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Power (Edexcel GCSE Physics)
Revision Note
Definition of 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
- 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 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
Calculating Power
- Power is defined as
energy transferred per unit time
- And
work done = energy transferred
- Therefore, power is also
work done per unit time
- Power can be calculated using the following equation:
- Where:
- P = power, measured in watts (W)
- E = energy transferred or work done, measured in joules (J)
- t = time, measured in seconds (s)
- This equation can be rearranged with the help of a formula triangle:
Work, power, time formula triangle
Worked example
Calculate the energy transferred if an oven of power 2500 W is used for 50 minutes.
Step 1: List the known values
-
- Power, P = 2500 W
- Time, t = 50 minutes = 50 × 60 = 3000 s
Step 2: Write down the power equation
Step 3: Rearrange to make energy the subject
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.
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 W = 1 J / s
- 1 kilowatt (1 kW) is equal to 1000 watts, or 1000 joules of energy transferred per second (1 kJ / s)
Examiner Tip
One way to remember this unit is it remember the saying “watt is the unit of power?”
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