AQA Physics (8463) 4.1.1.4 · Quantitative
Power
Two motors lift the same load to the same height, and one takes twice as long. If they transfer the same energy, what is different about them?
Triple Science · Foundation tier
Start here
Two kettles, one cup of tea.
A 3000 W kettle and a 1500 W kettle each heat 0.5 kg of water from 20 °C to 100 °C. The 3000 W kettle finishes first.
If you had to guess, does the 3000 W kettle transfer more energy to the water, or the same energy, faster?
Power is the rate at which energy is transferred, or the rate at which work is done. Its unit is the watt (W): a transfer of 1 joule every second. So the 3000 W kettle transfers 3000 J each second, and the 1500 W kettle 1500 J. When a motor lifts a crate, the work it does goes into the crate’s gravitational potential store, Ep = m g h. On the bench, two motors lift crates side by side. Call the result before they run.
The lift test · race 1 of 3
Motors A and B each lift a 20 kg crate through 3.0 m. A takes 4.0 s; B takes 8.0 s.
How do their powers compare?
Think again · power is not force
“Motor X has the bigger power, so it can lift the heavier load.”
Spot the flaw
ExplainMotor X is rated 500 W and motor Y 250 W. What is wrong with the student’s claim?
Equations
P = E ÷ t
power (W) = energy transferred (J) ÷ time (s)
E = P × t · t = E ÷ P
P = W ÷ t
power (W) = work done (J) ÷ time (s). Here W on the right is work done, not watts.
W = P × t · t = W ÷ P
Ep = m g h
gravitational potential energy (J) = mass (kg) × gravitational field strength (N/kg) × height (m): the work done lifting a load.
Joules and seconds go in; watts come out. Convert anything else before the equation.
1 kW = 1000 W · 1 MW = 1 000 000 W · 1 kJ = 1000 J · 1 MJ = 1 000 000 J · 1 minute = 60 s · 1 hour = 3600 s
Rate the machines
How powerful is each machine?
Each card gives the energy one machine transfers and the time it takes. Work out its power, then place it.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Key fact
Power is how fast energy is transferred: 1 watt is 1 joule every second. The same work done in less time means more power.
Command words in this lesson
- Define
- Give the exact meaning in one sentence. An example is not a definition.
- Calculate
- Convert, write the equation, substitute, rearrange, then give the answer with its unit.
- Explain
- Link the work done and the time taken to the rate of energy transfer.
- Describe
- Give the steps in order: what is measured, with what, and what is calculated.
Key note · AQA 4.1.1.4 (8463)
Power
- Power = rate of energy transfer.
- P = E/t = W/t.
- Unit: watt (W) = J/s.
- E = Pt.
- Always convert time to seconds.
- Higher power = same work done faster.
- P = mgh/t for climbing problems.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Mixing up power and energy, or stuck on a unit?
The lift test uses g = 9.8 N/kg and treats each motor as lifting at a steady speed with nothing dissipated, so all the work done goes into the crate’s gravitational potential store. In the hook, each kettle is treated as transferring all of its energy to the water.