- #1
thisisfudd
- 56
- 0
Hi,
I understand generally how to do problems with capacitors, but I am faced here with a problem about 2 capacitors and I am not sure what the appropriate calculations are for combining their capacitance, etc. Here is the problem:
A 7.7 uF capacitor is charged by a 125-V battery and then is disconnected from the battery. When this capacitor is the connected to a second (initially uncharged) capacitor, C2, the final voltage on each capacitor is 15 V. What is the value of C2?
So here is what I did:
Q = C V
Q = 7.7E-6 F x 125 V
Q = 9.625E-4 C per plate (first capacitor)
PE = 1/2 Q^2/C
PE = 1/2 x (9.625E-4)^2/7.7E-6 F
PE = .06
From this, I used PE = 1/2 CV^2
.06 = 1/2 (7.7E-6 F + x)(15^2)
To get: 5.3E-4 F which I am pretty sure is way too large a number
I understand generally how to do problems with capacitors, but I am faced here with a problem about 2 capacitors and I am not sure what the appropriate calculations are for combining their capacitance, etc. Here is the problem:
A 7.7 uF capacitor is charged by a 125-V battery and then is disconnected from the battery. When this capacitor is the connected to a second (initially uncharged) capacitor, C2, the final voltage on each capacitor is 15 V. What is the value of C2?
So here is what I did:
Q = C V
Q = 7.7E-6 F x 125 V
Q = 9.625E-4 C per plate (first capacitor)
PE = 1/2 Q^2/C
PE = 1/2 x (9.625E-4)^2/7.7E-6 F
PE = .06
From this, I used PE = 1/2 CV^2
.06 = 1/2 (7.7E-6 F + x)(15^2)
To get: 5.3E-4 F which I am pretty sure is way too large a number