High school physics
Voltmeters and Ammeters
Learn about the instruments we use to measure voltage and current. Created by David SantoPietro.
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- At6:50, how does the current "know" which path has less resistance without passing through there?(42 votes)
- (I'm a student just like you so I might not be right) From what Sal said in an earlier video, the flow of electrons in a circuit is probabilistic. And current is the measure of how many electrons (charge) flows through a point per unit time (seconds). a path with high resistance would make the current flow through it much slower than a path with no resistance. So the current is lower for a high resistor because it lets fewer charge through it per second, not because the electrons intuitively "know" which path has least resistance. Electrons will go through every path, but lower resistance means some paths will let electrons through at a higher rate and therefore have a higher current(7 votes)
- Are the measurements of the Ammeter and the Voltmeter given considering the inner resistance in it or is it like a ideal voltmeter/ammeter (no resistance)?(19 votes)
- I don't know of any ammeters or voltmeters that would incorporate their inner resistance in their measurements. Since their relative resistance is so small (or so large, in the case of the voltmeter), their resistance can be considered negligible for all but the most delicate experiments.(15 votes)
- Hey, sorry I'm really new to physics, I'm just learning casually. Quick question though: why would the amps be the same on either side of R3? I thought resistors reduced the current flow meaning that the amps on the input side would be greater than the output?1:52(6 votes)
- Ah, but think about what would happen if this was true. If there are more electrons flowing into the resistor than flowing out of it (flowing electrons being what current is), then there would have to be a growing pile of electrons forming in the resistor. Since electrons are negatively charged and repel each other, this pile of electrons would quickly form a voltage greater than the one driving the current and stop it, or if the driving current was strong enough, cause the resistor to shoot out lightning. We don't see this, so we conclude that the current is the same before and after the resistor. This is one half of Kirchhoff's rules, which you can read about here for example: http://www2.ignatius.edu/faculty/decarlo/kirchhoff.htm(6 votes)
- What is shunt? (it is not a part of the video)(3 votes)
- Hello Anusha,
Before I answer your question consider the attributes of an ideal voltmeter and an ideal ammeter:
The voltmeter is connected in parallel with the circuit to be measured. We do not want the voltmeter to load the circuit. Consequently an ideal voltmeter will have infinite resistance.
An ammeter is connected in series with the circuit to be measured. The ideal ammeter will have zero resistance so as not to disturb the circuit.
We will find the shunt as part of the ammeter circuit. The "shunt" is nothing more than a resistor with a very low resistance. This shunt is connected in parallel with the ammeter - hence the name "to shunt". Nearly all of the current will flow thought the shunt. The little that remains will flow through the ammeter.
Some useful links:
- @6:18he says "a small amount of current will flow through the voltmeter BECAUSE it has to take a reading". Does that mean the voltmeter is powered by the small current that is running through it? I thought it used a battery?(4 votes)
- The small amount of current flow through the voltmeter is needed for voltage measurement, isn't powering the device. It is the voltmeter's working principle.
So, the analog voltmeter doesn't require an additional power supply, because the voltage is reflected by moving a pointer across a scale, which is moving due the magnetic field changes, but digital voltmeter requires battery for powering its electronic parts — display for example. Good article: https://en.wikipedia.org/wiki/Voltmeter(4 votes)
- What would happen if we put the Voltmeter in series before the R2 resistor? Would it stop the flow of current in the whole circuit due to its high resistance?(2 votes)
- Ideal voltmeter has infinite resistance so no current will flow. This is why voltmeters are placed in parallel to the circuit, not in series. Ammeter is placed in series however and has low resistance.(5 votes)
Could someone explain to me what a fuse is and why is it useful to have them?
Thank you!(2 votes)
- Since electricity creates heat, too much current could melt critical components. Fuses are cheap and easy to replace. The purposes of fuses is transfer current normally when there is not too much current. If there is excess current, the fuse melts and breaks the circuit, keeping the excess current from traveling to and causing damage to important components.(4 votes)
- Can someone explain how a resistor is measured, when it is hot?(2 votes)
- Hi Samuel,
The hottest resistor I can think of is the filament of a light bulb. Here you would measure the resistor under actual operating conditions. If you measured it cold you would have a completely different reading...
