Energy Changes in a System (AQA GCSE Physics): Revision Notes
Power
What is power?
Power tells us how quickly energy moves from one place to another. It measures the rate at which work gets done.
The bigger the power rating of a device, the more energy it moves each second. For example, a hairdryer with higher power will dry your hair faster because it transfers more energy per second.
Measuring power
Power is measured in watts (W).
One watt means one joule of energy is being moved every second. So:
- (1 joule per second)
The watt is named after James Watt, the Scottish inventor who improved the steam engine. This unit directly connects energy and time, making it perfect for measuring how quickly energy transformations occur.
Power formulas
There are two main ways to calculate power:
When you know energy transferred:
- Power = Energy transferred ÷ Time
When you know work done:
- Power = Work done ÷ Time
You can rearrange these formulas using a triangle method. Put at the top, and (or ) and at the bottom. Cover the quantity you want to find, and the remaining two show you the calculation needed.
Typical power ratings
Different devices have different power ratings:
| Device | Power rating (W) |
|---|---|
| Hairdryer | 1500 |
| TV | 50 |
| Microwave oven | 850 |
| Jet engine |
Key fact: A hairdryer with 1800W power transfers 1800J of energy each second.
Huge numbers: A nuclear power station generates about 1 GW (1 gigawatt = W).
Energy transferred and work done
The way you calculate energy transferred depends on what type of energy change is happening:
- For height changes: Use (gravitational potential energy)
- For forces moving objects: Use (work done by force)
- For electrical energy: Use and (electrical energy equations)
Remember to identify the type of energy transformation first, then choose the appropriate formula. This step is crucial for solving power problems correctly.
Worked examples
Worked Example: Microwave Power
A microwave transfers 48,000J of energy in 1 minute. What is its power rating?
Given:
- Energy transferred = 48,000J
- Time = 1 minute = 60s
Solution: Power = Energy ÷ Time =
Answer: The microwave's power rating is 800W.
Worked Example: Climbing Power
A 78kg climber scales a 30m wall in 5 minutes. Calculate the climber's power.
Step 1: Calculate energy transferred Energy transferred =
Step 2: Convert time to seconds Time = 5 minutes = 300s
Step 3: Calculate power Power =
Answer: The climber's power is 78W.
Always convert minutes to seconds when calculating power in watts. This is a common source of errors in power calculations.
Remember!
Key Points to Remember:
- Power measures how quickly energy moves or work gets done
- Power is measured in watts (W), where
- Use when you know energy transferred
- Use when you know work done
- Always convert time to seconds for calculations in watts
- Higher power means more energy transferred per second