>> So now that we've got the concept of

voltage, current and resistance under our belts, let's just talk about Ohm's Law.

So Ohm's Law is the relationship that you see right there.

V = I*R, voltage equals current times resistance.

This is a useful relationship to keep in mind.

It's simple, but we need it for lots of little things.

So it expresses the relationship between voltage, current,

resistance, and it tells you a few things about the behavior of circuit.

So for instance, say you got a circuit and

the voltage is fixed, it's five volts, running at five volts.

If you increase the resistance then current will go down, right?

That makes sense because the voltage is fixed, right.

Our Arduino is running at five volts, it's driving five volts under its output pin.

So anything you connect to it, if the resistance goes up,

I has to go down to keep the relationship the same, right?

So that's the most useful thing about this law,

is that it gives you this intuition about current flow, right?

Current flow, and voltage, and resistance, and how they're all related.

And we need to know that in certain cases.

So it's used, so one way to do it,

to use this relationship is to compute one variable when you're given the other two.

So say you know the voltage, you know the current, you can solve for resistance.

Or you can do it the other way.

Say I know the current and I know the voltage, or I know the current and

I know the resistance, I can solve for voltage, and so on.

So given two, you can solve for the other one.

So when would you need to do this?

So, one use is what resistor do I need to limit current flow?

Now this is gonna happen a lot, where your Arduino is connected to a component and

this component, it only accepts a certain amount of current.

If you give it too much current, it will blow up.

Something bad will happen.

So you know that when you connect it to this component, you have to

add some resistor just to reduce the current flow to the appropriate level.

So, if you want to figure out what resistance do I need to make sure that my

current is low enough, you plug in the voltage say, Arduino is five volts.

The current limit that you want, you plug that in.

And then, you can solve for the resistance and say okay,

this is the type of resistor that I need.

So, that's one way to use this, that you will need to use this.

Another one is, what voltage can I expect for a given resistance?

And this is more on the, this is commonly something you need to figure

out on the input side, so I will get to this in a later module.

But, the idea is that say that you are doing this connector to some sensor of

some kind, okay.

And this sensor, so

our Arduino, it's gonna be reading the voltage that this senor is outputting.

But what voltage do I expect?

So, say I have it connected to a light sensor, right?

When the brightness is a certain level, I'll get a certain resistance in my

sensor, so what voltage would I expect to see in my Arduino?

So I can use this type of equation to figure that out.

Well I know what the current has to be and

I know the voltage will be this, the resistance will be such and

such at this light level, so the voltage has to be.

And then you could use that inside your program and say, if the voltage I measure

is greater than this threshold, then do this, else do that, and so on.

So V = I * R is sort of a very common law and you use it a lot for

lots of little things.

Thank you.

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