# Voltmetre problem.

Discussion in 'Homework Help' started by drao, Mar 23, 2012.

1. ### drao Thread Starter New Member

Mar 11, 2012
16
0
I must find the voltage indicated by the voltmetre for this circuit.

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This how i solved (after reading the voltmetre`s theory from allboutcircuits ) and i want to be sure its correct.
R2 || Ri
I=E/(R1+R2||Ri)=3.746 *10^-6 A
So the voltage indicated by the voltmetre is I*(R2||Ri)=8.243 V
And, if the voltmetre is ideal, the voltage indicated is 7.5 V.
Did i make any mistakes???
And i want to ask you one more thing. Im a bit confused about this circuit. I learned that when you want to use a voltmetre,you put it in ||, but in this figure, it considered in || with R1 and in fact it indicates the voltage PT2???

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Last edited: Mar 23, 2012
2. ### #12 Expert

Nov 30, 2010
16,703
7,341
Your 8.243 number seems wrong and the meter is in parallel with R2.

3. ### drao Thread Starter New Member

Mar 11, 2012
16
0
Ohh, yeah, its with R2.
I did a mistake here "And i want to ask you one more thing. Im a bit confused about this circuit. I learned that when you want to use a voltmetre,you put it in ||, but in this figure, it considered in || with R1 and in fact it indicates the voltage PT2???", but i calculated with R2.
Why its wrong???
Opps, its 6.75 V??? It was late night when i made the thread and i was tired, im from Europe .
And for ideal meter its 7.5 V??

Last edited: Mar 24, 2012
4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,991
1,116
The voltmeter is connect between PT2 and GND.
R2 is also connect between PT2 and GND.
So R2 and voltmeter are connect in parallel.
And R1 is connect between PT1 and PT2 so for sure voltmeter is connect parallel to R2 and show voltage across R2.

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,991
1,116
Vout = 15V* (R2||Ri) / ( R1 + (R2||Ri)) = 15V * 1.8MΩ/ ( 1.8MΩ + 2.2MΩ) = 15V * 1.8/4 = 15V * 0.45 = 6.75V

and if Ri = ∞ then Vout = 15V/2 = 7.5V