Power and efficiency

What you'll learn

What power tells you about how quickly energy is transferred.
How energy becomes dissipated into less useful stores.
How to calculate efficiency using the equation you must recall.
How insulation and lubrication reduce unwanted energy transfers.

Starting point: energy is transferred, not “used up”

In physics, a system is the object or group of objects you are focusing on, such as a kettle, a motor, or a whole house.

Energy can be stored in different ways, such as in a thermal energy store, kinetic energy store, or chemical energy store. When a device is working, energy is transferred from one store to another.

Key Idea

Energy is conserved

Energy cannot be created or destroyed. If an appliance seems to “lose” energy, that energy has usually been transferred to the surroundings in a less useful way.

Useful and wasted energy transfers

A useful energy transfer is one that does what the device is designed to do.

A wasted energy transfer is energy transferred in a way that is not useful for the device’s purpose. It is not destroyed — it is just less useful.

Definition

Dissipated energy

Energy is dissipated when it spreads out into the surroundings, often warming them, so it is stored in less useful ways and is harder to use again.

For example, in an electric fan, electrical energy is transferred to the fan. Some energy is usefully transferred to the kinetic energy store of the moving air, but some is dissipated by heating the motor and the surroundings, and some is transferred by sound.

Sankey diagram showing electrical energy input to an electric fan split into useful kinetic energy and wasted thermal and sound energy

Example

Tracing energy transfer in a motor

A battery-powered toy car uses a motor to move forwards. Describe the useful and wasted energy transfers.

The input energy comes from the chemical energy store of the battery, transferred electrically to the motor.
The useful output is energy transferred to the kinetic energy store of the car, because the car moves.
Some energy is dissipated to the thermal energy stores of the motor and surroundings due to friction and electrical resistance, and some is transferred by sound.

Common Mistake

Saying energy is lost

Avoid saying “energy is lost” on its own. A better answer is: energy is dissipated to the surroundings, usually increasing their thermal energy store.

Power: how fast energy is transferred

Power is the rate of energy transfer. That means it tells you how much energy is transferred each second.

Definition

Power

Power is energy transferred per second. The unit of power is the watt (W), where $J/s1\ \text{W} = 1\ \text{J/s}$ .

The equation is:

\frac{\Delta E}{t}

where:

$P$ is power in watts (W)
$ΔE\Delta E$ is energy transferred in joules (J)
$t$ is time in seconds (s)

You can rearrange it as:

ΔE=Pt\Delta E = P t

Power ratings on appliances

A power rating tells you the rate at which an appliance transfers energy when it is working normally.

For example:

an LED lamp might be rated at about 8 W
a phone charger might be rated at about 10 W
a kettle might be rated at about 2000 W

A 2000 W kettle transfers 2000 J of energy every second. This is why high-power appliances often heat things quickly.

Domestic devices can get energy from:

batteries, which store energy chemically and transfer it electrically as direct current
the mains supply, which transfers energy electrically using alternating current, or a.c.

Example

Calculating energy transferred from a power rating

A kettle has a power rating of 2.0 kW. It is switched on for 3 minutes. Calculate the energy transferred.

Convert the units: 2.0 kW is 2000 W, and 3 minutes is 180 s.
Use the power equation rearranged for energy: $ΔE=Pt\Delta E = P t$ .
Substitute the values: $J\Delta E = 2000\ \text{W} \times 180\ \text{s} = 360000\ \text{J}$ , so the kettle transfers 360 kJ.

Tip

Sanity check for power

If the power is large or the time is long, the energy transferred should be large. A kettle running for minutes should transfer hundreds of thousands of joules, not just a few joules.

Common Mistake

Power is not the same as efficiency

A higher-power appliance transfers energy faster, but that does not automatically mean it is more efficient. Efficiency depends on the fraction of input energy that becomes useful output energy.

Efficiency: how much input becomes useful output

Efficiency tells you what fraction of the input energy transfer becomes useful output energy transfer.

The OCR Gateway Physics spec says you must recall and apply this equation:

transfer\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{input energy transfer}}

Efficiency can be written as a decimal or as a percentage:

efficiency=efficiency×100\text{percentage efficiency} = \text{efficiency} \times 100

So an efficiency of 0.75 is the same as 75%.

Key Idea

Efficiency range

Efficiency cannot be greater than 1, or greater than 100%, because useful output energy cannot be more than the total input energy.

Example

Calculating efficiency

A kettle transfers 450 kJ of electrical energy. 360 kJ is usefully transferred to the thermal energy store of the water. Calculate the efficiency.

