Electrical Power (OCR Gateway GCSE Physics: Combined Science)

• 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

• 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

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

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