Lesson 8 of 10 · 7 min
Power
NCERT § "Power"
The fair's overhead tank holds 1500 litres, and the pump fills it from the well in 10 minutes. The pump is labelled 500 W. Is that the power going into the water?
The lesson in notes
In short
Power is the rate of doing work. Average power = W/t; instantaneous power P = dW/dt.
Instantaneous power can also be written P = F·v, the dot product of force and velocity.
The SI unit of power is the watt, 1 W = 1 J s⁻¹. One horsepower is 746 W.
The kilowatt hour is a unit of energy, not power: 1 kWh = 3.6 × 10⁶ J.
A pump lifting mass m of water through height h in time t does useful work at the rate mgh/t; if its efficiency is η, the input power is (mgh/t)/η.
Efficiency is useful output power divided by input power and is always less than 1 for real machines.