The watt
Suppose 600 J of work is done in 10 seconds. Dividing one by the other gives the rate, and that rate has a unit of its own.
rate of doing work
= 600 J ÷ 10 s
= 60 J s-1
One joule per second has a name
A rate of one joule per second is defined as one watt. The power worked out above is therefore 60 W.
The watt
1 J s-1 = 1 W
The watt — the unit in which power is measured.
You have read this unit before
Unlike the joule, the watt is printed on things you own.
- A light bulb marked 9 W converts 9 J of electrical energy every second.
- An electric kettle marked 2000 W does its work two thousand joules at a time, each second — which is why it boils water so quickly.
- A motor rated in kilowatts is rated in thousands of joules per second.
A watt is a joule per second
1 W = 1 J s-1. The watt is not a quantity of energy; it is a rate, and it means nothing until you say for how long.