2.4.1 Power

2.4.1 Electrical power

Power: how fast a device transfers energy

Definition

Power

The rate at which energy is transferred, measured in watts ( $W\text{W}$ ).

Every circuit device transfers energy from one store to another; a lamp transfers energy electrically from the supply and raises the thermal energy store of the lamp and its surroundings, while also transferring some energy by light.
The power of a device tells you how quickly it makes that transfer.
A power of one watt means energy is transferred at a rate of one joule per second.
So a higher-power device transfers more energy every second than a lower-power one.

Power from potential difference and current

The power transferred by an electrical device depends on the potential difference across it and the current through it.
These are linked by $P = V I$ , where $P$ is power in watts ( $W\text{W}$ ), $V$ is potential difference in volts ( $V\text{V}$ ), and $I$ is current in amperes ( $A\text{A}$ ).
Current is the rate of flow of charge, so a larger current means more charge passes each second.
Potential difference is the energy per unit charge, so a larger potential difference means each unit of charge carries more energy.
Increasing either quantity therefore increases the power transferred.

Example

A motor has a potential difference of $V12\ \text{V}$ across it and a current of $A3.5\ \text{A}$ through it. Calculate its power.

Write the equation: $P = V I$ .
Substitute the values: $\times 3.5$ .
Calculate, with the unit: $42\ \text{W}$ .
The motor transfers energy at a rate of $J42\ \text{J}$ each second.

Power from current and resistance

Power can also be found from the current through a device and its resistance: $P = I^2R$ , where $P$ is power in watts ( $W\text{W}$ ), $I$ is current in amperes ( $A\text{A}$ ), and $R$ is resistance in ohms ( $Ω\Omega$ ).
The current is squared, so changing the current has a large effect on the power.
If the current doubles while the resistance stays the same, the power becomes four times larger because $2^2 = 4$ .

Example

A heating element has a resistance of $Ω20\ \Omega$ and a current of $A2.0\ \text{A}$ through it. Calculate the power.

Write the equation: $P = I^2R$ .
Substitute the values: $(2.0)^2 \times 20$ .
Square the current first: $2.0)^2 = 4.0$ .
Complete the calculation: $\times 20 = 80\ \text{W}$ .
The element transfers energy at a rate of $J80\ \text{J}$ each second.

Common Mistake

Do not confuse power with energy: energy is how much is transferred, power is how quickly.
In $P = I^2R$ , only the current is squared, so work out $I^2$ before multiplying by the resistance, and never square the resistance.
Use the potential difference across the device and the current through the device.

Exam technique

Pick the equation that matches the quantities you are given: use $P = V I$ for potential difference and current, and $P = I^2R$ for current and resistance.
Write the equation, substitute with correct units, show each step, and give the answer in watts. Both equations must be recalled and may need rearranging.

Self review

What does the power of an electrical device tell you?
Write the equation linking power, potential difference and current.
Write the equation linking power, current and resistance.
State the units of power, potential difference, current and resistance.
If the current doubles at constant resistance, by what factor does the power change?

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Power is the rate at which energy is transferred by a device. It is measured in watts, $W\text{W}$ .

A power of $\, \text{W}$ means that energy is transferred at a rate of $\, \text{J}$ every second. A higher-power device transfers more energy each second than a lower-power device.

For example, a lamp transfers energy electrically from the supply, raising the thermal energy store of the lamp and its surroundings while also transferring some energy by light.

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Power is the [ ].

2.4.1 Power

2.4.1 Electrical power

Power: how fast a device transfers energy

Definition

Power

The rate at which energy is transferred, measured in watts ( $W\text{W}$ ).

Every circuit device transfers energy from one store to another; a lamp transfers energy electrically from the supply and raises the thermal energy store of the lamp and its surroundings, while also transferring some energy by light.
The power of a device tells you how quickly it makes that transfer.
A power of one watt means energy is transferred at a rate of one joule per second.
So a higher-power device transfers more energy every second than a lower-power one.

Power from potential difference and current

The power transferred by an electrical device depends on the potential difference across it and the current through it.
These are linked by $P = V I$ , where $P$ is power in watts ( $W\text{W}$ ), $V$ is potential difference in volts ( $V\text{V}$ ), and $I$ is current in amperes ( $A\text{A}$ ).
Current is the rate of flow of charge, so a larger current means more charge passes each second.
Potential difference is the energy per unit charge, so a larger potential difference means each unit of charge carries more energy.
Increasing either quantity therefore increases the power transferred.

Example

A motor has a potential difference of $V12\ \text{V}$ across it and a current of $A3.5\ \text{A}$ through it. Calculate its power.

Write the equation: $P = V I$ .
Substitute the values: $\times 3.5$ .
Calculate, with the unit: $42\ \text{W}$ .
The motor transfers energy at a rate of $J42\ \text{J}$ each second.

Power from current and resistance

Power can also be found from the current through a device and its resistance: $P = I^2R$ , where $P$ is power in watts ( $W\text{W}$ ), $I$ is current in amperes ( $A\text{A}$ ), and $R$ is resistance in ohms ( $Ω\Omega$ ).
The current is squared, so changing the current has a large effect on the power.
If the current doubles while the resistance stays the same, the power becomes four times larger because $2^2 = 4$ .

Example

A heating element has a resistance of $Ω20\ \Omega$ and a current of $A2.0\ \text{A}$ through it. Calculate the power.

Write the equation: $P = I^2R$ .
Substitute the values: $(2.0)^2 \times 20$ .
Square the current first: $2.0)^2 = 4.0$ .
Complete the calculation: $\times 20 = 80\ \text{W}$ .
The element transfers energy at a rate of $J80\ \text{J}$ each second.

Common Mistake

Do not confuse power with energy: energy is how much is transferred, power is how quickly.
In $P = I^2R$ , only the current is squared, so work out $I^2$ before multiplying by the resistance, and never square the resistance.
Use the potential difference across the device and the current through the device.

Exam technique

Pick the equation that matches the quantities you are given: use $P = V I$ for potential difference and current, and $P = I^2R$ for current and resistance.
Write the equation, substitute with correct units, show each step, and give the answer in watts. Both equations must be recalled and may need rearranging.

Self review

What does the power of an electrical device tell you?
Write the equation linking power, potential difference and current.
Write the equation linking power, current and resistance.
State the units of power, potential difference, current and resistance.
If the current doubles at constant resistance, by what factor does the power change?

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How was this guide?

Teach Genie

Review 2.4.1 Power by teaching Genie

Teach it back in your own words, spot gaps, and remember it better.

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Lesson

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8 minute activity

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Power is the rate at which energy is transferred by a device. It is measured in watts, $W\text{W}$ .

A power of $\, \text{W}$ means that energy is transferred at a rate of $\, \text{J}$ every second. A higher-power device transfers more energy each second than a lower-power device.

For example, a lamp transfers energy electrically from the supply, raising the thermal energy store of the lamp and its surroundings while also transferring some energy by light.

Flashcards

Remember key concepts with flashcards

21 flashcards

Practice flashcards

Power is the [ ].

2.4.1 Power

2.4.1 Electrical power

Power: how fast a device transfers energy

Power from potential difference and current

Power from current and resistance

Review 2.4.1 Power by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

2.4.1 Power Revision Guide

2.4.1 Power

2.4.1 Electrical power

Power: how fast a device transfers energy

Power from potential difference and current

Power from current and resistance

Review 2.4.1 Power by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

2.4.1 Power Revision Guide