Physics Homework Help

[phystudent]

I need help on what formulas to use and how to start this physics problem.

We estimate that there are 260 million plug-in electric clocks in the United States, approximately one clock for each person. The clocks convert energy at the average rate of 3.6W.
To supply this energy, how many metric tons of coal are burned per hour in coal fired electrci-generating plants that are, on average, 13.4% efficient? The heat of combustion for coal is 33MJ/kg. Answer in units of metric tons.

Another question, how is Watts a rate?

[jamesrc]

Watts is a rate because it is a measure of power and power is the rate of doing work (or using energy). 1 Watt = 1 Joule/second

For this problem:

The total power consumed is (260,000,000)3.6W = 936 MW

So:

(13.4%)*(33MJ/kg)*(mass of coal)/(1hour) = 936 MW

The amount of useful energy is given by the efficiency multiplied by the energy content of the coal multiplied by the amount of coal.

Take care of your units properly and solve for the mass of coal burned in an hour.