A 2.0 kW geyser heats water for 5 hours…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A geyser heats water for hours. How much energy does it use, in kilowatt-hours (units on the bill) and in joules?
Need a reminder? The key ideaShow
Power is the rate of doing work: , in watts. For a force at speed , . Efficiency useful power out power in. One kilowatt-hour is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Multiply power by time in the units of the bill, then change to joules.
Step 1. In units. .
Step 2. In joules. .
Answer: units, or .
Check. In SI: . Same.
Watch out. The kilowatt-hour is energy, not power. The bill charges for energy used.
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