Flor hauls a box of tiles up a ladder, doing 600 J of work in 10 s.
Comparing her with a second mover needs the second mover. A single number for her own transfer would settle every compare at once. What is that number?
Split her climb into seconds. 600 ÷ 10 = 60. Flor transfers 60 J each second.
That number, the joules transferred each second, measures how fast her transfer runs: it is her rate of energy transfer.
The rate of energy transfer is called power.
Power is measured in watts. One watt is one joule transferred each second. Flor's power is 60 W.
A 60 W light bulb converts 60 J every second: the same rate as Flor's climb.
Dov's hoist does 1800 J of work in 24 s. Calculate the hoist's power.
Write down the values in the question:
W = 1800 J, t = 24 s; P = ?
Write down the equation:
P = W/t
Substitute in the values, and calculate:
P = 1800 ÷ 24 = 75
The hoist's power is 75 W.
Edda's winch does 240 J of work in 30 s. Enter the winch's power in watts.
A 500 W motor is running. How much energy does it convert each second?
One watt is one joule transferred each second.
The motor's power is 500 W.
So the motor converts 500 J each second.
The motor converts 500 J every second it runs.
A desk fan is rated 45 W. Gael says: 'The fan converts 45 J of energy every second.' Is he correct?
A space heater converts 4400 J of energy in 80 s. Enter its power in watts.
Careful with the symbol W: it does two jobs. In P = W/t, the capital W is the work done, in joules: the same W as in W = Fd.
The watt's symbol W is different: it is written only after a number, like 60 W. A W after a number is the unit; the W in the equation is the work.
A dishwasher's heating element transfers 4500 J to the water in 90 s. Enter the element's power in watts.
A hoist's test sheet carries two lines: 'motor rating: 60 W' and 'lift record: W = 300 J, t = 5 s'. Which entry is the work done during the lift?
Which statement says what one watt is?