OpenStax College Physics, Chapter 20, Problem 51 (Problems & Exercises)

This is College Physics Answers with Shaun Dychko. The power of our toaster is 1200 watts and the times it takes to toast a piece of bread is one minute which we will convert into seconds by multiplying by 60 seconds per minute. The energy consumed will be the power multiplied by time. So that's 1,200 watts times 60 seconds, which is 72,000 joules. Now the cost for using up 72,000 joules is going to be the energy multiplied by the rate that the power utility is charging, dollars per joule or usually cents per kilowatt hour is how they actually -- those are the units they typically use for the cost of electricity. So we have 72,000 joules but we know we're going to be multiplying by nine cents per kilowatt hour and so we're going to convert this into kilowatt hours. So a joule is a watt second and then I find it convenient to make that conversion because then it becomes clear that to turn this watts into kilowatts we multiply by one kilowatt for every 1,000 watts. To convert the seconds into hours we multiply by one hour for every 3,600 seconds. This leaves us with kilowatt hours units here. Then we multiply by nine cents per kilowatt hour and then those cancel, leaving us with cents. So all this works out to 72,000 divided by 1,000, divided by 3,600 times nine, giving us 0.18 cents.