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### Course: High school physics>Unit 12

Lesson 4: DC Ammeters and voltmeters

# Voltmeters and Ammeters

Learn how to measure voltage and current in electrical circuits using voltmeters and ammeters. Discover why voltmeters are connected in parallel and ammeters in series. Understand the importance of resistance in these devices to ensure accurate readings and prevent damage. Created by David SantoPietro.

## Want to join the conversation?

• At , how does the current "know" which path has less resistance without passing through there?
• (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
• 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)?
• 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.
• 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?
• 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
• @ he 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?
• 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
• What is shunt? (it is not a part of the video)
• 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.

http://hobohome.com/news/?p=537

Regards,

APD
• 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?
• 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.
• Hello,
Could someone explain to me what a fuse is and why is it useful to have them?
Thank you!
• 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.
• At , 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.
• An ideal voltmeter has infinite internal resistance, so no current at all goes through it. Real voltmeters have very very high internal resistance.
• Can someone explain how a resistor is measured, when it is hot?
• 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.

Regards,

APD