OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain how the power transfer in any circuit device is related to the…
Full specification wording
Simple circuits: explain how the power transfer in any circuit device is related to the potential difference across it and the current, and to the energy changes over a given time
Power transfer in a device is the rate at which energy is transferred to or from it.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 13 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Power transfer in a device is the rate at which energy is transferred to or from it. For a device carrying current I with potential difference V across it, the power is P = V × I. A 12 V lamp drawing 0.5 A transfers 12 × 0.5 = 6 W, so 6 J each second. Over a time t the energy transferred is E = P × t = V × I × t; the same lamp run for 30 s transfers 6 × 30 = 180 J. Rearranging gives I = P ÷ V and V = P ÷ I. A larger p.d. across the same current, or a larger current at the same p.d., means more power and faster energy transfer. In a resistor the energy becomes thermal energy; in a motor most becomes kinetic energy; in a lamp most becomes light and thermal energy.
What earns the marks
- Selects P = V × I as the relationship linking power transfer to potential difference and current.
- Calculates power by multiplying the potential difference in volts by the current in amperes, giving watts.
- Uses E = P × t to find the energy transferred over a stated time, giving joules when time is in seconds.
- Links the size of the power to the rate of energy transfer, so a larger V or I gives faster energy transfer.
- Identifies the energy change produced by the device, such as thermal energy in a resistor or light in a lamp.
Common mistakes and how to put them right
Adds potential difference and current instead of multiplying them.
Put it right: Multiply V by I to obtain power.
Divides energy by time when asked for power.
Put it right: Divide energy by time only when converting from energy to power, and use P = V × I directly when V and I are given.
Treats power in watts as energy in joules.
Put it right: Keep power as a rate and multiply by time to obtain energy.
Exam tips
- Write the equation P = V × I before substituting numbers so the relationship is visible.
- Check units: volts × amperes gives watts, and watts × seconds gives joules.
- When a question asks about energy over a given time, show the multiplication by time explicitly rather than stopping at power.
Try the lesson
The start of the Simple circuits: explain how the power transfer in any circuit device is related to the potential difference across it and the current, and to the energy changes over a given time lesson, made from checked teaching notes for this specification point.
Have a go first
A lamp has a potential difference of 12 V across it and a current of 2 A through it. What is the power transferred by the lamp?
Listen
Power transfer in a device is the rate at which energy is transferred to or from it. For a device carrying current I with potential difference V across it, the power is P = V × I. A 12 V lamp drawing 0.5 A transfers 12 × 0.5 = 6 W, so 6 J each second. Over a time t the energy transferred is E = P × t = V × I × t; the same lamp run for 30 s transfers 6 × 30 = 180 J. Rearranging gives I = P ÷ V and V = P ÷ I. A larger p.d. across the same current, or a larger current at the same p.d., means more power and faster energy transfer. In a resistor the energy becomes thermal energy; in a motor most becomes kinetic energy; in a lamp most becomes light and thermal energy. A strong answer earns marks for these points. Selects P = V × I as the relationship linking power transfer to potential difference and current. Calculates power by multiplying the potential difference in volts by the current in amperes, giving watts. Uses E = P × t to find the energy transferred over a stated time, giving joules when time is in seconds. Links the size of the power to the rate of energy transfer, so a larger V or I gives faster energy transfer. Exam tip: Write the equation P = V × I before substituting numbers so the relationship is visible.
See it
Key words
Say what each word means before you turn the card.
Power transfer
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P = V × I
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E = P × t
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Energy change
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See every definition
- Power transfer
- The rate at which energy is transferred to or from a circuit device.
- P = V × I
- The relationship linking power transfer to the potential difference across a device and the current through it.
- E = P × t
- The relationship used to find the energy transferred over a given time, giving joules when time is in seconds.
- Energy change
- The change produced by a device, such as thermal energy in a resistor or light and thermal energy in a lamp.
Label the diagram
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1Box 1
2Box 2
3Box 3
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