Charged capacitors connected together, find the final potential difference

In summary, the conversation discusses the concept of capacitance and how it relates to potential difference and charge. The individual attempts at solving the problem involve using the equations C=Q/V and Q1=Q2, as well as understanding that two capacitors in series will have the same charge. The conversation also addresses the misunderstanding of how the capacitors are prepared before being connected.
  • #1
The Blind Watchmaker
17
0

Homework Statement


upload_2018-3-6_21-55-34.png


Homework Equations


C = Q/V

The Attempt at a Solution


Q1 = Q2
C1*V1 = C2*V2
2*V1 = 5*V2
V2 = 2/5*V1

V1+V2 =100
7/5*V1 = 100
V1 = 71.4 V

Can someone verify my working? Thanks!
 

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  • #2
The Blind Watchmaker said:

The Attempt at a Solution


Q1 = Q2 ← How do you conclude this?
C1*V1 = C2*V2
2*V1 = 5*V2
V2 = 2/5*V1

Two different valued capacitors with the same potential difference will have the same charge?
 
  • #3
Note that before the switches are closed, Q1 ≠ Q2. What is the total charge on the two bottom plates before the switches are closed? What is the total charge on the two bottom plates after the switches are closed?
 
  • #4
gneill said:
Two different valued capacitors with the same potential difference will have the same charge?
They are in series thus Q1=Q2. I do not get why they have the same potential difference as I assume only 1 battery is used
 
  • #5
The Blind Watchmaker said:
They are in series thus Q1=Q2. I do not get why they have the same potential difference as I assume only 1 battery is used
There are two issues in your understanding. When we connect two capacitors in series using only one battery q for each is taken to be the same. Just make such a connection with two different capacitors and then argue why we take charge stored by these capacitors to be the same. But in this problem they have been charged separately to the same potential difference.
 
  • #6
The Blind Watchmaker said:
They are in series thus Q1=Q2. I do not get why they have the same potential difference as I assume only 1 battery is used
Read the problem statement carefully as to how the capacitors are prepared *before* they are connected to each other.
 

1. How do you calculate the final potential difference of charged capacitors connected together?

To calculate the final potential difference, you need to add the charges of the capacitors together and divide by the total capacitance. This will give you the final voltage across the capacitors.

2. What is the formula for calculating the final potential difference in a series circuit of charged capacitors?

The formula for calculating the final potential difference in a series circuit is Vf = Q/C, where Vf is the final potential difference, Q is the total charge of the capacitors, and C is the total capacitance.

3. Can charged capacitors be connected together in parallel?

Yes, charged capacitors can be connected together in parallel. When connected in parallel, the final potential difference will be the same as the potential difference of the individual capacitors.

4. What happens to the potential difference when charged capacitors are connected together in series?

When charged capacitors are connected together in series, the potential difference across each capacitor is divided among them. This means that the final potential difference will be less than the potential difference of the individual capacitors.

5. How does the capacitance affect the final potential difference in a circuit of connected capacitors?

The capacitance affects the final potential difference by determining how much charge can be stored on the capacitors. A higher capacitance will result in a larger final potential difference, while a lower capacitance will result in a smaller final potential difference.

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