What a learner can do afterwards
- States the definition of power and gives its unit (watt)
- Uses P = E/t to calculate power, energy, or time given the other two quantities
- Compares the power ratings of common appliances and explains what the rating means
- Explains that greater useful power completes the same useful-energy transfer sooner, while greater input power means more energy per second rather than necessarily more total energy
1 · Read
Power is energy transferred per second. It is a rate, not an amount stored in a battery. Energy amounts are measured in joules. Total energy used also depends on running time. One minute is 60 seconds; one hour is 3600 seconds.
Average power is P = E ÷ t, with energy E in joules and time t in seconds. For several intervals, add their energies and times before dividing. A heater transferring 2400 J in 2 s has power 1200 W.
At constant power, energy E = P × t. A 60 W lamp running for 10 s transfers 600 J. For changing settings, calculate the energy for each interval and add. Use seconds when power is in watts and energy is in joules.
At constant input ratings, a larger watt number means more energy used each second. Divide the larger rating by the smaller rating to find how many times the rate is as large. Use the same units for both ratings. Over equal times, the higher-rated device uses more energy. Over different times, compare E = P × t; rating alone does not decide total energy.
Useful power is energy reaching the intended job each second. Efficiency is the fraction of input energy that becomes useful energy. At equal efficiency, useful-power and input-power ratios are equal because the same fraction of each input is useful. For the same useful energy, multiplying useful power by a factor divides time by that factor. At constant useful powers of 3 W and 6 W, delivering 30 J takes 30 ÷ 3 = 10 s and 30 ÷ 6 = 5 s.
At constant power, time t = E ÷ P. A 250 W device transfers 5000 J in 20 s. If some target energy has already transferred, subtract it before finding remaining time. Compare two durations by subtracting the shorter from the longer.
Power is how fast energy transfers, measured in watts: one watt is one joule each second. P = E ÷ t, and you can rearrange it to find energy (E = P × t) or time (t = E ÷ P).
Adapted from OpenStax College Physics, openstax.org.
Original reading and extra questions written for this collection. The linked power video is from Khan Academy.
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