Identify the useful output energy transfer and the input energy transfer: useful output is 360 kJ, input is 450 kJ.
Substitute into the efficiency equation: $efficiency=360450\text{efficiency} = \frac{360}{450}$ .
Calculate the fraction: $efficiency=0.80\text{efficiency} = 0.80$ , which is 80%.

Common Mistake

Mixing decimal and percentage efficiency

Do not write 80 as the efficiency unless you mean 80%. As a decimal, 80% is 0.80.

Energy transfers in domestic devices

Different domestic appliances are designed for different useful energy transfers.

Motors

A motor is designed to transfer energy electrically to the kinetic energy store of moving parts.

Examples include:

fans
washing machines
electric drills
food mixers

In motors, energy may be wasted because:

friction between moving parts increases thermal energy stores
electrical resistance in wires and coils causes heating
sound is produced by vibration

Heating devices

A heating device is designed to increase the thermal energy store of something useful.

Examples include:

kettles heating water
toasters heating bread
electric ovens heating food and air inside the oven

Even in heating devices, energy can be wasted. For example, a kettle usefully heats the water, but it also warms the kettle body and the surrounding air.

Increasing efficiency and reducing unwanted transfers

For Higher Tier, you should be able to describe ways to increase efficiency. Everyone should be able to explain ways of reducing unwanted energy transfer.

To increase efficiency, you reduce the energy dissipated to the surroundings, so a larger fraction of the input energy becomes useful output energy.

Lubrication

Friction is a force that opposes motion when surfaces rub together. Friction can cause unwanted heating and wear.

Lubrication means adding a substance such as oil or grease between moving surfaces. This reduces friction, so less energy is dissipated to thermal energy stores.

Example: oiling a bicycle chain helps more of the rider’s energy be transferred usefully to the bike’s movement, rather than heating the chain and gears.

Thermal insulation

Thermal insulation reduces unwanted energy transfer by heating. It is especially important in buildings, because you want to keep energy inside during cold weather.

Common examples include:

loft insulation
cavity wall insulation
double glazing
draught excluders
insulating jackets around hot water tanks

Definition

Thermal conductivity

Thermal conductivity describes how easily energy is transferred through a material by conduction. A low thermal conductivity means the material is a good insulator.

A building cools more slowly if its walls are:

thicker, because energy has further to transfer through the wall
made from materials with lower thermal conductivity, because energy is transferred through them more slowly

Comparison of heat loss through a thin high-conductivity wall and a thick low-conductivity insulated wall

Example

Comparing wall designs

Two houses are identical except for their walls. House A has thin brick walls. House B has thicker insulated walls with lower thermal conductivity. Which house cools more slowly?

Compare wall thickness: House B has thicker walls, so energy takes longer to transfer through them.
Compare thermal conductivity: House B has lower thermal conductivity, so energy is transferred through its walls more slowly.
Combine both effects: House B has the lower rate of cooling because less energy is transferred from inside to outside each second.

Tip

Cooling rate wording

If a question asks about “rate of cooling”, think about energy transferred per second from the building to the surroundings. Faster energy transfer means faster cooling.

Practical link: measuring energy and power

In practical work, such as using a joulemeter with domestic appliances, you can compare how much energy different devices transfer.

A joulemeter measures energy transferred in joules. If you also measure the time, you can calculate power using:

\frac{\Delta E}{t}

This helps you compare appliances fairly. For example, two lamps may produce similar light, but the lamp with the lower energy transfer for the same useful output is more efficient.

Exam technique

In the exam

For efficiency questions, clearly identify the useful output and the total input before substituting numbers.
Convert units before calculating: kW to W, minutes to seconds, and kJ to J if needed.
Use precise wording: energy is dissipated to the surroundings, not destroyed or simply “used up”.

Self review

Check yourself

A 60 W lamp is switched on for 300 s. What energy is transferred?
Why can an appliance never be more than 100% efficient?
How do lubrication and thermal insulation reduce unwanted energy transfers?

Power and efficiency

What you'll learn

What power tells you about how quickly energy is transferred.
How energy becomes dissipated into less useful stores.
How to calculate efficiency using the equation you must recall.
How insulation and lubrication reduce unwanted energy transfers.

Starting point: energy is transferred, not “used up”

In physics, a system is the object or group of objects you are focusing on, such as a kettle, a motor, or a whole house.

Key Idea

Energy is conserved

Energy cannot be created or destroyed. If an appliance seems to “lose” energy, that energy has usually been transferred to the surroundings in a less useful way.

Useful and wasted energy transfers

A useful energy transfer is one that does what the device is designed to do.

