Section 1 of 6What power means
Definition
- Definition Power is the rate at which energy is transferred, measured in watts (W), where 1 W = 1 J/s.
Power rating
- Power rating The rating printed on an appliance is the energy it transfers each second when working normally.
- A 2000 W kettle transfers 2000 J every second.
Same energy, different time
- Same energy, different time A higher-power device transfers a given amount of energy in a shorter time,
- but it does not necessarily use more energy overall.
Section 2 of 6Equations for electrical power
| Equation | Symbols and units | Use when you know |
|---|---|---|
| P = I × V | P in watts (W), I in amperes (A), V in volts (V) | Current and p.d. |
| P = I2 × R | R in ohms (Ω) | Current and resistance |
| P = E ÷ t | E in joules (J), t in seconds (s) | Energy and time |
| E = P × t | Same units | Power and time |
Section 3 of 6Worked method
1
Write down the quantities given and convert units, for example 2.3 kW = 2300 W or 5 minutes = 300 s.
2
Choose the equation that links the known quantities with the one wanted.
3
Rearrange before substituting, e.g. I = P ÷ V; for a 2300 W kettle on 230 V, I = 2300 ÷ 230 = 10 A.
4
State the answer with its unit and check it is sensible.
Section 4 of 6Why a hot wire wastes energy
1Current flows through a resistor
- Current flows through a resistor Electrons collide with ions in the conductor.
2Energy transferred thermally
- Energy transferred thermally Kinetic energy of the electrons is passed to the lattice, heating the component.
3Power loss depends on I2R
- Power loss depends on I2R Doubling the current quadruples the power wasted for the same resistance.
4Design response
- Design response Use low-resistance, thick wires and lower currents to reduce energy wasted as heat.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Power | The energy transferred per second, measured in watts. |
| Watt | The unit of power; one joule transferred each second. |
| Kilowatt | 1000 watts. |
| Power rating | The maximum or normal power at which an appliance is designed to operate. |
| Energy transferred | The amount of energy moved from one store to another, in joules. |
Section 6 of 6Exam technique
1
Leaving power in kilowatts or time in minutes when the equation needs watts and seconds.
2
Confusing power with energypower is the rate, energy is the total amount.
3
Using P = I × V when the question gives current and resistance but no p.d.; use P = I2 × R.
4
Forgetting to square the current in P = I2 × R.