# The voltage across a voltmeter Olymp 09

My attempt : 2.36 V

Vb1 / (R + r1 + r2) = 2 / 2.5 = 0.8 A
Vb2 / (R + r1 + r2) = 5 / 2.5 = 2 A

The reading of the voltmeter = the voltage across r1 + the voltage across r2 and R = (0.8)(2.2) + (2)(0.3)
= 2.36 V

I'm not sure of my answer, and even if it's correct I'm not completely convinced how is my solution is right.

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rude man
Homework Helper
Gold Member
The voltmeter sits across a 2V battery. What do you suppose it will read?

mfb
Mentor
The voltmeter sits across a 2V battery. What do you suppose it will read?
Not 2 V, as we have to consider the internal resistances.

@ElmorshedyDr: What is meant with "in the opposite circuit [...]"?
I don't understand your approach to convert those currents to voltages across the 0.3 Ohm resistor.

rude man
Homework Helper
Gold Member
Not 2 V, as we have to consider the internal resistances.
Right. Didn't see that.

So the answer is voltmeter reading = VB + V across the internal resistance = 2 + (1.2)(0.3) = 2.36 V ??

rude man
Homework Helper
Gold Member
So the answer is voltmeter reading = VB + V across the internal resistance = 2 + (1.2)(0.3) = 2.36 V ??
Correct.

NascentOxygen
Staff Emeritus
View attachment 70418

My attempt : 2.36 V

Vb1 / (R + r1 + r2) = 2 / 2.5 = 0.8 A
Vb2 / (R + r1 + r2) = 5 / 2.5 = 2 A

The reading of the voltmeter = the voltage across r1 + the voltage across r2 and R = (0.8)(2.2) + (2)(0.3)
= 2.36 V

I'm not sure of my answer, and even if it's correct I'm not completely convinced how is my solution is right.
Your working is basically correct, but your technique is not what I was expecting. Can you explain the method you have used?

Correct.

But I'm still confused, why can't we just calculate the total voltage and the total resistance and then find the total current ad multiply it by the internal resistance of B1 to find the reading, since it is connected in parallel with internal resistance of the battery which would be 0.36V

mfb
Mentor
Sure, and the result is the same.

NascentOxygen
Staff Emeritus