Electrical Power (WJEC GCSE Physics: Combined Science)

• Power is defined as 

The amount of energy transferred per unit of time

• Power can be calculated using 

• Where:
  • P = power measured in watts (W)
    • The watt is equivalent to joules per second (J / s)
  • E = energy transferred measured in joules (J)
  • t = time measured in seconds (s)

• Energy can be calculated by rearranging this equation into the following form

• This equation triangle can be used to rearrange the equation

Energy, Power, Time Equation Triangle

Cover up the variable you need to calculate, and the triangle shows you how the equation is arranged

 

• Two electric motors:
  • lift the same weight
  • by the same height
  • but one motor lifts it faster than the other

• The motor that lifts the weight faster has more power

Comparison of Two Motors with Different Amounts of Power

The motor with the most power lifts the weight faster for a given heigh

• Power ratings are given to appliances to show the amount of energy transferred per unit time
• Common power ratings are shown in the table below:

Power Ratings Table

Appliance	Power Rating
Torch	1 W
Light bulb	10 - 100 W
Electric cooker	10 000 W or 10 kW (1 kW = 1000 W)
Railway engine	1 000 000 W or 1 MW (megawatt)
Saturn V space rocket	100 MW
Large power station	10 000 MW
Global demand for power	10 000 000 MW
Star (similar in size to the Sun)	100 000 000 000 000 000 000 MW

• 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:

• Where:
  • P = power measured in watts (W)
  • V = voltage measured in volts (V)
  • = current measured in amps (A)

• This equation can be rearranged with the help of a formula triangle:

Equation Triangle for The Electrical Power Equation

Cover up the variable you need to calculate, and the triangle shows you how the equation is arranged