Work, Power & Energy Resources
Energy Resources & Energy Transfers
Definition
- Power is the rate at which work is done, or equivalently the rate at which energy is transferred
- The SI unit is the watt (W), where 1 W = 1 J/s (one joule transferred per second)
- Common multiples: 1 kW = 1000 W; 1 MW = 1 000 000 W
Common exam question
Defining power
Question: State what the term power means (1 mark).
Asked in 2 of the 24 papers. Power is the rate of doing work, or the rate at which energy is transferred. "Energy transferred per second" and "per unit time" are accepted, and one scheme lets the equation P = W/t or its word form stand in for the sentence. Both schemes reject P = I × V, which is a way of calculating electrical power rather than what power means, and each rejects an "in" wording, "energy transferred in a unit time" in one and "the amount of energy in a given time" in the other: "in" describes an amount, "per" describes a rate.
Why time matters
- Two machines that do the same total work over different times have different powers. A weightlifter and a forklift may both lift a 200 kg crate onto a shelf, but if the forklift does it in 1 s while the weightlifter takes 30 s, the forklift's power is 30 times larger
- High-power devices either deliver large amounts of useful work very quickly (a rocket engine) or low amounts of useful work continuously over long periods (a wall-mounted clock running for years)
The power equation
P = W / t
- where:
- P = power (W)
- W = work done, or energy transferred (J)
- t = time taken for the transfer (s)
- Useful rearrangements:
- W = P × t (the energy a device transfers in a given time)
- t = W / P (how long a device of fixed power needs to deliver a target amount of energy)
Typical power ratings
| Device | Approximate power |
|---|---|
| Pocket torch | ~1 W |
| Incandescent light bulb | ~60–100 W |
| Hair dryer | ~1500 W |
| Electric oven | ~3 kW |
| Family car at cruise | ~25 kW |
| Large coal-fired power station | ~2 GW (2000 MW) |
| Saturn V rocket at launch | ~60 GW |
Worked example
Calculating time from power and energy
A pump transfers 1 350 000 J of energy at a constant power of 4500 W. Calculate the time taken.
Solution:
- Write the equation: P = W ÷ t
- Rearrange: t = W ÷ P
- Substitute: t = 1 350 000 ÷ 4500
- t = 300 s
If a time is given in minutes, turn it into seconds before you substitute: that conversion is a mark in its own right. A power or energy written in standard form earns the same substitution, rearrangement and evaluation marks, and a powers-of-ten slip loses one of them.