Section 1 of 5What Power Means
Two devices can transfer the same amount of energy but at different rates, and power measures that rate.
Power
- The rate at which energy is transferred or work is done, measured in watts.
Watt
- One watt is equal to one joule of energy transferred each second.
Higher power
- A device with a higher power transfers the same amount of energy in a shorter time.
Energy and time
- Energy transferred equals power multiplied by time, so E = P × t.
Section 2 of 5Typical Power Ratings
Power ratings help to compare appliances and machines of very different size.
Typical Power Ratings
Kilowatt
- One kilowatt equals 1000 W, a unit often used for household appliances such as kettles.
Megawatt
- One megawatt equals 1 000 000 W, a unit used for large machines and trains.
Efficiency link
- A more efficient device has a larger proportion of its power output transferred usefully.
Reading ratings
- A power rating on an appliance gives the rate at which it transfers energy when working normally.
| Device | Approximate power |
|---|---|
| Light bulb | 60 W |
| Kettle | 2 kW (2000 W) |
| Electric oven | 10 kW (10 000 W) |
| Train | 1 MW (1 000 000 W) |
Section 3 of 5Calculating Power
Always convert the time into seconds before dividing.
1
Identify the energy transferred or work done, in joules, which is the top part of the power equation.
2
Convert the time into seconds, since power is measured in joules per second, or watts.
3
Divide the energy by the time to find the power in watts.
4
Convert the answer into kilowatts or megawatts only if the question asks for a larger unit.
Section 4 of 5Worked Example
Question
- A heater transfers 600 000 J of energy in 5 minutes.
- Calculate its power in watts.
- Convert the time into secondst = 5 × 60 = 300 s.
- Use P = W ÷ t = 600 000 J ÷ 300 s.
- P = 2000 W.
Answer The power of the heater is 2000 W, which is 2 kW.
Section 5 of 5Exam Checklist & Tips
4.2.5 – Key Points for the Exam
- Power The rate of energy transfer or work done, measured in watts.
- Watt The unit of power, equal to one joule per second.
- Kilowatt A unit of power equal to 1000 watts.
- Megawatt A unit of power equal to one million watts.
- Energy transferred The amount of energy moved from one store to another, in joules.
Common Mistakes
- Leaving the time in minutes instead of converting it into seconds before dividing.
- Confusing power with energy; power is the rate of transfer while energy is the total transferred.
- Giving the answer in kW when the calculation gives watts, or failing to convert kilowatts into watts.
- Using energy in kJ without first converting it into J before dividing by the time.