Electrical Power (AQA GCSE Physics)
Revision Note
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Electrical Power
Power is defined as
The rate of energy transfer or the amount of energy transferred per second
The power of a device depends on:
The voltage of the device
The current of the device
The power of an electrical component (or appliance) is given by the equation:
This equation can be rearranged with the help of a formula triangle:
Power, current, voltage formula triangle
The unit of power is the watt (W), which is the same as a joule per second (J/s)
Worked Example
Calculate the potential difference through a 48 W electric motor with a current of 4 A.
Answer:
Step 1: List the known quantities
Power, P = 48 W
Current, I = 4 A
Step 2: Write down the relevant equation
P = VI
Step 3: Rearrange for potential difference, V
Step 4: Substitute in the values
Examiner Tips and Tricks
Remember: Power is just energy per second.
Power & Resistance
The voltage across an electric device depends upon the current and resistance of that device
Using the equations P = VI with V = IR, power can be written in terms of resistance, R:
Worked Example
Calculate the power through a 20 Ω resistor with a current of 6 A through it.
Answer:
Step 1: List the known quantities
Resistance, R = 20 Ω
Current, I = 6 A
Step 2: Write down the relevant equation
P = I2R
Step 3: Substitute in the values
P = (6)2 × 20 = 720 W
Examiner Tips and Tricks
You can use the mnemonic
“Twinkle Twinkle Little Star, Power equals I squared R!”
to remember whether to multiply or divide by resistance in the power equations.
Which equation to use will depend on whether the value of current or voltage has been given in the question
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