Electricity - 2.4.1 Power (GCSE Physics AQA)

Power

Power

The power of an appliance is the amount of energy (J) that it transfers per second

In other words, power is the rate of energy transfer.

Power is measured in Watts (W).

1W means 1J of energy transferred in 1 second.

Calculating Power

Using Current and Potential Difference

We can calculate power in one of two ways. Firstly, we can use potential difference and current.

Remember, current tells us the rate of movement of charge. Whilst potential difference tells us the energy transferred per unit charge. Therefore to find the total energy transferred per second, power, you need to multiply current by voltage.

Where:

• power, P, in watts, W
• potential difference, V, in volts, V
• current, I, in amperes, A

Power is measured in Watts (W). 1W means 1J of energy transferred in 1 second.

Question: Billy is using a lamp. The current in the lamp is 0.4A, and the potential difference across the lamp is 5V. Calculate the power supplied to the lamp.

1. Write out the appropriate equation.

P = VI

2. Substitute in the numbers.

P = 5 x 0.4
P = 2 W

Question: There is just one bulb and one cell in a series circuit. The cell provides a voltage of 6V. A current of 3A flows in the circuit. What is the power of the bulb?

The bulb is the only component, so gets the maximum voltage of 6V.

Power = Current x Voltage = 3 x 6 = 18W

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Using Current and Resistance

We can also calculate power using current and resistance. Let us look at the first equation, P = VI.

If we don’t know the voltage, but we only know the resistance and current, we can still work out the power.

We know that P = VI, and from before we know that V = IR. We can substitute IR in the place of V, to give P = (IR)I = P = I² R.

Where:

• power, P, in watts, W
• current, I, in amperes, A (amp is acceptable for ampere)
• resistance, R, in ohms, Ω

Question: Katie is using her PlayStation 4. It has a power rating of 920 watts and uses a current of 4 amps. Calculate the resistance of the PlayStation.

1. Write out the equation.
For this question, we actually need to rearrange the equation to make resistance the subject of the formula.

P = I² R
R = P / I²

2. Substitute in the numbers.
Now that we have the equation, we can substitute in the numbers.

R = P / I²
R = 920 / (4)²
R = 920 / 16
R = 57.5 Ohms