Solving an Electric Circuit: Potential Difference & Capacitor Charge

[Jay9313]

Homework Statement

I have this circuit here.. http://s1167.photobucket.com/albums/q639/jay9313/?action=view&current=EM.jpg
The question says that when switch S is open in the circuit shown above, the potential difference between points a and b would be..
A) 0V
B) 6V
C) 12V
D) 24V
E) 36V

Part 2 says when switch S is closed, the carge on the 3 microfarad capacitor would be..
A) 0 μF
B) 12 μF
C) 18 μF
D) 36 μF
E) 72 μF

Homework Equations

The Attempt at a Solution

My professor ways that the answer is A. I need somebody to help me understand why.

 

[SammyS]

Homework Statement

I have this circuit here.. http://s1167.photobucket.com/albums/q639/jay9313/?action=view&current=EM.jpg

The question says that when switch S is open in the circuit shown above, the potential difference between points a and b would be..
A) 0V
B) 6V
C) 12V
D) 24V
E) 36V

Homework Equations

The Attempt at a Solution

My professor ways that the answer is A. I need somebody to help me understand why.

Where's the switch ?

Also, the rules for Homework Help in this Forum state:

You MUST show that you have attempted to answer your question in order to receive help.​

So, what do you think the answer should be --- and why ?

 

[Jay9313]

I redrew it and added the switch, and I don't know how he got 0V. My logic was that all of the current would go through the capacitors, so there would be a potential difference of 36V, the same as the EMF source.

 

[Curious3141]

I redrew it and added the switch, and I don't know how he got 0V. My logic was that all of the current would go through the capacitors, so there would be a potential difference of 36V, the same as the EMF source.

Addressing the first part, A is the correct answer.

You may assume that the capacitors charge up very quickly and both capacitors are fully charged.

Take the negative terminal of the power source as being ground (0V).

What is the potential at a? Hint: voltage divider.

What is the potential at b? Hint: what do you know about capacitors in series? Compare the charges across the plates of each capacitor. Now compare the voltages across the plates of each capacitor.

 

[Jay9313]

Oh, ok. It's 0v because both drop 24V in the first resistor or capacitor! Nnow what about the second part, where do I even start for that?

 

[Jay9313]

Is it just 72 μC?

 

[Curious3141]

Is it just 72 μC?

Yes, since a and b are equipotentials. Closing that switch makes no difference. The capacitors don't discharge and the charges (and voltages) across both of them don't change.

 

[Jay9313]

Yes, since a and b are equipotentials. Closing that switch makes no difference. The capacitors don't discharge and the charges (and voltages) across both of them don't change.

Oh thank you so much, you helped me understand it so much =D