Power (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Power

• 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

• 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

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

Energy and power

• Power is defined as:

The rate at which energy is transferred

• In other words, power is the energy transferred per unit time (e.g., per second)

• Therefore, power can be calculated using the equation:

$P=\frac{E}{t}$

• Where:

  • P = power in watts (W)

  • E = energy transferred in joules (J)

  • t = time in seconds (s)

Work and power

• When work is done on an object, energy is transferred from one store to another, so:

energy transferred = work done

• This means power can also be defined as:

The rate at which work is done

• Therefore, power can also be calculated using the equation:

$P=\frac{W}{t}$

• 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

• 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 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

• 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

$1\text{ W}=1\text{ J/s}$

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

Comparing Power Outputs

• 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

Two motors with different powers

Maths Tip: Proportionality

• 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