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We're still talking about Ohm's Law, which is commonly written as V equals IR,

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and we're going to work through three simple example problems with Ohm's Law.

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Now, even though it's commonly written this way, remember that there are other ways we can write Ohm's Law.

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We could take the equation V equals IR, and we can divide both sides by R, as we did before,

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and when we do that, the R's cancel out on the right, and that leaves us with an I on the right equal to V over R.

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So instead of being written like this, V equals IR, it could also be written like this, I equals V over R.

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And there's another thing we can do.

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We can take the equation V equals IR, and we can divide both sides by I.

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And when we do that, the I's on that side cancel out, and we're left with R equals V over I.

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So Ohm's Law could also be written like that, R equals V over I.

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these three equations are all mathematically equivalent they're all really the same equation

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just algebraically arranged differently and we'll do an example problem with with the equation in

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each of those three forms here's the first one a 12 volt battery is connected to a light bulb

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with a resistance of three ohms how much current flows through the light bulb well the 12 volts

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when it says there. That's the voltage. So I'm going to write V equals 12 volts. And it's connected

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to a light bulb with the resistance of 3 ohms. So that's the resistance. So I'm going to write

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R equals 3 ohms. And you see the symbol omega is the symbol for the resistance, for the unit of

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resistance, the ohms. And we're trying to find the current, how much current. So I is what we don't

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know, we can use the equation in this form, I equals V over R. And we just put in the

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numbers that we know for V. V is 12 volts, R is 3 ohms, and when we divide we get 4,

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and the current is in amps, and we use the symbol A for amps or amperes. In this example

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we're told that a large electric heater is designed to draw 40 amps of current, and 40

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is a lot of current. Just to give you an idea, I hooked up a meter to my stove.

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I have a regular, it's a regular electric stove with four burners, and I put them

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all on high, and with all burners on high it was drawing 28 amps of current. So, and

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you typically don't run the stove with all four burners on high, but that gives

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you an idea of how much 28 amps is. Your electric stove, turn everything on high,

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that's it. So this is 40 amps of current, that's a large heater. Alright, let's look

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the problem. If the resistance of the heating element is 5.5 ohms, so that's the resistance

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R, 5.5 ohms, and we're also given here the 40 amps, that's the current. So I'll write

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I for current is 40 amps. And we're asked how much voltage, the voltage is what we're

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looking for. So we just use the equation in this form, V equals IR. So it's just a simple

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multiplication. 40 amps times 5.5 ohms, and it comes out to 220 volts. That's our answer.

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Now that's a lot of voltage. Now you might remember I said that a regular electrical

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household outlet has 120 volts, or sometimes it's 110. So one of these outlets on your

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wall that looks like this is 120 volts. So you obviously couldn't plug this heater into

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that outlet you need it to match this heater is designed to use 220 volts but if you go in your

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house and you look say behind your dryer your dryer uses a lot of electricity there's a heating

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element in there and a big fan and it takes a lot of energy to heat those clothes up enough to dry

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them and there's a plug probably on the wall behind the dryer that looks something like this

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and it's got three holes right there and the plug on the back of your dryer that you plug

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into the to the socket is shaped to fit there and it won't plug into a regular outlet this is a 220

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volt outlet and you most people have one of those in their home and this is 120 volt outlet here

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so this you could have a heater that runs on 220 volts but you would need to plug it into a 220

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volt outlet like this most of the stuff in your home though goes on the regular 120 or 110

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and one more when a certain light bulb is connected to a 1.5 volt battery okay so that's

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our voltage v is 1.5 volts 0.25 amps of current flow so that's the current and i is our symbol

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for current and it's 0.25 amps 0.25 amps of current flow through the bulb what is the resistance the

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resistance is what i don't know so i'll use the equation in this form r equals v over i

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and I put in 1.5 volts and 0.25 amps

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and I do the division

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and it comes out to 6

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and when you divide volts by amps

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you get ohms

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so this is 6 ohms

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that's the resistance in this particular case