Electrical Power (WJEC GCSE Science (Double Award)): Revision Note

Energy & Power

• 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

• Time is an important consideration when it comes to power

• Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance

• If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time

Comparison of Two Cars with Different Amounts of Power

Two cars accelerate to the same final speed, but the one with the most power will reach that speed sooner

• 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

Electrical Power

• 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