OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Power: (a) power; the unit watt; P = W/t
Power is the rate at which work is done or energy is transferred.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 6
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Power is the rate at which work is done or energy is transferred. It is defined as the work done W divided by the time taken t, so P = W/t. The unit of power is the watt (W), where 1 W = 1 J s⁻¹. Power can also be expressed as the rate of energy transfer, so P = E/t. For example, if a machine does 600 J of work in 3 s, its power output is 600 / 3 = 200 W. In MCQ questions, you may calculate power, compare power ratings, or relate power to work and time. Remember that power is a scalar quantity and can be calculated from the gradient of a work-time graph, while the area under a power-time graph represents the total work done or energy transferred.
What earns the marks
- Power is defined as the rate of doing work or the rate of energy transfer.
- The equation for power is P = W/t, where W is work done in joules and t is time in seconds.
- The unit of power is the watt (W), which is equivalent to one joule per second (J s⁻¹).
- Power can also be calculated using P = E/t, where E is energy transferred.
- Power is a scalar quantity and its unit can be expressed in base units as kg m² s⁻³.
Common mistakes and how to put them right
Confusing power with energy.
Put it right: Check the units; energy is in joules, power in watts.
Using time in minutes or hours without converting to seconds.
Put it right: Convert time to seconds before calculating power.
Forgetting that the watt is a derived unit.
Put it right: Use consistent units in calculations.
Assuming power is always constant.
Put it right: For varying power, use average power or calculus if required.
Exam tips
- In multiple-choice questions, check the units of the given quantities; if time is in minutes, convert to seconds before using P = W/t.
- Remember that power can be calculated from the gradient of a work-time graph, and the area under a power-time graph gives the work done.
- If a question asks for the power of a device, ensure you use the work done by the device, not the total energy input if efficiency is involved.
Try the lesson
The start of the Power: (a) power; the unit watt; P = W/t lesson, made from checked teaching notes for this specification point.
Listen
Power is the rate at which work is done or energy is transferred. It is defined as the work done W divided by the time taken t, so P = W/t. The unit of power is the watt (W), where 1 W = 1 J s⁻¹. Power can also be expressed as the rate of energy transfer, so P = E/t. For example, if a machine does 600 J of work in 3 s, its power output is 600 / 3 = 200 W. In MCQ questions, you may calculate power, compare power ratings, or relate power to work and time. Remember that power is a scalar quantity and can be calculated from the gradient of a work-time graph, while the area under a power-time graph represents the total work done or energy transferred. A strong answer earns marks for these points. Power is defined as the rate of doing work or the rate of energy transfer. The equation for power is P = W/t, where W is work done in joules and t is time in seconds. The unit of power is the watt (W), which is equivalent to one joule per second (J s⁻¹). Power can also be calculated using P = E/t, where E is energy transferred. A common slip: Confusing power with energy: power is the rate of energy transfer, not the total energy. Instead, check the units; energy is in joules, power in watts. Exam tip: In multiple-choice questions, check the units of the given quantities; if time is in minutes, convert to seconds before using P = W/t.
See it
Key words
Say what each word means before you turn the card.
Power
Try answering aloud or in your head before turning the card.
Watt
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See every definition
- Power
- The rate at which work is done or energy is transferred, defined as the work done W divided by the time taken t, so P = W/t.
- Watt
- The unit of power, where 1 W = 1 J s⁻¹.
Label the diagram
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