Electricity & Power (Edexcel GCSE Physics)
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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 (potential difference) of the device
The current of the device
Examiner Tips and Tricks
Remember: Power is just energy per second. Think of it this way will help you to remember the relationship between power and energy
Calculating Electrical Power
The power of an electrical component (or appliance) is given by the equation:
The unit of power is the watt (W), which is the same as a joule per second (J/s)
Combining the equations P = IV with V = IR, power can be written in terms of resistance, R:
Where:
P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V)
R = resistance in ohms (Ω)
This equation can be rearranged with the help of a formula triangle:
Power, current, voltage formula triangle
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 = IV
Step 3: Rearrange for potential difference, V
Step 4: Substitute in the values
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 You can remember the unit of power by the phrase: “Watt is the unit of power?”
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