Infinite resistance of a Voltmeter

In summary, a voltmeter has a large resistance compared to the resistance across which it is connected, otherwise an appreciable current will flow through the voltmeter. This concept is easily understood by considering two circuits each with a battery and a two equal resistors. If you measure the voltage across a resistor with a high resistance voltmeter, the voltage should be higher than what you measure with a low resistance voltmeter. By doing this, you minimize the amount of current that flows through the voltmeter, which in turn minimizes the change to the circuit conditions. Additionally, reading this article on physicsforums.com may help you understand the concept better.
  • #1
shehri
30
0
While studying about 'Potentiometer' it's said :
"The resistance of the voltmeter be large compared to the circuit resistance across which the voltmeter is connected.Otherwise an appreciable current will flow through voltmeter which will alter the circuit current & the P.D to be measured". Can anybody explain this concept?.Thanks.
 
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  • #2
The concept is actually straight forward. You need to show some thought about your question before we can help you. (see https://www.physicsforums.com/showthread.php?t=94379)

If you are truly lost, you might begin by defining some of the terms or if it is a measuring device, how do you normally use this in the lab. Have you done any research about this, on the web? (hint: use search engines)..
 
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  • #3
Think of two circuits each with a battery and a two equal resistors of say 1kohm, one where you are measuring the voltage across a resistor with a high resistance voltmeter say 1Mohm and one where you measure with a low resistance voltmeter say 1ohm. Draw out the circuits and see if you can work out what the voltage measured in each case would be and what the actual voltage should be.
 
  • #4
shehri said:
While studying about 'Potentiometer' it's said :
"The resistance of the voltmeter be large compared to the circuit resistance across which the voltmeter is connected.Otherwise an appreciable current will flow through voltmeter which will alter the circuit current & the P.D to be measured". Can anybody explain this concept?.Thanks.

Because "an observer disturbs the system he/she is trying to observe", it is important to include a measuring device in a such way that only minimal change to the original system results. In the voltmeter case, you are providing the current an alternative pathway to flow once you have introduced the voltmeter in the circuit. Hence, in order to minimise the amount of currrent using this path (created by the inclusion of the voltmeter) you want the resistance of the voltmeter to be very large compared to the circuit component it is measuring. Higher resistance means that most of the current will follow the original path and hence leads to minimal change to the circuit conditions.
 
  • #5
I know both of you, are trying to be helpful to shehri, and you have not earned "homework helper" badges yet. It would be more helpful to the person asking the question, to show they have given some thought to them and not just seeking answers from the PF community.

You are encouraged to read this https://www.physicsforums.com/showthread.php?t=94379 which outlines what is expected of posters to the homework section.
 
  • #6
Thanks for clearing me about 'infinite resistance' in a simple manner.Especially 'mjsd'.Thanks once again.
 

1. What is infinite resistance in a voltmeter?

Infinite resistance refers to the maximum resistance that a voltmeter can have. It means that the voltmeter has an extremely high resistance, which allows it to measure voltage without drawing any current from the circuit.

2. Why is infinite resistance important in a voltmeter?

Infinite resistance is important because it ensures that the voltmeter does not alter the voltage being measured. This is because the high resistance prevents any current from flowing through the voltmeter, making it an ideal instrument for accurately measuring voltage.

3. How is infinite resistance achieved in a voltmeter?

Infinite resistance is achieved by using a high resistance component, such as a resistor or a vacuum tube, in the voltmeter's circuit. This high resistance prevents any current from passing through the voltmeter, allowing it to measure voltage without altering the circuit.

4. Can a voltmeter have infinite resistance in all ranges?

No, a voltmeter cannot have infinite resistance in all ranges. In most cases, a voltmeter will have a set range of measurements and will have a finite resistance in each range. However, the highest range on a voltmeter may have an extremely high resistance, approaching infinity.

5. What is the difference between infinite resistance and open circuit?

Infinite resistance refers to the maximum resistance that a voltmeter can have, while open circuit refers to a complete break in the circuit. In other words, infinite resistance means that the voltmeter has a high resistance and is still connected to the circuit, while open circuit means that the circuit is incomplete and no current can flow through it.

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