# Capacitance (Potential Difference and Energy)

1. Sep 30, 2015

### kilnvzol

1. The problem statement, all variables and given/known data
In the figure C1 = 10.0 μF,C2 = 20.0 μF and C3 = 27.0 μF. If no capacitor can withstand a potential difference of more than 130 V without failure, what is (a) the magnitude of the maximum potential difference that can exist between points A and B and (b) the maximum energy that can be stored in the three-capacitor arrangement?
http://edugen.wileyplus.com/edugen/courses/crs7165/art/qb/qu/c25/q37.jpg
2. Relevant equations
1/Cseries= 1/C1 + 1/C2 ...
U = q2/(2C) = (0.5)CV2

3. The attempt at a solution
For part A, I know that 130V goes into C1 and I think half that goes into C2 so 65V but I'm not sure how much goes into C3.
For part B, I need the solution to part A to figure that out.

Thank you!

2. Sep 30, 2015

### Staff: Mentor

Can you justify that claim?
Consider how charge is distributed amongst series-connected capacitors (when they are charged simultaneously by the same current).

What relationship is there between voltage, charge, and capacitance?

Edit: For part (b), consider a clever way to put as much energy as possible onto the arrangement.

3. Sep 30, 2015

### kilnvzol

Actually I think I got part A. C = Q/V => V=Q/C the voltage for the first one is 130 so Q = (130)(10E-6) = 1.3E-3
Then you use the Q value to find the rest? So C3 is 48.15?

For part B, I just use the equation U = q2/(2C) = (0.5)CV2?

4. Sep 30, 2015

### Staff: Mentor

That looks good.
Yes,... but the question as posed does not restrict you to using the charges or voltages from part A...

5. Sep 30, 2015

### kilnvzol

Do you just plug in each voltage with its respective capacitance to the equation and see which is higher or what is the other way?

6. Sep 30, 2015

### Staff: Mentor

You want to maximize the total energy stored (while respecting the voltage tolerance of the individual components). So what would be the ideal situation?

7. Oct 1, 2015

### kilnvzol

Only using the potential energy from the first capacitor?

8. Oct 1, 2015

### Staff: Mentor

Wouldn't that be a waste of the energy storage potential of the other two?

9. Oct 1, 2015

### kilnvzol

Add all of the potential energies together?

10. Oct 1, 2015

### Staff: Mentor

Sure. After all, you have three "containers" and each can store some. Otherwise it's like having three boxes to put things in and only using the smallest one.

11. Oct 1, 2015

### kilnvzol

Thank you i got it!