Power (AQA GCSE Physics Combined Science): Revision Notes
Power
What is power?
Power tells us how quickly energy moves from one place to another. It's also how fast work gets done. Think of it like this - a more powerful device can transfer more energy each second.
Power is fundamentally about the rate of energy transfer or work completion. The faster the transfer, the higher the power.
Power is measured in watts (W). One watt means one joule of energy is being transferred every second.
Key formulas for power
There are two main ways to calculate power:
When you know energy and time:
- Power = Energy transferred ÷ Time taken
When you know work and time:
- Power = Work done ÷ Time taken
The triangle relationship shows: Energy = Power × Time or
This means you can rearrange to find any of the three quantities if you know the other two!
Typical power ratings
Here are some everyday devices and their power ratings:
| Device | Power rating (W) |
|---|---|
| Hairdryer | 1500 |
| TV | 50 |
| Microwave oven | 850 |
| Jet engine |
Key fact: A typical nuclear power station generates 1 GW (that's W - one billion watts!).
Power in Perspective
A 1500W hairdryer transfers 1500J of energy every single second. That's equivalent to lifting a 1kg object to a height of about 150m every second!
Energy transfer and work done
The way you calculate energy or work depends on what type of energy change is happening:
- Gravitational potential energy:
- Work done by a force:
- Electrical energy: and
Understanding which formula to use is essential for calculating power in different situations.
Worked examples
Worked Example 1: Microwave Oven Power
A microwave transfers 48000J of energy in 1 minute. What's its power rating?
Solution:
- Energy transferred = 48000J
- Time = 1 minute = 60s
- Power = Energy ÷ Time =
Worked Example 2: Force and Motion
A 120N force moves an object 40m in 2 minutes. Calculate the power.
Solution:
- Work done = Force × Distance =
- Time = 2 minutes = 120s
- Power = Work done ÷ Time =
Worked Example 3: Climbing Power
A 78kg climber scales a 30m wall in 5 minutes. Find the power used.
Solution:
- Energy transferred =
- Time = 5 minutes = 300s
- Power =
Important tip: Always convert minutes to seconds when calculating power in watts! This is a common source of errors in power calculations.
Key Points to Remember:
- Power measures how quickly energy is transferred or work is done
- Power is measured in watts (W)
- Use when you know energy and time
- Use when you know work and time
- Always convert minutes to seconds in power calculations
- Higher power rating means faster energy transfer