A wasted energy transfer is energy transferred in a way that is not useful for the device’s purpose. It is not destroyed — it is just less useful.

Definition

Dissipated energy

Energy is dissipated when it spreads out into the surroundings, often warming them, so it is stored in less useful ways and is harder to use again.

Sankey diagram showing electrical energy input to an electric fan split into useful kinetic energy and wasted thermal and sound energy

Example

Tracing energy transfer in a motor

A battery-powered toy car uses a motor to move forwards. Describe the useful and wasted energy transfers.

The input energy comes from the chemical energy store of the battery, transferred electrically to the motor.
The useful output is energy transferred to the kinetic energy store of the car, because the car moves.
Some energy is dissipated to the thermal energy stores of the motor and surroundings due to friction and electrical resistance, and some is transferred by sound.

Common Mistake

Saying energy is lost

Avoid saying “energy is lost” on its own. A better answer is: energy is dissipated to the surroundings, usually increasing their thermal energy store.

Power: how fast energy is transferred

Power is the rate of energy transfer. That means it tells you how much energy is transferred each second.

Definition

Power

Power is energy transferred per second. The unit of power is the watt (W), where $J/s1\ \text{W} = 1\ \text{J/s}$ .

The equation is:

\frac{\Delta E}{t}

where:

$P$ is power in watts (W)
$ΔE\Delta E$ is energy transferred in joules (J)
$t$ is time in seconds (s)

You can rearrange it as:

ΔE=Pt\Delta E = P t

Power ratings on appliances

A power rating tells you the rate at which an appliance transfers energy when it is working normally.

For example:

an LED lamp might be rated at about 8 W
a phone charger might be rated at about 10 W
a kettle might be rated at about 2000 W

A 2000 W kettle transfers 2000 J of energy every second. This is why high-power appliances often heat things quickly.

Domestic devices can get energy from:

batteries, which store energy chemically and transfer it electrically as direct current
the mains supply, which transfers energy electrically using alternating current, or a.c.

Example

Calculating energy transferred from a power rating

A kettle has a power rating of 2.0 kW. It is switched on for 3 minutes. Calculate the energy transferred.

Convert the units: 2.0 kW is 2000 W, and 3 minutes is 180 s.
Use the power equation rearranged for energy: $ΔE=Pt\Delta E = P t$ .
Substitute the values: $J\Delta E = 2000\ \text{W} \times 180\ \text{s} = 360000\ \text{J}$ , so the kettle transfers 360 kJ.

Tip

Sanity check for power

If the power is large or the time is long, the energy transferred should be large. A kettle running for minutes should transfer hundreds of thousands of joules, not just a few joules.

Common Mistake

Power is not the same as efficiency

A higher-power appliance transfers energy faster, but that does not automatically mean it is more efficient. Efficiency depends on the fraction of input energy that becomes useful output energy.

Efficiency: how much input becomes useful output

Efficiency tells you what fraction of the input energy transfer becomes useful output energy transfer.

The OCR Gateway Physics spec says you must recall and apply this equation:

transfer\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{input energy transfer}}

Efficiency can be written as a decimal or as a percentage:

efficiency=efficiency×100\text{percentage efficiency} = \text{efficiency} \times 100

So an efficiency of 0.75 is the same as 75%.

Key Idea

Efficiency range

Efficiency cannot be greater than 1, or greater than 100%, because useful output energy cannot be more than the total input energy.

Example

Calculating efficiency

A kettle transfers 450 kJ of electrical energy. 360 kJ is usefully transferred to the thermal energy store of the water. Calculate the efficiency.

Identify the useful output energy transfer and the input energy transfer: useful output is 360 kJ, input is 450 kJ.
Substitute into the efficiency equation: $efficiency=360450\text{efficiency} = \frac{360}{450}$ .
Calculate the fraction: $efficiency=0.80\text{efficiency} = 0.80$ , which is 80%.

Common Mistake

Mixing decimal and percentage efficiency

Do not write 80 as the efficiency unless you mean 80%. As a decimal, 80% is 0.80.

Energy transfers in domestic devices

Different domestic appliances are designed for different useful energy transfers.

Motors

A motor is designed to transfer energy electrically to the kinetic energy store of moving parts.

Examples include:

fans
washing machines
electric drills
food mixers

In motors, energy may be wasted because:

friction between moving parts increases thermal energy stores
electrical resistance in wires and coils causes heating
sound is produced by vibration

Heating devices

A heating device is designed to increase the thermal energy store of something useful.

Examples include:

kettles heating water
toasters heating bread
electric ovens heating food and air inside the oven

Even in heating devices, energy can be wasted. For example, a kettle usefully heats the water, but it also warms the kettle body and the surrounding air.

