Watt

Also called W

The SI unit of power, symbol W, equal to one joule per second or one kilogram meter squared per second cubed. A watt measures how fast energy is being transferred, so it describes a rate and never an amount.

1 W=1 Js=1 kg⋅m2s31\ \text{W} = 1\ \frac{\text{J}}{\text{s}} = 1\ \frac{\text{kg}\cdot\text{m}^2}{\text{s}^3}

The defining relation is Pavg=W/Δt=ΔE/ΔtP_{\text{avg}} = W/\Delta t = \Delta E/\Delta t, printed on all four AP sheets, and the circuits version P=IΔVP = I\Delta V lands in the same unit. Every table of information lists watt, W, in its UNIT SYMBOLS box.

How big is one. Raising a 1.0 kg object steadily at 1.0 m/s takes 9.8 W against gravity. A person walking upstairs at a normal pace is doing a few hundred watts of it.

The category error. "It used 60 watts" is not a quantity of anything until a time is attached. Watts are not consumed; joules are. A 60 W bulb left on for one hour transfers (60)(3600)=216,000(60)(3600) = 216{,}000 J, which is 216 kJ. Change the hour and the energy changes while the 60 W does not.

That is also why the electricity bill is in kilowatt hours rather than kilowatts. One kilowatt hour is (1000)(3600)=3.6×106(1000)(3600) = 3.6 \times 10^6 J, a power multiplied by a time, so it is an energy. And "watts per hour" is not a unit of anything an AP problem asks for: the division by time is already inside the watt.

On the exam it reads as a signal word. Any question phrased around how long, how quickly, or what rate is a power question even when the word never appears.

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