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Power (Cambridge O Level 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
Vehicles with Different Powers
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 is the rate of transfer of energy, or rate of work done. The higher power motor will do the same work in a shorter time.
- 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 Rating Table
| Appliance or Object | Power rating / W |
| Torch | 1 |
| Electric lightbulb | 100 = 1 × 102 |
| Electric cooker | 10 000 = 1 × 104 |
| Railway engine | 1 000 000 = 1 × 106 |
| Saturn V space rocket | 100 000 000 = 1 × 108 |
| Very large power station | 10 000 000 000 = 1 × 1010 |
| World demand for power | 10 000 000 000 000 = 1 × 1013 |
| A star similar to the Sun | 1 00 000 000 000 000 000 000 000 000 = 1 × 1026 |
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
or
- Where:
- W or ΔE = The work done, or energy transferred, measured in joules (J)
- t = time measured in seconds (s)
- P = power measured in watts (W)
- Note that these two equations may be written slightly differently, but are representing the same thing - a transfer of energy over time
- This equation can be rearranged with the help of a formula triangle:
Formula Triangle for Power
Work, power, time formula triangle
Worked example
Calculate the energy transferred when an iron with a power rating of 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
Step 3: Rearrange for energy transferred, ΔE
Step 4: Substitute in the known values
Examiner Tip
Think of power as “energy per second”. Thinking of it this way will help you to remember the relationship between power and energy.
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