 Now, let's talk about the relationship between power, energy, and time. And this can be used in a lot of different contexts. The general definition of power is that it's the energy rate, or the energy per time. And in some situations, that's the energy being used. In some situations, it's the energy being generated, or it might even be the rate at which energy has changed from one format into another format. For my students this semester, I'm using this for the electrical power used. So let's take an example here. If you know that something is using up 400 joules of energy every 10 seconds, that would be a rate of 40 watts. Maybe it's a light bulb. A 40 watt light bulb uses 40 joules of energy every 10 seconds. Now I could rearrange this equation to solve for the amount of energy. So if I take that same 40 watt light bulb, but now I run it for 15 seconds, it's going to use up 600 joules of energy. I could also rearrange this equation in order to calculate the amount of time. And this is an example if I've got the amount of energy being used, and I've got the rate it's being used. So let's say I only have 600 joules of energy and I'm using it up at a rate of 20 watts. It'll take me 30 seconds to use up that energy. Coming back to my equation here for the energy. I want to do a couple other examples and show you some things about some units you may have heard of. So let's say I have that 40 watt light bulb and I want to use it for a whole hour. Well I could work out the equation and notice that one hour is 60 minutes, but each minute has 60 seconds. So 60 times 60 gives me the number of seconds in one hour. And when I work out that math I'd get 144,000 joules. Now let's say instead of it being a 40 watt, it's something that uses 40 kilowatts. And I use that for one hour. Well again I could multiply everything out, and I could use that a kilowatt is times 10 to the third watts and the hour again is 60 times 60 seconds. And I'd find out I've got 144 million joules. But there's another unit we can use. Which is if I just leave it in kilowatts in hours, I find that using a 40 kilowatt device for one hour uses 40 kilowatt hours of power. So a kilowatt hour is actually in unit of energy. So these are some examples of the relationship between tower, energy, and time.