OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe, with examples, the relationship between the power ratings for…
Full specification wording
Power and efficiency: describe, with examples, the relationship between the power ratings for domestic electrical appliances and how this is linked to the changes in stored energy when they are in use
A power rating tells you the rate at which an appliance transfers energy: a 2000 W kettle transfers 2000 J each second, while a 100 W lamp transfers 100 J each second.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A power rating tells you the rate at which an appliance transfers energy: a 2000 W kettle transfers 2000 J each second, while a 100 W lamp transfers 100 J each second. The energy transferred equals power multiplied by time, so a higher-rated appliance changes the stored energy of its surroundings faster. A 2 kW kettle switched on for 300 s transfers 2000 × 300 = 600 000 J, heating the water's thermal store; the same kettle for 60 s transfers 120 000 J. Compare appliances by their ratings: a 2500 W heater warms a room faster than a 1000 W heater because it transfers energy at a greater rate. Match each rating to the store that changes, such as thermal energy of water or kinetic energy of a fan, and use E = P × t to compare.
What earns the marks
- States that the power rating is the rate at which an appliance transfers energy, measured in watts where 1 W = 1 J/s.
- Uses the relationship energy transferred = power × time to compare the energy transferred by two appliances.
- Gives an example in which a higher-rated appliance changes a named stored energy faster, such as a 2 kW kettle heating water more quickly than a 1 kW kettle.
- Links the energy transferred to a named store that changes, such as thermal energy of water or kinetic energy of a fan.
- Calculates the energy transferred correctly for a stated power and time, for example 2000 W × 300 s = 600 000 J.
Common mistakes and how to put them right
Confuses power with energy.
Put it right: State that power is the rate of transfer and energy is the total transferred.
Multiplies power by time in the wrong units.
Put it right: Convert minutes to seconds before calculating.
Assumes a higher rating always means more total energy.
Put it right: Note that a low-rated appliance left on longer can transfer more energy.
Ignores which store changes.
Put it right: Name the store, such as thermal energy of the water, that the transfer fills.
Exam tips
- Convert any time given in minutes to seconds before using E = P × t.
- Check the units of the answer and give energy in joules.
- Compare two appliances by stating both the rating and the time to decide which transfers more energy.
- Name the store that changes so the link between rating and stored energy is explicit.
Try the lesson
The start of the Power and efficiency: describe, with examples, the relationship between the power ratings for domestic electrical appliances and how this is linked to the changes in stored energy when they are in use lesson, made from checked teaching notes for this specification point.
Have a go first
A 2000 W kettle and a 100 W lamp are both switched on for the same length of time. Which statement is correct?
Listen
A power rating tells you the rate at which an appliance transfers energy: a 2000 W kettle transfers 2000 J each second, while a 100 W lamp transfers 100 J each second. The energy transferred equals power multiplied by time, so a higher-rated appliance changes the stored energy of its surroundings faster. A 2 kW kettle switched on for 300 s transfers 2000 × 300 = 600 000 J, heating the water's thermal store; the same kettle for 60 s transfers 120 000 J. Compare appliances by their ratings: a 2500 W heater warms a room faster than a 1000 W heater because it transfers energy at a greater rate. Match each rating to the store that changes, such as thermal energy of water or kinetic energy of a fan, and use E = P × t to compare. A strong answer earns marks for these points. States that the power rating is the rate at which an appliance transfers energy, measured in watts where 1 W = 1 J/s. Uses the relationship energy transferred = power × time to compare the energy transferred by two appliances. Gives an example in which a higher-rated appliance changes a named stored energy faster, such as a 2 kW kettle heating water more quickly than a 1 kW kettle. Links the energy transferred to a named store that changes, such as thermal energy of water or kinetic energy of a fan. Exam tip: Convert any time given in minutes to seconds before using E = P × t.
Key words
Say what each word means before you turn the card.
Power rating
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Energy transferred
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Comparing appliance ratings
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Changes in stored energy during use
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See every definition
- Power rating
- The rate at which an appliance transfers energy, measured in watts where 1 W = 1 J/s.
- Energy transferred
- Equal to power multiplied by time, so a higher-rated appliance changes the stored energy of its surroundings faster.
- Comparing appliance ratings
- A 2500 W heater warms a room faster than a 1000 W heater because it transfers energy at a greater rate.
- Changes in stored energy during use
- Match each rating to the store that changes, such as thermal energy of water or kinetic energy of a fan.
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