# Homework Help: Electricity: voltage calculation in serial

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1. Apr 14, 2017

### Epoch

1. The problem statement, all variables and given/known data
Calculate the voltage U.
The voltmeter gives a value of 80V.

3. The attempt at a solution

3RI + 4RI - U = -80
How can I create a System of Linear equations if I have only one equation?
Do I miss something?

My answer sheet says U = 150V.

2. Apr 14, 2017

### TomHart

What is 'R'? Is it a variable, or does it represent "resistor"? I guess it really doesn't matter. If it is a variable, the R's should cancel out. You know that the voltage across the 2 inside resistors is 80 V, so you should be able to come up with a second equation for that.

3. Apr 14, 2017

### D Nguyen

Let I be the current through the circuit. The V to the drop at 2R and 6R, so
2RI + 6RI = 80
8RI = 80
RI = 10

For the whole circuit,
U - 3RI - 2RI - 6RI - 4RI = 0

Try to take it from there!

4. Apr 14, 2017

### Epoch

Thanks, I got it.

5. Apr 14, 2017

### Epoch

RI = 10
U - 3RI - 2RI - 6RI - 4RI = 0
U - 3*10 - 2*10 - 6*10 - 4*10 = 0
U - 30 - 20 - 60 - 40 = 0
U - 150 =0
U = 150V

I found it, thanks for your help.