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Anderson Video - Measuring Current and Voltage

Professor Anderson
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Let's talk about how you might measure the stuff. Okay, you have done this in your lab, you've measured voltages, you've probably even measured current, and what you use is various meters. It's usually one meter that has a bunch of different settings on it, but let's say we go back to our basic circuit for a second. Battery and resistor. And I want to measure how much current flows. The way I measure current is I insert something called an ammeter. It's called an ammeter because it measures amps. The ammeter has to go in line with that wire. So I'm going to take a device and I'm going to stick it right there and I'm gonna label it I for current. That thing is my ammeter. It has to be in line with the wire, meaning if you had a solid wire you would have to break it, connect one end to the ammeter, connect the other end to the other side of the ammeter. It's gotta measure how much current is actually going through it. Okay? If you have a circuit like this, and now you take your ammeter and you simply connect it with wires like that this will measure zero current. It's got to be in this circuit wire itself. You can't just attach it in parallel somewhere else, it has to be in series. Okay. An ideal ammeter, in fact, has zero resistance, because you don't want the ammeter to affect the circuit. if it did have resistance, then of course it would change how much current is going to flow through that circuit without the ammeter. Usually you want an idea of how much current is going to flow in that circuit normally. And if you put in another resistor right there, that's going to change how much current flows. If the ammeter has zero resistance, then it won't change how much current flows. Now, ideal ammeters, of course, don't really exist, so there's always some small amount of resistance that you're going to add to your circuit when you put the ammeter in. And so just by measuring it, you're going to change that current. And this is sort of an important consideration, whenever you're doing physics, always remember the act of measuring it is going to change the experiment a little bit. And there's basically no way around that. How do we measure voltage, right? If we were measuring current, we'd stick it in series, but now we want to measure voltage which is something different, of course, than current. And now let's say we want to measure voltage. How do we measure voltage? Well, we use a voltmeter. Took them a long time to come up with that name- what should we call this meter that measures volts? How about a voltmeter? Where do you put the voltmeter? You put it across the resistor. You are interested in measuring voltage across this resistor, and so you have to take wires and connect it to either side of that resistor. If current I is flowing then we know that there is a voltage drop across the resistor and you can measure that with your voltmeter. Now, we want all that current to go through R. We want hardly any current to go through this arm over here, through the voltmeter. So an ideal voltmeter has a resistance equal to infinity. So it's the exact opposite of the ammeter. Ammeter has zero resistance, voltmeter has infinite resistance. Because you don't want any current going through here. If you have an infinite amount of resistance there, no current can flow in that arm of the circuit, it won't change the measurement. Now you guys have played with these things, and you in fact probably have a device that has an ammeter and a voltmeter, and can also- what do they call it, they called an ohm meter because it measures resistance. Anybody know what that device is called, the ones that you guys have been using in lab? It's called a multimeter because it's a meter that measures multiple things, right? It can measure current, it can measure volts, it can measure resistance. You just have to change the knobs, on it change the settings and then change the connections on the front panel. So this is what you guys have been playing with in lab: multimeters. and typically whenever you go to Home Depot or someplace like that, you say 'I need a voltmeter, ' they're gonna sell you a multimeter it's got everything built into it, all in one