9. Work & Energy
Power
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>> When you get your SDG&E bill at the end of the month, what does it say on your bill? One, it says how much money you owe. But two, it tells you something else. What does it tell you? [ Inaudible Speaker ] >> Yeah. It tells you kilowatt hours that you've used. It says, oh, and this month you used 100 kilowatt hours. Is that a measure of power? No. In fact, it's a measure of energy that you used that month. OK. But we just said that kilowatts, that's power. Power is of course time. And so there's a general relationship between energy and power. Energy is equal to power times time. OK. If you turn on your lightbulb, it's at 60 watts, that's how much power it is dissipating. If you do that for a long time, you use more energy. If you only do it for a short time, you don't use very much energy at all, OK? So power is in units of watts. And watts we write with a capital W. Again, we've run out of letters, so W is watts but W is also work and W is also width, and it's a big nightmare. But you guys will hopefully figure it out. OK. Time is of course seconds. Energy is of course joules. So a joule is equal to a watt second. So let's ask you a question about different devices that use power, that use electricity in your home. And let's see if we can figure out sort of roughly how much power they use. So what's an item in your home that uses electricity, that uses power? [ Inaudible Speaker ] >> What's it? A lamp, OK. How much does a lamp use in terms of power? [ Inaudible Speaker ] >> Well, you put the bulb in there, right? And right up on top of the bulb it tells you a number, right? 60. 60 watts. So we'll do power here in terms of watts. So that's a lamp, right? 60 watts for a lamp. OK, what else? [ Inaudible Speaker ] >> What's that? >> (student speaking) Toaster. >> Toaster. OK. We'll put a toaster down here. How much does a toaster use? It it less than a lightbulb or more than a lightbulb? >> (student speaking) More than a lightbulb. >> More. How much more? [ Inaudible Speaker ] >> OK. Almost. It's probably closer to, like -- [ Laughter ] -- 1,200, OK? A toaster actually uses a lot. There's something else that you use, which I'm sure you guys have one, which is a hairdryer. I don't know what kind you guys have, but I have the Vidal Sassoon Pro Stylus 1500. How much does a hairdryer use? [ Inaudible Speaker ] >> I just gave it away in the name. >> (student speaking) 1,500. >> 1,500. 1,500 watts. OK. What about your cell phone charger? How much does a cell phone charger use? That's probably on the order of 1 watt. OK. So look at the scale here, right? We've gone from 1 watt up to 1,500 watts, OK, there's a big range there. And you know that a cell phone charger doesn't use anywhere near 60 watts. Why? Because you can put a cellphone charger in your pocket and it won't burn you up, right? You can't put a lamp in your pocket or any of these other things, right? So when you turn on your lamp -- let's ask the following question. Let's say you're going to leave the lamp on for 10 hours. How much does that cost? How much are you going to have to pay SDG&E for that electricity that you use? Well, let's figure it out. The lamp is 60 watts. The time, we just said, was ten hours. Sixty watts is 0.06 kilowatts. And we're going to multiply that by ten hours. And so we get 0.6 kilowatt hours. The reason I put it in these units is because SDG&E bills you for kilowatt hours. These are the units that they send you on your bill. So anybody know how much 1 kilowatt hour costs? What's the price for 1 kilowatt hour, anybody have an idea? Is it $100? Is it a fraction of a penny? Yeah, it's on the order of a dime or 20 cents. So the cost is about 20 cents per kilowatt hour. So how much did this thing cost? That was about a dime. It cost you a dime to leave your lightbulb on for ten hours. So at the end of the month, you get your bill and it's like, why is this so much? Because it's stuff like heaters and coolers. A heater is just like a hairdryer, it uses a lot of electricity. An air conditioner is just like a hair dryer in reverse. It uses a lot of electricity, OK? Those are the things that are really dominating your electric bill, it's not the little bit of lightbulb or the cell phone charger here and there. So one way to do it is to just stop turning on your heat and your air conditioning. That'll take care of it. Or you can also go solar on your house. That's what we did on our house, put solar panels and now the sun makes electricity for you. Kind of cool. All right. Any questions before we get out of here? You guys feeling all right? All right, good. Let's wrap it up. And if you have any major issues, come see me in office hours. Cheers.
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