tyres and surroundings warm up.
Electric kettle boiling water: energy is transferred electrically; the thermal energy store of the water increases, but some energy heats the kettle and surroundings.

Example

Identifying energy changes in a vehicle slowing down

A car brakes and comes to rest.

At the start, the car has a large kinetic energy store because it is moving.
Friction in the brakes and tyres transfers energy by heating, so the thermal energy stores of the brakes, tyres, road and air increase.
At the end, the car’s kinetic energy store is zero, but the energy has not vanished; it has been dissipated into less useful thermal stores.

Dissipated energy and wasteful processes

Key Idea

Dissipated does not mean destroyed

When energy is dissipated, it is still conserved, but it is harder to transfer back into a useful store.

Reducing unwanted energy transfers

A lubricant is a slippery substance, such as oil or grease, placed between moving surfaces. Lubrication reduces friction, so less energy is transferred to thermal stores.

Thermal insulation reduces unwanted energy transfer by heating. This is useful in homes, kettles, flasks and warm clothing.

Definition

Thermal conductivity

Thermal conductivity describes how easily energy is transferred through a material by conduction. A material with low thermal conductivity is a better insulator.

For a building wall:

A thicker wall reduces the rate of cooling because energy has further to transfer through the wall.
A wall with lower thermal conductivity reduces the rate of cooling because energy is transferred through it less easily.

Example

Choosing a wall to reduce cooling

A house can be built with either a thin concrete wall or a thick wall containing insulating foam. Which reduces cooling more?

To reduce cooling, you want to reduce the rate of energy transfer from the warm inside to the colder outside.
The thicker wall helps because energy must transfer through a greater thickness of material.
The insulating foam helps because it has low thermal conductivity, so the thick foam wall reduces cooling more than the thin concrete wall.

Exam technique

In the exam

Start by naming the system and the energy stores at the beginning and end.
For GPE, use the vertical height change; for kinetic energy, square the speed first.
Never say energy is “lost” or “used up”; say it is dissipated to thermal stores of the surroundings.
In insulation questions, compare thickness and thermal conductivity qualitatively.

Self review

Check yourself

A 5 kg object is lifted by 2 m. Which energy store increases, and which equation would you use?
If a cyclist’s speed doubles, what happens to their kinetic energy?
Why does adding oil to a machine reduce wasted energy transfer?

Energy stores, transfers and conservation

What you'll learn

Energy is stored, then transferred

Gravitational potential energy

Kinetic energy

Conservation of energy

Common system changes

Dissipated energy and wasteful processes

Reducing unwanted energy transfers

Recap questions

Energy stores, transfers and conservation

What you'll learn

Energy is stored, then transferred

Gravitational potential energy

Kinetic energy

Conservation of energy

Common system changes

Dissipated energy and wasteful processes

Reducing unwanted energy transfers

Recap questions

Energy stores, transfers and conservation

What you'll learn

Energy is stored, then transferred

Gravitational potential energy

Kinetic energy

Conservation of energy

Common system changes

Dissipated energy and wasteful processes

Reducing unwanted energy transfers

Recap questions

Energy stores, transfers and conservation Revision Guide

Energy stores, transfers and conservation

What you'll learn

Energy is stored, then transferred

Gravitational potential energy

Kinetic energy

Conservation of energy

Common system changes

Dissipated energy and wasteful processes

Reducing unwanted energy transfers

Recap questions

Energy stores, transfers and conservation Revision Guide