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Anderson Video - Current in Everyday Devices

Professor Anderson
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Okay let's talk about current for a second this idea that different devices are gonna use different amounts of current. And let's see if we can come up with some typical numbers here for different items- how much current they're going to use. So let's say I have an LED. Right, if I turn on just a little red LED green LED, how much current does it draw? Any ideas? And we're talking about in the scale of amps right, that's our SI unit. Well it turns out an LED draws somewhere around 10 milliamps. Okay, there's kind of a wide scale but this is a typical number there for an LED: 10 milliamps. What about your smart phone? When you're walking around with your smart phone how much power does it draw? Let's say you're not even talking on the phone right now it's just doing its normal thing talking to the cellular towers, doing some internal processing. A smartphone is about a hundred milliamps. Now that should make sense to you because we just talked about a double-a battery, right a double-a battery running a hundred milliamps will last about 10 hours. Your smartphone doing its normal thing will last the full day, right? 10 hours, or 2 batteries in there, 20 hours. Okay? What about a light bulb? Let's say you go to Home Depot in the past because you can't buy a hundred watt light bulb anymore but let's say you go to Home Depot, right and you buy a hundred watt light bulb, the incandescent light bulb not the new compact fluorescents, not the new LED you buy a normal hundred watt light bulb. How much current does that draw? It's about one amp. Okay, so we've gone up by a order of ten each time. 10 milliamps, a hundred milliamps, a thousand milliamps which is one amp. What about your hairdryer? When you fire up your hairdryer how much current does it draw? Well we talked about the power of the hairdryer and when you turn on your hairdryer you can actually see the lights dim in your house a little bit so it seems like it's got to be considerably more than the light bulb and in fact it is, it's on the order of 10 amps. Look, we've already gone four orders of magnitude here up to a hair dryer. What about the total current that's available coming into your house? Right, there's a big power line on the telephone pole there's a big metal wire that comes down to your house and there is some amount of current that is available to your house and that's it . If you draw more than that the electricity company is gonna have a problem with you. So how much is that total house current available? Well it's basically another factor of 10. A hundred amps. Okay? When you go out in the morning and you start your car there's a 12 volt battery in there and there's cables that are connected to the starter motor in your car and when you turn the ignition, what happens is it powers up the starter motor that starter motor pushes against a gear that attaches to your engine and that electric motor actually spins your whole engine for a little bit until the engine starts firing and the gas takes over, okay? And then the thing retracts and then the electric motor just sits there idle until you need to start the car again. But during that start the battery has to provide a lot of current. And you know this right? Because if you're sitting there holding the key and your car's not starting you can only do that for a few minutes before your battery just drains. Doesn't work anymore, right? So you're hoping, you're crossing your fingers, "come on, start on this one, I know this is gonna be it" then you try and decide should I push real hard on the accelerator should I not? You know. That's whole science of flooding the engine or not. How much current is available during the start of your car? Is it really more than your whole house current? It is. It's like 300 amps. Okay, they call this the cold cranking power of your battery and it can be hundreds of amps. How do you know this? Well, when you look at your car battery those cables that are attaching from the terminals of the battery down to the starter motor? Those are big heavy thick cables. They need to be carrying a lot of current a lot of electrons per second to get that car to start. Okay, so that can be like 300 amps. All right let's get to something really big. Lightning strike. A lightning strike is when there's a big voltage difference between the cloud and the ground so much so that the electric field strips the electrons off of the molecules that are floating around in the air all the nitrogen, the oxygen, the carbon dioxide, carbon monoxide and it creates this huge amount of current and that's what we see as a lightning strike and that can be a hundred thousand amps up to a million amps, okay. 10 to the 5 to 10 to the 6 amps. Alright so this is the sort of scale of things that you're talking about. We've got you know eight orders of magnitude here. It's interesting thinking about stuff like LEDs right, how little current they draw compared to things like your hairdryer. And this is why there's this big push for LED lighting. Okay, because LED lights just draw a lot less current and still produce the same amount of light. A 100 watt light bulb, an incandescent light bulb is not very good at converting electric power into optical power. Whereas something like an LED is much more efficient at converting electrical power into optical power. And so now you should all be buying LED bulbs. When you go to Home Depot, right buy the LED bulb and it'll say, "oh it's a 75 watt equivalent and it only uses 7 watts." Something like that, okay. It's like 10 times as efficient as an incandescent light bulb. All right. When you think about types of current you also need to think about is it DC or is it AC?