Lesson focus: powerHow quickly energy is transferred, measured in watts (W). measures the rateHow fast something happens, such as how much changes each second or each minute. of energyWhat is needed to make things happen; it can be stored and moved but never made or destroyed, and it is measured in joules (J). transfer, so devices can transfer the same energy in different times.
Calculate power, energy and time using consistent SI units.
Build on earlier learning
Energy dissipationEfficiency described a fraction of energy; power now describes the rate of transfer.
You will be successful when you can…
- Define power.
- Use P = E ÷ t.
- Convert minutes and kilojoules.
- Distinguish power from energy.
Words that carry the science
- power
- Rate of energy transfer or rate of doing work.
- watt
- One joule transferred per second.
- kilowatt
- One thousand watts.
- energy
- A conserved quantity measured in joules.
- rate
- Change in a quantity per unit time.
Power is energy per second
01Power is energy per second
powerHow quickly energy is transferred, measured in watts (W). = energyWhat is needed to make things happen; it can be stored and moved but never made or destroyed, and it is measured in joules (J). transferred ÷ time. One wattThe unit of power (W); 1 watt means 1 joule of energy transferred every second. equals one joule per second.
A high-power device transfers energy rapidly. It does not necessarily transfer more energy overall because operating time also matters.
02Units must be compatible
Use joules, seconds and watts unless deliberately working in another consistent set. Convert kilojoules to joules and minutes to seconds.
Rearrange to energy = power × time or time = energy ÷ power. Include units at every stage.
03Same work can have different power
Two people climbing the same stairs can increase gravitational stores by the same amount. The person taking less time has the greater average power.
Instantaneous power can vary during a movement; a whole-task calculation gives average power.
Energy and power
Power is how quickly energy is transferred. 1 watt (W) is 1 joule per second. Energy transferred (J) = power (W) × time (s). Bills use kilowatt-hours: 1 kWh is a 1000 W appliance running for an hour.
Controls
Power ratings are typical values. Cost = energy (kWh) × price per kWh.
Calculate kettle power
A kettle transfers 108 kJ in 60 s.
- Convert 108 kJ to 108,000 J.
- Use P = E ÷ t.
- Calculate 108,000 ÷ 60 = 1,800.
- State power = 1,800 W or 1.8 kW.
How to make the reasoning visible: The kettle transfers energy at an average rateHow fast something happens, such as how much changes each second or each minute. of 1.8 kJ each second.
Compare transfer rates
Convert units before calculating.
| Device | Energy | Time | Power |
|---|---|---|---|
| lamp | 7200 J | 120 s | ? |
| motor | 45 kJ | 30 s | ? |
| heater | 360 kJ | 180 s | ? |
| phone | 18 kJ | 3600 s | ? |
Calculate all four powers.
Which transfers most energy and which has highest power?
Check your understanding
Answer from memory, then check against the suggested answer.
Power equation?
One watt equals?
Unit of energy?
Two motors each transfer 12 kJ. One takes 8 s and one 20 s. Which is more powerful?
Why can a low-power appliance use more energy than a high-power one?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A 1.5 kW pump runs for 4 minutes.
Calculate the energy transferred, in joules. Show your working.
(5)Total for this question: 5 marks
Command word: Unit conversions earn the calculation; write them before substituting.