Ammeter and Voltmeter in Series

[Devs]

Homework Statement

Hi! A battery of emf 12 V and negligible internal resistance is connected to a resistor of constant resistance 6 Ω, an ideal ammeter and an ideal voltmeter. The voltmeter and ammeter are in series with the cell and the resistor. What is the reading on each?

Homework Equations

The Attempt at a Solution

I realize that an ammeter is always in series but a voltmeter is always in parallel.

 

[CWatters]

I realize that an ammeter is always in series but a voltmeter is always in parallel.

Correct. A volt meter is normally connected in parallel... but not in this question. What effect would that have?

 

[Devs]

Correct. A volt meter is normally connected in parallel... but not in this question. What effect would that have?

I thought that the reading on the voltmeter would be 0 and the ammeter will have a reading of 2A. But that isn't correct

 

[CWatters]

No that's not correct. What are the properties of an _ideal_ voltmeter?

 

[Devs]

No that's not correct. What are the properties of an _ideal_ voltmeter?

An ideal voltmeter has infinite resistance while an ideal ammeter has zero resistance.

 

[CWatters]

So what happens if you put an infinite resistance in series with a 12V source and a 6R resistor?

 

[Devs]

So what happens if you put an infinite resistance in series with a 12V source and a 6R resistor?

I am not sure but it will have infinite voltage and 2A current?

 

[CWatters]

No. Revise how to add resistors that are in series. For example a 100R and a 200R in series is equivalent to a 300R.

 

[Devs]

No. Revise how to add resistors that are in series. For example a 100R and a 200R in series is equivalent to a 300R.

So we add resistance when in series which gives us 0A because anything over infinity will limit to 0. And the voltage will equal 12?

 

[CWatters]

Correct.

The infinite resistance of the ideal voltmeter stops any current flowing. So the ammeter reads 0A.
If no current is flowing there is no voltage drop across the 6R so the voltmeter "sees" the full 12V.

If you fancy an exercise... What would happen if the 12V battery, 6R, ammeter and voltmeter were all in parallel with each other rather than series? (Hint: Don't try it at home).

 

[Devs]

Correct.

The infinite resistance of the ideal voltmeter stops any current flowing. So the ammeter reads 0A.
If no current is flowing there is no voltage drop across the 6R so the voltmeter "sees" the full 12V.

If you fancy an exercise... What would happen if the 12V battery, 6R, ammeter and voltmeter were all in parallel with each other rather than series? (Hint: Don't try it at home).

Thanks a lot!

Regarding the exercise...since they are all in parallel, an extremely high level of voltage will pass through the ammeter causing it to blow up.

 

[CWatters]

Regarding the exercise...since they are all in parallel, an extremely high level of voltage current will pass through the ammeter causing it to blow up.

Yes. This is one reason why it is good practice to leave a multi-meter on a voltage setting when you finish using it. It would be very easy to leave it set on Amps and then next time you pick it up and go to measure the voltage of a battery and...