6.3.2Generating and Distributing Electricity and Household Use
Energy and Power in Appliances — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 8
Part 1 of 5What power means
Rate of energy transfer
- Rate of energy transfer Power is the rate at which energy is transferred, or the rate at which work is done.
- A high-power device transfers a given amount of energy in a shorter time.
The watt
- The watt Power is measured in watts (W). One watt is one joule per second (1 W = 1 J/s).
Same energy, different power
- Same energy, different power Two motors that lift the same weight through the same height transfer the same energy,
- but the more powerful motor does it faster.
Part 2 of 5Equations for power
| Equation | Symbols and units |
|---|---|
| P = E ÷ t | P power in watts (W); E energy transferred in joules (J); t time in seconds (s) |
| P = W ÷ t | W work done in joules (J), which equals the energy transferred |
| P = I × V | I current in amperes (A); V potential difference in volts (V) |
| P = I2 × R | Combines P = I × V with V = I × R; R in ohms (Ω) |
Part 3 of 5Using P = E ÷ t
1
Write down the known quantities and check their units. Energy must be in joules and time in seconds.
2
Choose the equation P = E ÷ t.
3
Substitute the values. For example a kettle transfers 360 000 J in 150 s, so P = 360 000 ÷ 150.
4
Calculate the answer: P = 2400 W, which is 2.4 kW.
5
Give the unit with the answer, and rearrange if needed, for example E = P × t.
Part 4 of 5Power from current and p.d.
1Potential difference
- Potential difference The p.d. across a device tells you how much energy is transferred to it for each coulomb of charge.
2Current
- Current The current tells you how many coulombs pass each second.
3Multiply
- Multiply Energy per coulomb × coulombs per second gives energy per second, so P = I × V.
4Example
- Example A 230 V heater with a current of 5.0 A has a power of 230 × 5.0 = 1150 W.
Part 5 of 5Typical power ratings
| Device | Approximate power |
|---|---|
| Torch | 1 W |
| Light bulb | 10 to 100 W |
| Electric cooker | 10 000 W (10 kW) |
| Railway engine | 1 000 000 W (1 MW) |
| Large power station | 10 000 MW |
Key terms
| Term | Meaning |
|---|---|
| Power | The rate at which energy is transferred or work is done. |
| Watt (W) | The unit of power; one joule transferred each second. |
| Joule (J) | The unit of energy and of work done. |
| Power rating | The power an appliance transfers when working normally, shown on its label. |
| Kilowatt (kW) | 1000 watts. |
Avoid these mistakes
1
Using minutes or hours in P = E ÷ t. Convert to seconds when using joules and watts.
2
Thinking a higher power rating means more energy in total. Total energy depends on the power and the length of time.
3
Writing the unit of power as J instead of W (or J/s).
4
Forgetting to convert kilowatts and megawatts to watts: 1 kW = 1000 W and 1 MW = 1 000 000 W.
5
Confusing P = I × V with V = I × R. Both are needed but they calculate different quantities.
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