Anderson Video - DC vs. AC

Professor Anderson
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Okay, let's talk about DC current. DC means direct current versus AC current which is alternating current. And this goes back to the early days of electricity. People like Thomas Edison and Ben Franklin and Tesla and there was this big debate about, "should we just power the United States by DC current or AC current?" What won out, of course, was AC, and when you plug into the wall it is AC current, but batteries are all direct current. So direct current just means the following. The current I doesn't change as a function of time. It's that. Okay, it's all heading in one direction. AC current of course means that it alternates. So as a function of time AC current looks like this. It's sort of sinusoidal. It's going positive for part of the time and then it's going negative for part of the time. Positive and then negative. Okay, so when you fire up your double-a battery you get 1.5 volts and that's it. But when you plug something into the wall you get 120 volts AC. And that AC means that it's going positive for part of the time and then it's going negative for part of the time. Positive and negative. And so the current is actually going this way and then that way, this way and that way. It's pushing and pulling all the time. All right. How do we think about current in terms of other things that we like to understand, like gravity? Well, for DC currents at least there is a nice waterfall analogy and it's this. If I take a battery and remember the long side is positive, the short side is negative. And I hook it up to a resistor. Then there is current that's going to run around that circuit. Current I, like that. We know that it's electrons going the other way but forget about that, whenever you draw I it's always referring to positive current, which way is the positive sign. This is just like a waterfall, okay and the analogy is the following. Here is our waterfall. And here comes the river and it goes over the waterfall and then it goes down across the rocks. V is the height of the waterfall. Okay, that's the battery. R is the rocks. And what rocks do is they try to slow down the current, they try to prevent the current. The water is, of course, the current. All right? Height of the waterfall is the battery the amount of water flowing is the current the rocks in the river those are the resistance. Put a bunch more rocks in there it's going to agitate the water and slow it down a whole bunch. No rocks, the water is gonna flow much more smoothly. Okay? So when you think about a battery one thing that you might consider is, well how do I recharge the battery? Right, the battery in your smartphone drains, okay. I need to refill it and that is the pumping station that's gonna pump the water back up to the top of the waterfall.
Okay, let's talk about DC current. DC means direct current versus AC current which is alternating current. And this goes back to the early days of electricity. People like Thomas Edison and Ben Franklin and Tesla and there was this big debate about, "should we just power the United States by DC current or AC current?" What won out, of course, was AC, and when you plug into the wall it is AC current, but batteries are all direct current. So direct current just means the following. The current I doesn't change as a function of time. It's that. Okay, it's all heading in one direction. AC current of course means that it alternates. So as a function of time AC current looks like this. It's sort of sinusoidal. It's going positive for part of the time and then it's going negative for part of the time. Positive and then negative. Okay, so when you fire up your double-a battery you get 1.5 volts and that's it. But when you plug something into the wall you get 120 volts AC. And that AC means that it's going positive for part of the time and then it's going negative for part of the time. Positive and negative. And so the current is actually going this way and then that way, this way and that way. It's pushing and pulling all the time. All right. How do we think about current in terms of other things that we like to understand, like gravity? Well, for DC currents at least there is a nice waterfall analogy and it's this. If I take a battery and remember the long side is positive, the short side is negative. And I hook it up to a resistor. Then there is current that's going to run around that circuit. Current I, like that. We know that it's electrons going the other way but forget about that, whenever you draw I it's always referring to positive current, which way is the positive sign. This is just like a waterfall, okay and the analogy is the following. Here is our waterfall. And here comes the river and it goes over the waterfall and then it goes down across the rocks. V is the height of the waterfall. Okay, that's the battery. R is the rocks. And what rocks do is they try to slow down the current, they try to prevent the current. The water is, of course, the current. All right? Height of the waterfall is the battery the amount of water flowing is the current the rocks in the river those are the resistance. Put a bunch more rocks in there it's going to agitate the water and slow it down a whole bunch. No rocks, the water is gonna flow much more smoothly. Okay? So when you think about a battery one thing that you might consider is, well how do I recharge the battery? Right, the battery in your smartphone drains, okay. I need to refill it and that is the pumping station that's gonna pump the water back up to the top of the waterfall.