All we need to do is measure the voltage across and the current flowing through the light bulb. Then use Ohm's law to calculate the resistance.
- At6:47, David says the current would completely skip the voltmeter and the resistor, but if you think about it, he's connecting it in parallel to the other 2 resistors at the bottom, so some current should flow through the circuit, but the voltmeter would end up measuring the difference in electrical potential across R_1 and R_2.(2 votes)
- An ideal voltmeter has infinite internal resistance, so no current at all goes through it. Real voltmeters have very very high internal resistance.(2 votes)
- Why wouldn’t the ammeter fry if you placed it in series before R3 like he did in the video? Couldn’t all the current still potentially run directly through it without hitting a resistor first?(2 votes)
- Resistors affect the current for the entire series circuit. The current doesn't go fast before the resistor and then slow down when it gets to the resistor. The charges move through the circuit together, like the cars of a train move together(1 vote)
- [Voiceover] Let's say you have a circuit here and you had a battery with a voltage v and there were resistors one, resistor two, and resistor three up here, and there was current flowing through here. What if you wanted to experimentally measure the voltage across some of these elements? You'd have to use a voltmeter. Voltmeter looks like this. So a circle with a v in it is the symbol we use for a voltmeter. How do you use it? You take that voltmeter, you bring it over to here. I can't plug it in the circuit like that. What I do is I take the leads of the voltmeter and I just connect them to either side of the circuit element that I want to determine the voltage across. So if I do this and I connect those leads right here, this voltmeter will tell me the voltage across R three. Or take the voltmeter, put it over here, and if I connect the leads across R one in parallel, notice I'm hooking up the voltmeter in parallel. Voltmeters you always hook up in parallel. This now will tell me the voltage across R one and if I wanted to make sure my battery was functioning correctly, I could take my voltmeter and I can hook up the leads across the positive and negative terminals of the battery and see if the voltage across the battery is what I think it is. That's how you use a voltmeter: always hooked up in parallel. But if I wanted to measure the current, I don't use a voltmeter, I use an ammeter. And for an ammeter you do not hook up an ammeter in parallel with the element you're trying to measure. You will probably blow out the ammeter. I've done it a few times. It's embarrassing. Don't hook up the ammeter in parallel, tell you why in a minute. But what you have to do is hook it up in series. So if I wanted to know the current going through R three, I could just stick the ammeter right in here. One lead would plug into one side of the ammeter, the other lead would plug into the other side. This current would have to flow straight through the ammeter and this is telling me how much current goes through R three. It doesn't matter what side I put it on, the current going into R three will equal the current going out. So you can put it over here too, but it's gotta be hooked up in series. So you have to disconnect, it's kind of a pain to hook up an ammeter sometimes. You have to disconnect something here, then connect that connection to the one side of the ammeter, connect to the other side of the ammeter. For a voltmeter, you didn't have to do that. For a voltmeter, just kept it out here and just touch those leads wherever you needed to touch them. But for an ammeter, you have to break the circuit to let this ammeter in. But I can move it wherever I want. I could put it down here, that tells me the current in this strip. Again, ammeters always hooked up in series with the element that you're trying to measure. So this ammeter position will let me measure the current that's flowing through the battery. But why is the voltmeter always hooked up in parallel and the ammeter always hooked up in series? We want the ammeter to be hooked up in series because we want to measure the current through a line in the circuit. We want to measure the current flowing through this resistor. So if we want to measure the current flowing through something, we need to make sure that the current flows through our ammeter and that's how we get our reading. Because of this, people design ammeters with very little resistance. An ammeter has very little resistance. And the reason is, if you took this ammeter and it had a big resistance and you stuck it in here, you'd be changing how much current flowed through this part of the circuit. We don't want to do that. Whenever we measure something, we don't want to disturb it. So when I stick my ammeter in here, I don't want to disturb how much current was going through here. I wanted to know how much current flows without my ammeter being in there. So when I put my ammeter in there, it better have very little affect on this circuit. That's why we make this ammeter have a very small resistance. And that's also why you