Why are these voltages the same?

  • #1
Why does the R3 and voltmeter share the same voltage drop? They arent parallel because of the R1 and R2 resistors so how can they have equal voltage?
circuit.png
 

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  • #2
To clarify, im speaking about the voltage across R3 and the voltmeter U1.
 
  • #3
gneill
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I presume that the designations "DC 10 MOhm" implies that the meters present a 10 MegOhm impedance. If that's the case then the two rather smaller resistances of R1 and R2 don't make much of an impact to the the U1 branch. Nearly no current will pass through that branch, so the voltage drops across those two resistors will be negligible.

You should be able to work out the current through the branch and the voltage drops across those resistors.
 
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  • #4
I presume that the designations "DC 10 MOhm" implies that the meters present a 10 MegOhm impedance. If that's the case then the two rather smaller resistances of R1 and R2 don't make much of an impact to the the U1 branch. Nearly no current will pass through that branch, so the voltage drops across those two resistors will be negligible.

You should be able to work out the current through the branch and the voltage drops across those resistors.
Hmm. What if instead of the voltmeter then I had an open?
 
  • #5
gneill
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Hmm. What if instead of the voltmeter then I had an open?
Then no current would flow. No voltage drops.
 
  • #6
Tom.G
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Have you learned about voltage dividers yet? If so then you can treat R1 and R2 as being in series with U1 and calculate the voltages across each.

If voltage dividers have not been covered yet, be aware that when resistors are connected in series across a voltage source, the voltage across each resistor is proportional to their resistances.

Cheers,
Tom
 
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  • #7
jim hardy
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They arent parallel because of the R1 and R2 resistors so how can they have equal voltage?
Have you studied Ohm's :law ?

If so, here's a hint:
Have you done the arithmetic to find voltage across the meter ??
To how many significant figures did you calculate ?
 
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  • #8
NascentOxygen
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Why does the R3 and voltmeter share the same voltage drop? They arent parallel because of the R1 and R2 resistors so how can they have equal voltage?
The figure shows a source of 8.000V and the meter U1 reading is marked as 7.989V so it's indicating the voltages are not identical.
 
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  • #9
Merlin3189
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Perhaps your question should be, why should the voltages be different? If you answer that question correctly, you'll see that the reasons for any difference don't apply here.

They arent parallel because of the R1 and R2
Is that so? Why do the resistors make it not parallel?
If the voltmeter were in parallel, it would have to be joined by two wires - otherwise known as two resistors, albeit of very low values. (I don't know whether they're still sold, but you used to be able to buy zero ohm resistors. I'm pretty confident they weren't exactly 0 Ω, but they were bits of wire with encapsulation the size of a standard resistor.)
Your two resistors are not 0 Ω, but you need to ask yourself what difference that makes and be very rigourous in your answer.
 
  • #10
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Consider the two circuit branches individually.

U1 and R3 are in parallel with the voltage source, so the voltages across them must be the same i.e. 8.000 V.
Similarly, the combination of R1, R2 and U2 is in parallel with the battery, so the voltage across the combination is 8.000 V.

R1, R2 and U2 are in series, so the total resistance is 10,000,000 + 10,000 + 4,000 = 10,014,000 Ω.
The current through this branch is 8.000 / 10,014,000 = 7.989 x10‾⁷ V = 0.7989 μV.
Therefore, the voltage across U2 is 10,000,000 x 7.989 x 10‾⁷ = 7.989 V.
 

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