Increasing efficiency and reducing unwanted transfers

For Higher Tier, you should be able to describe ways to increase efficiency. Everyone should be able to explain ways of reducing unwanted energy transfer.

To increase efficiency, you reduce the energy dissipated to the surroundings, so a larger fraction of the input energy becomes useful output energy.

Lubrication

Friction is a force that opposes motion when surfaces rub together. Friction can cause unwanted heating and wear.

Lubrication means adding a substance such as oil or grease between moving surfaces. This reduces friction, so less energy is dissipated to thermal energy stores.

Example: oiling a bicycle chain helps more of the rider’s energy be transferred usefully to the bike’s movement, rather than heating the chain and gears.

Thermal insulation

Thermal insulation reduces unwanted energy transfer by heating. It is especially important in buildings, because you want to keep energy inside during cold weather.

Common examples include:

loft insulation
cavity wall insulation
double glazing
draught excluders
insulating jackets around hot water tanks

Definition

Thermal conductivity

Thermal conductivity describes how easily energy is transferred through a material by conduction. A low thermal conductivity means the material is a good insulator.

A building cools more slowly if its walls are:

thicker, because energy has further to transfer through the wall
made from materials with lower thermal conductivity, because energy is transferred through them more slowly

Comparison of heat loss through a thin high-conductivity wall and a thick low-conductivity insulated wall

Example

Comparing wall designs

Two houses are identical except for their walls. House A has thin brick walls. House B has thicker insulated walls with lower thermal conductivity. Which house cools more slowly?

Compare wall thickness: House B has thicker walls, so energy takes longer to transfer through them.
Compare thermal conductivity: House B has lower thermal conductivity, so energy is transferred through its walls more slowly.
Combine both effects: House B has the lower rate of cooling because less energy is transferred from inside to outside each second.

Tip

Cooling rate wording

If a question asks about “rate of cooling”, think about energy transferred per second from the building to the surroundings. Faster energy transfer means faster cooling.

Practical link: measuring energy and power

In practical work, such as using a joulemeter with domestic appliances, you can compare how much energy different devices transfer.

A joulemeter measures energy transferred in joules. If you also measure the time, you can calculate power using:

\frac{\Delta E}{t}

This helps you compare appliances fairly. For example, two lamps may produce similar light, but the lamp with the lower energy transfer for the same useful output is more efficient.

Exam technique

In the exam

For efficiency questions, clearly identify the useful output and the total input before substituting numbers.
Convert units before calculating: kW to W, minutes to seconds, and kJ to J if needed.
Use precise wording: energy is dissipated to the surroundings, not destroyed or simply “used up”.

Self review

Check yourself

A 60 W lamp is switched on for 300 s. What energy is transferred?
Why can an appliance never be more than 100% efficient?
How do lubrication and thermal insulation reduce unwanted energy transfers?

Power and efficiency

What you'll learn

Starting point: energy is transferred, not “used up”

Useful and wasted energy transfers

Power: how fast energy is transferred

Power ratings on appliances

Efficiency: how much input becomes useful output

Energy transfers in domestic devices

Motors

Heating devices

Increasing efficiency and reducing unwanted transfers

Lubrication

Thermal insulation

Practical link: measuring energy and power

Recap questions

Power and efficiency

What you'll learn

Starting point: energy is transferred, not “used up”

Useful and wasted energy transfers

Power: how fast energy is transferred

Power ratings on appliances

Efficiency: how much input becomes useful output

Energy transfers in domestic devices

Motors

Heating devices

Increasing efficiency and reducing unwanted transfers

Lubrication

Thermal insulation

Practical link: measuring energy and power

Recap questions

Power and efficiency

What you'll learn

Starting point: energy is transferred, not “used up”

Useful and wasted energy transfers

Power: how fast energy is transferred

Power ratings on appliances

Efficiency: how much input becomes useful output

Energy transfers in domestic devices

Motors

Heating devices

Increasing efficiency and reducing unwanted transfers

Lubrication

Thermal insulation

Practical link: measuring energy and power

Recap questions

Power and efficiency Revision Guide

Power and efficiency

What you'll learn

Starting point: energy is transferred, not “used up”

Useful and wasted energy transfers

Power: how fast energy is transferred

Power ratings on appliances

Efficiency: how much input becomes useful output

Energy transfers in domestic devices

Motors

Heating devices

Increasing efficiency and reducing unwanted transfers

Lubrication

Thermal insulation

Practical link: measuring energy and power

Recap questions

Power and efficiency Revision Guide