can't hook this ammeter up in parallel, cause if you did, look at what would happen. This is why it's bad. If I took this ammeter and I hooked it up right here, and I hooked the other side up right here, look what the current's gonna do. I've got current flowing through here, current comes this way, goes this way, reaches this fork in the road and it's got a choice. It can go to the left or flow up through here and go through R three or flow through my ammeter, but my ammeter has very little resistance. I mean small, maybe on the order of a milliohm. So all of this current that's flowing through here, all this current's gonna choose to go through my ammeter. It's gonna just skip all those resistors, forget that. it just goes through the ammeter. If you've got a normal-sized voltage, maybe nine volts, three volts, hooked up to a milliohm, you're gonna burn out your ammeter. There's usually a fuse in here because they know people are gonna hook it up wrong. I've done that, and you burn out a fuse, you gotta go replace the fuse and it's a pain. So don't hook up your ammeter in parallel. What about voltmeters? Why do we hook those up in parallel? Well, a voltmeter is hooked up in parallel because we want to know the voltage across a circuit element, so on either side. Voltage, remember, is defined to be the difference between electric potential at two points in space. It makes no sense to ask what's the voltage through a certain point in a circuit. You can ask what current flows through that point in the circuit. But asking what the voltage is at a particular point in a circuit makes no sense. The only thing that would make sense is asking what's the voltage across two points in a circuit. So I can ask what's the voltage between this point and that point, that makes sense, or I can ask what's the voltage between this point and that point, that makes sense. But asking what's the voltage at a point or through a point, makes no sense. That's what current is. Current flows through a point, voltage is across two points. The difference in electric potential between two points. That's why we hook up voltmeters in parallel and because we hook up voltmeters in parallel, voltmeters have to have a huge resistance. Sometimes on the order of hundreds of thousands of ohms or even millions of ohms. So this can be big, big number of ohms. And the reason is, think about it, again our key idea is that we don't want to disturb the thing we're measuring. I'm measuring the voltage across this resistor. If I were to hook up a voltmeter with very little resistance, I just told you what would happen. This current that's flowing out of the battery, would all try to go through this voltmeter. Not only would it try to mess up the voltmeter, but that's current that's not flowing through R three anymore, and so I wouldn't get a correct reading for the voltage through R three. So we want to make sure our voltmeter has a big resistance so that yes, technically a very, very small amount of current, maybe a milliamp, will flow through this voltmeter, because it's gotta take a reading. But, we want as small amount as possible, because we want to keep this current flowing through R three the same as it was before we were measuring it, because I know v equals IR. And if I can measure this voltage across here, I want to make sure the current's the same, or I won't be getting an accurate measurement for the voltage. You could ask what would happen if we did hook the voltmeter in series instead of parallel. Voltmeters have a huge resistance, so if I stuck that here, the voltmeter has a huge resistance, you wouldn't break it, it's just that, think about what the current's gonna do. Current comes out of this battery, it's got a choice, it can go up here through R three and the voltmeter or through R one and R two. I said the voltmeter has hundreds of thousands, even millions of ohms, so this current's just all gonna go this way. Forget that. It's gonna skip this entirely. If you hook up a voltmeter in series instead of in parallel, you just kill off any current through this portion of the circuit that the voltmeter was hooked up in. You probably won't break it, so it's not as delicate as the ammeter, but you still mess up your measurement because it wasn't designed to be used that way. So remember voltmeters are hooked up in paralled to the circuit element that you want to determine the voltage across. But ammeters are connected in series to the circuit element that you want to measure. And if you're sitting there thinking, "Pfft, I'm never gonna hook up my ammeter in parallel. "How dumb do you think I am?" Well, gotta be careful, cause most multimeters are both voltmeters and ammeters, depending on where you set the dial. So if you're sitting there all day measuring current with your ammeter setting. Everything's going well. And then you go to measure a voltage, but you forget to switch the dial to volt instead of amps, you'll be hooking up an ammeter in parallel erroneously. That's what happened to me. Don't let it happen to you. Check the dial on your multimeter. Make sure it's on the function that you want it to be so you don't burn out a fuse.