A kettle is rated 1200 W. It runs for 90 s.
How much energy does the kettle transfer to the water?
Start with what the rating means: 1200 W is 1200 J transferred each second.
Now count 90 seconds of that: 1200 × 90 = 108000. The kettle transfers 108000 J.
That move is P = W/t solved for the work: W = P × t.
P = W/t rearranges for either unknown.
To find the work: W = P × t.
To find the time: t = W/P.
A kettle rated 1200 W runs for 90 s. Calculate the energy it transfers.
Write down the values in the question:
P = 1200 W, t = 90 s; W = ?
Write down the arrangement whose unknown stands alone:
W = P × t
Substitute in the values, and calculate:
W = 1200 × 90 = 108000
The kettle transfers 108000 J.
A 400 W sander must do 5200 J of work. How long does this take?
Write down the values in the question:
P = 400 W, W = 5200 J; t = ?
Write down the arrangement whose unknown stands alone:
t = W/P
Substitute in the values, and calculate:
t = 5200 ÷ 400 = 13
The sander takes 13 s.
The mixed run below asks all three ways; read each question for its unknown before reaching for an arrangement.
A 150 W pump runs for 40 s. Enter the energy it transfers, in J.
A motor must transfer 3000 J. Its power is 250 W. Enter the time this takes, in s.
A 75 W fan runs for 120 s. Enter the energy it transfers, in J.
A winch does 1600 J of work in 32 s. Enter its power, in watts.
A heater must transfer 8400 J. Its power is 350 W. Enter the time this takes, in s.
A 2000 W oven element runs for 45 s. Enter the energy it transfers, in J.
A 900 W toaster element runs for one 150 s toasting cycle. Enter the energy it transfers during the cycle, in J.
A 1500 W space heater must deliver 45000 J to a cold workshop. Enter the time this takes, in s.
A laptop charger transfers 7200 J to a battery while charging at a steady 120 W. Enter the time this takes, in s.