Voltmeter must cause disconnnection in order to measure the volt?

[Femme_physics]

We learned yesterday that a voltmeter's resistance approaches infinity. And that infinite resistance means total disconnect. I'm trying to understand why is that? Why must a voltmeter's resistance approach infinity? Doesn't it mean that the voltage will be 0?


BTW - I hope I'm translating the word "disconnection" correctly, I'm studying it with a Hebrew text.

 

[Metaleer]

No, the voltage will be whatever it needs to be so that Kirchhoff's laws hold.

If it weren't 0, some of the current will branch into the voltmeter itself, and the voltage drop you're measuring is now somewhat different than what you're supposed to be measuring, as the current going through the original element isn't the same as it was before.

Think of it this way, we have V = IR. If R goes to infinity, then V also goes to infinity unless I goes to 0, which is what we require so that no current goes into the voltmeter.

Hope this helps. :)

 

[Femme_physics]

If it weren't 0, some of the current will branch into the voltmeter itself

Well, that's what it's supposed to do for the voltmeter to give a reading, no?

and the voltage drop you're measuring is now somewhat different than what you're supposed to be measuring

That's inevitable, no? You're putting the voltmeter in the circuits so it naturally alters the result a bit.

Think of it this way, we have V = IR. If R goes to infinity, then V also goes to infinity unless I goes to 0, which is what we require so that no current goes into the voltmeter.

If I goes to 0, then everything is zeo!

 

[I like Serena]

We learned yesterday that a voltmeter's resistance approaches infinity. And that infinite resistance means total disconnect. I'm trying to understand why is that? Why must a voltmeter's resistance approach infinity? Doesn't it mean that the voltage will be 0?


BTW - I hope I'm translating the word "disconnection" correctly, I'm studying it with a Hebrew text.

The resistance between 2 objects that are not connected by a copper wire is close to infinity, which is what is means that they are "disconnected".

The volt meter has a very large resistance, so it disturbs the circuit as little as possible.

You are quite right if you say that the voltage the volt meter actually measures is close to 0. Luckily the volt meter "knows" this, and multiplies the result to the proper reading.

Note that if the voltage in the circuit is twice as high, the voltage on the volt meter - although close to 0 - will be twice as high as well.

 

[Metaleer]

Well, that's what it's supposed to do for the voltmeter to give a reading, no?

That's inevitable, no? You're putting the voltmeter in the circuits so it naturally alters the result a bit.

This is the real case, of course, there is a very slight distortion in the original circuit, but this is only slight.

If I goes to 0, then everything is zeo!

No, the current in that branch can go 0, but the rest of the circuit remains functioning as usual, according to Kirchhoff's laws.

 

[Femme_physics]

Ah, I think I get it. I don't want to dive too deeply as to how it functions as we're in elementary electronics, but I think I get the gist of its principles. Thanks.

 

[jsgruszynski]

The summary answer:

The higher the impedance the less interaction and corruption of the circuit voltages being measured. In the limit, having infinitely large input impedance would have infinitely small interaction with the test circuit. It's a KVL/KCL thing.

Only an ideal voltmeter has infinite input impedance. But a real voltmeter does not; it is merely designed to have a very high input impedance - generally as high as possible economically. A cheap voltmeter (multimeter) may only have 1 Mohm input impedance but for most purposes that's quite good. Laboratory grade voltmeters might have 1 Gohm input impedance. I grew up with analog meters that had 1K-10K input impedances.

 

[maimonides]

Nitpick:
The input impedance of analog voltmeters is range dependent. It was often stated in kohms per Volt of range. So with 10k/V you´d get 10M in the 1000V range. (10k on a 1000V range would have been a problem).
(This does not apply to Voltmeters with amplifiers; there were Millivoltmeters with internal amplifiers.)