Physics Problem

Question

How many kilowatt-hours do you waste if you forget to turn off your 100. Watt bedroom lamp in the morning before school and it remains on for 9.0 hours? How many Joules of energy does this quantity represent? Show all of your work.

Answer 1

A 100 watt bulb which burns for 9 hours uses 900 watt hours of electricity. (100 * 9 = 900)

900 watt hours = 0.9 * 1000 watt hours = 0.9 kilowatt hours (1 kilowatt = 1000 watts).

Now, 1 watt is equivalent to 1 joule per second,

So our 100 watt bulb is using 100 joules every second.

Which means that it is using 100 * 60 joules every minute which is 6,000 joules every minute.

Which means that it is using 6,000 * 60 joules every hour which is 360,000 joules every hour.

So, in 9 hours, it would use 360,000 * 9 joules which is 3,240,000 joules which is 3,240 kilojoules.