# Finding voltage in a capacitors circuit

Discussion in 'Homework Help' started by gbox, Jan 24, 2016.

1. ### gbox Thread Starter Member

Dec 29, 2015
42
0

The voltmeter shows 1V
$q_1=c_1v_1=60*10^{-9}C$
$60*10^{-9}C=60*10^{-9}V_2\rightarrow V_2=1V$
$60*10^{-9}C=30*10^{-9}V_3\rightarrow V_2=2V$

$V_1+V_2+V_3=4V$

I need to find the voltage of the battery, I can not find a way to continue
Can I say that the voltage drop on C_4 is 4V?
Can I say that C_(1+2+3) must have the same voltage drop of 4V?

Last edited: Jan 24, 2016
2. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,432
703
It is not clear to me how you decided that there is 4 volts across C_4.

3. ### gbox Thread Starter Member

Dec 29, 2015
42
0
I didn't, I am asking if I can say that C_4 has 4V because V_1+V_2+V_3=4V ?

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,432
703
Yes. The two branches are in parallel. Therefore each branch has the same voltage across it.

5. ### J_Rod Member

Nov 4, 2014
109
6
Is the voltage supply (V on the right) 70Vdc; from line 2, $q_1 = c_1v_1 = 60 * 70^{-9}$ C ?

Recall the rules for combining capacitors in series and in parallel. How can you determine the total charge stored in capacitors $C_6, \, C_4, \,$ and $C_5$?

6. ### gbox Thread Starter Member

Dec 29, 2015
42
0
But first I must find the equivalent of C_1 and C_2 and C_3 and to find the voltage on it?

7. ### gbox Thread Starter Member

Dec 29, 2015
42
0
Sorry the voltmeter shows $1V$

In parallel is the sum is series it is like resistors in parallel

8. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
But what is the basis for claiming that V_1 + V_2 + V_3 = 4 V ?

Your very equation appears to be

q_7 = (C_7)(V_7)

yet you have given no indication what C_7 and V_7 are.

You don't give a voltage for the battery nor for the voltage source on the right hand side.

9. ### J_Rod Member

Nov 4, 2014
109
6
Capacitors in series have the same charge, but different voltages across them, whereas capacitors in parallel distribute charge, and have the same voltage between their terminals. All of the capacitances are written in the schematic, and you are asked to determine the voltage (across the battery).

10. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
So what information do you have that is not contained in your diagram?

Do you know what the voltage of the battery is?

Do you know what the reading on the voltmeter is?

11. ### gbox Thread Starter Member

Dec 29, 2015
42
0

Sorry for not mentioning there is no $C_7$ it is a battery

Dec 29, 2015
42
0

13. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
Then what does your second line mean, since it has a C_7 and a V_7 in it?

14. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
If you know how much voltage appears across C_1, does that tell you how much charge is on C_1?

If you know how much charge is on C_1, does that tell you how much charge is on one or more of the other capacitors? If so, which ones and why?

15. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,432
703
You misread q1 of C1 as q7, etc.

16. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
Okay -- so how do they write a 7? With a cross bar?

I'd recommend that the TS change how they write a 1 so that it isn't so much like a 7.

So now that you know how much charge is on C4 and C2, where did that charge have to come from? Which capacitor does that now tell you has how much charge?

17. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,432
703
Yeah, the writing is not very good. I hope you noticed that all I did in this thread is ask one simple question and provided one simple answer. I am not interested in deciphering someone else's writing. If the person wants help and does not want to waste time on answering 100 and 1 question, then they will write clearly so that everyone would understand WTF they wrote. Obviously OP did not think of that.

18. ### J_Rod Member

Nov 4, 2014
109
6
Recommend T.S. to post an updated schematic showing the nodes where the connected voltmeter measured 1V and the polarity of the measurement.

19. ### WBahn Moderator

Mar 31, 2012
23,141
6,966
That would be good, though the polarity is pretty easy to figure out (but it requires that we guess the polarity of their meter and engineering is not about guessing).

There is also an assumption being made that the TS needs to be aware of -- namely that the voltages on all the capacitors is consistent with them starting out uncharged.

20. ### gbox Thread Starter Member

Dec 29, 2015
42
0
Edited and wrote in Latex