Power (Edexcel IGCSE Physics)

Spec point — 4.16 · describe power as the rate of transfer of energy or the rate of doing work

Power

• Power is work done per unit time

• Since work done is equal to energy transferred, power is also energy transferred per unit time

• 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 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 with different power ratings doing the same amount of work

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

Two motors with different powers

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 ratings table

• 1 kW = 1000 W (1 kilowatt = 1000 watts)
• 1 MW = 1 000 000 W (1 megawatt = 1 million watts)

Calculating power

Spec point — 4.17 · use the relationship between power, work done (energy transferred) and time taken: power = work done / time taken P = W / T

Calculating power

• Since power is defined as The rate of doing work
• And work is

Work done = energy transferred

• Then, power can be expressed in equation form as

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

• Where:

  • W = The work done, measured in joules (J) or newton-metres (Nm)
  • t = time measured in seconds (s)
  • P = power measured in watts (W)

Power, work done and time formula triangle

The formula triangle can be used to rearrange the equation

• For a reminder on how to use formula triangles, see the revision note on Speed

Worked Example Calculate the work done if an iron of power 2000 W is used for 5 minutes.

Answer:

Step 1: List the known values

• Power, P = 2000 W
• Time, t = 5 minutes = 5 × 60 = 300 s

Step 2: Write down the relevant equation

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

Step 3: Rearrange for energy transferred, ΔE

$W=Pt$

Step 4: Substitute in the known values

$W=2000\times300$

$W=600000J$

Exam Hint Think of power as “energy per second”. Thinking of it this way will help you remember the relationship between power and energy.