Power (AQA GCSE Physics: Combined Science)

• Machines, such as car engines, transfer energy from one energy store to another constantly over a period of time

• The rate of this energy transfer, or the rate of work done, is called power

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

• Power is defined as

Energy transferred per unit time

• Therefore, power can be calculated using the equation

• Where:
  • P = power in watts (W)
  • E = energy transferred in joules (J)
  • t = time in seconds (s)

• Since

energy transferred = work done

• Power can also be calculated using the equation

• Where:
  • P = power in watts (W)
  • W = work done in joules (J) 
  • t = time in seconds (s)

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

Work, power and time formula triangle

How to Use Formula Triangles

• Formula triangles are really useful for knowing how to rearrange physics equations
• To use them:

1. Cover up the quantity to be calculated, this is known as the 'subject' of the equation
2. Look at the position of the other two quantities
   • If they are on the same line, this means they are multiplied
   • If one quantity is above the other, this means they are divided - make sure to keep the order of which is on the top and bottom of the fraction!

• In the example below, to calculate speed, cover-up 'speed' and only distance and time are left
  • This means it is equal to distance (on the top) ÷ time (on the bottom)

How to use formula triangles

• 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

• Power and power ratings are useful because power describes how fast the energy is transferred from one store to another

Two identical cars accelerating to the same final speed will both transfer the same amount of energy. But if one of them does it in a shorter time, it will have a greater power

• The watt is the unit of power
• Since power is energy transferred per second, the watt can also be defined as 1 joule per second

1 W = 1 J / s 

• 1 kilowatt (1 kW) is equal to 1000 watts, or 1000 joules of energy transferred per second (1 kJ / s)

Two motors with different powers

Maths Tip

• GCSE physics equations will mostly require fractions
  • These are made up of the numerator (the top number) and the denominator (the bottom number)

• If the denominator decreases and the numerator stays the same, the whole fraction increases
• If the denominator increases and the numerator stays the same, the whole fraction decreases
  • This is known as inverse proportionality

• If the denominator stays the same and the numerator increases, the whole fraction increases
• If the denominator stays the same and the numerator decreases, the whole fraction decreases
  • This is known as direct proportionality

How to know whether the value of a fraction increases or decreases