AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: Students should be able to explain how the power of a circuit device is related to:
The power of a circuit device is the rate at which it transfers energy, measured in watts, where one watt equals one joule per second.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The power of a circuit device is the rate at which it transfers energy, measured in watts, where one watt equals one joule per second. Power depends on the potential difference across the device and the current through it, as summarised by P = VI. For a fixed potential difference, increasing the current increases the power, and for a fixed current, increasing the potential difference increases the power. Everyday appliances illustrate this: a 230 V mains heater drawing a larger current transfers energy faster and has a higher power rating than one drawing a smaller current.
What earns the marks
- State that power is the rate of energy transfer, measured in watts, where 1 W = 1 J/s.
- Explain that power depends on both the potential difference across a device and the current through it.
- Use the equation P = VI to relate power, potential difference and current for a circuit device.
- Explain that for a fixed potential difference, a larger current gives a greater power, and for a fixed current, a larger potential difference gives a greater power.
- Link the power rating of an everyday appliance to the rate at which it transfers energy from the mains supply.
- Combine P = VI with E = Pt to calculate the energy transferred by a device over a given time.
Common mistakes and how to put them right
Treating power as the total energy transferred rather than the rate of transfer.
Put it right by dividing energy by time when comparing devices.
Assuming power depends only on current.
Put it right by using P = VI.
Mixing units, for example using time in minutes with power in watts.
Put it right by multiplying minutes by 60.
Exam tips
- Write the equation P = VI before substituting values, then rearrange only after identifying the unknown quantity.
- When comparing two appliances, calculate the power of each using the same method so the comparison is fair.
- Give the unit with every numerical answer, for example W for power or J for energy, to secure the unit mark.
Try the lesson
The start of the Energy transfers in everyday appliances: Students should be able to explain how the power of a circuit device is related to: lesson, made from checked teaching notes for this specification point.
Have a go first
A 12 V supply pushes a current of 2 A through a heater. Which equation gives the power of the heater in watts?
Listen
The power of a circuit device is the rate at which it transfers energy, measured in watts, where one watt equals one joule per second. Power depends on the potential difference across the device and the current through it, as summarised by P = VI. For a fixed potential difference, increasing the current increases the power, and for a fixed current, increasing the potential difference increases the power. Everyday appliances illustrate this: a 230 V mains heater drawing a larger current transfers energy faster and has a higher power rating than one drawing a smaller current. A strong answer earns marks for these points. State that power is the rate of energy transfer, measured in watts, where 1 W = 1 J/s. Explain that power depends on both the potential difference across a device and the current through it. Use the equation P = VI to relate power, potential difference and current for a circuit device. Explain that for a fixed potential difference, a larger current gives a greater power, and for a fixed current, a larger potential difference gives a greater power. A common slip: Treating power as the total energy transferred rather than the rate of transfer: power in watts is energy in joules per second. Instead, dividing energy by time when comparing devices. Exam tip: Write the equation P = VI before substituting values, then rearrange only after identifying the unknown quantity.
Key words
Say what each word means before you turn the card.
Power
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Potential difference and current
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Everyday appliance power ratings
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Energy transfer calculations
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See every definition
- Power
- The rate at which a circuit device transfers energy, measured in watts, where one watt equals one joule per second.
- Potential difference and current
- Power depends on both the potential difference across a device and the current through it, as summarised by P = VI.
- Everyday appliance power ratings
- The power rating of an everyday appliance is linked to the rate at which it transfers energy from the mains supply.
- Energy transfer calculations
- Combining P = VI with E = Pt allows calculation of the energy transferred by a device over a given time.
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