Energy transfer and electric power
Distinguish a rate of energy transfer from the amount transferred.
Before you begin
- Multiplication with units
- Time conversions
Describe energy transfers
Energy is conserved overall but can be transferred between stores, including chemical, kinetic and thermal stores. A device's useful output is only part of the total transfer; other energy often spreads into the surroundings. Calling it wasted describes its usefulness for a task, not its disappearance. Define the system and the useful output before calculating efficiency.
Use power as a rate
Power is energy transferred per unit time. One watt means one joule per second. Multiply constant power by time in seconds to obtain energy in joules. Electricity bills often use kilowatt-hours, another energy unit: multiply power in kilowatts by time in hours. A high-power device can use less total energy if it runs for much less time.
Worked example
A 60 W lamp operates for 120 seconds. How much energy does it transfer?
Show the worked solution
- 60 W means 60 joules each second.
- Multiply by 120 seconds: 60 × 120 = 7,200.
- The time units cancel in J/s × s.
7,200 J.
Try it yourself
Which quantity is measured in watts?
Common mistakes
- A kilowatt-hour is energy, not power per hour.
What can you explain now?
A 2 kW heater runs for 30 minutes. Find its energy use in kWh.
Compare with the explanation
1 kWh.
Thirty minutes is 0.5 hours. Multiply 2 kW by 0.5 h. This equals 3.6 million joules, but the requested unit is kWh.
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