Calculate Voltage Across Resistor Without Voltmeter

AI Thread Summary
A voltmeter with an internal resistance of 100k ohm reads 40V across a 200k ohm resistor, prompting a discussion on calculating the actual voltage across the resistor without the voltmeter. Participants suggest breaking down the voltmeter into an ideal voltmeter and its internal resistance to analyze the circuit configuration. The importance of understanding the arrangement, particularly that the voltmeter is connected in parallel with the resistor, is emphasized. Clarification on the circuit diagram is requested to facilitate accurate calculations. The conversation highlights the need to consider the impact of the voltmeter's resistance on the overall circuit voltage.
kyip
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A voltmeter with internal resistance of 100k ohm reads 40V across the 200k ohm resistor of the circuit. Determine the actual voltage across the resistor( without the voltmeter)

my anawer:
across 100k ohm = 13V
across 200k ohm = 26.7 V
is it correct ?
but it seems to be wrong..
so anyone can help ?
thx
 
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Try breaking the "real" voltmeter, into an ideal voltmeter and the internal resistance of it. How would this configuration look like ;)
 
actuallt the diagram is

Code:
┌------W (100k ohm) -------------┐
│                                 │
│                                 │
---                                      (200k ohm)  
 -                                              │
---                                            │
 -                                              │
│                                 │
│                                 │
└---------------------------------┘
well, the diagram is shown in series
but the question is saying '' across'' ?
so... i don't know ...
 
Last edited by a moderator:
Well, you can measure voltage between the two nodes of the resistor, hence you have to put parallel with the resistor, that's what it means by across.

Now how can you split the voltmeter into an ideal voltmeter and its internal resistance?
 
kyip said:
A voltmeter with internal resistance of 100k ohm reads 40V across the 200k ohm resistor of the circuit. Determine the actual voltage across the resistor( without the voltmeter)

my anawer:
across 100k ohm = 13V
across 200k ohm = 26.7 V
is it correct ?
but it seems to be wrong..
so anyone can help ?
thx

We need to see the actual circuit in order to answer the question.

The circuit in post #3 is not displaying properly. Could you do it this way instead:
[noparse]
Code:
{post ciruit diagram here}
[/noparse]

Then we should be able to see what is going on.
 
kyip, please do all correspondence about this problem by posting here in this thread.

If you have an online image of the circuit diagram:

[noparse][/noparse][i]the URL of the image goes here[/size][/i]

For example, typing

[noparse][PLAIN]https://www.physicsforums.com/customavatars/avatar122961_23.gif[/noparse]

will display this:
https://www.physicsforums.com/customavatars/avatar122961_23.gif
 
Last edited by a moderator:
Okay, now I can see the updated circuit diagram in post #3.

When the 100 kΩ voltmeter is connected across (i.e., in parallel with the 200 kΩ resistor, that will change the net resistance of that component.

So, first try solving the circuit with the extra 100 kΩ voltmeter there.
 
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