Charge redistribution on capacitors

  • Thread starter Jahnavi
  • Start date
  • #1
848
102

Homework Statement


capacitor.jpg


Homework Equations




The Attempt at a Solution


I will name the top left capacitor A , bottom left capacitor B and right capacitor C .

Before the switch is closed , charges on A and B will be 20 μC each and that on C will be 40 μC .

After the switch is closed ,both A and B will be shorted . Charge on C will be 60μC .

A and B will get discharged i.e there will be no charge on A and B .

On doing further calculations I am getting all four options correct .

The correct answer i.e incorrect statement is 3) .

But if charge on C changes from 40μC to 60μC , doesn't this mean 20 μC has flown through the battery and battery has done 0.6 mJ work or 0.6 mJ energy is supplied by the battery ?
 

Attachments

Answers and Replies

  • #2
cnh1995
Homework Helper
Gold Member
3,387
1,126
But if charge on C changes from 40μC to 60μC , doesn't this mean 20 μC has flown through the battery and battery has done 0.6 mJ work or 0.6 mJ energy is supplied by the battery ?
Find the total energy in the circuit before and after the switch is closed. The difference between the two energies would be the energy supplied by the battery.
 
  • #3
848
102
Find the total energy in the circuit before and after the switch is closed. The difference between the two energies would be the energy supplied by the battery.
I don't think this is correct :smile:

You are forgetting energy dissipated as heat .
 
  • Like
Likes cnh1995
  • #4
cnh1995
Homework Helper
Gold Member
3,387
1,126
Also, the second option is confusing (or should I say 'tricky') because there is no resistance shown in the circuit. I'm afraid that makes this statment incorrect.
The difference between the two energies would be the energy supplied by the battery.
Can you correct this statement?
Hint: The '1/2' in the equation for the energy stored in the capacitor.
 
  • #5
848
102
Also, the second option is confusing (or should I say 'tricky') because there is no resistance shown in the circuit. I'm afraid that makes this statment incorrect.
It doesn't make a difference whether a resistance is shown in the circuit or not :smile: .

Heat is dissipated in the wires .
 
  • #6
cnh1995
Homework Helper
Gold Member
3,387
1,126
Heat is dissipated in the wires .
And the wires are shown to be ideal (although you can assume them to have some resistance).
 
  • #7
848
102
Anyways , I am finding all four options correct .

Which option do you find incorrect ?

Answer given is 3)
 
  • #8
cnh1995
Homework Helper
Gold Member
3,387
1,126
Anyways , I am finding all four options correct
Yes, I misread option C as 30mJ. I agree that all the options are correct.
(I still find dc capacitive circuits very tricky!).
 
  • Like
Likes Jahnavi

Related Threads on Charge redistribution on capacitors

  • Last Post
Replies
7
Views
694
Replies
40
Views
6K
Replies
3
Views
2K
  • Last Post
Replies
5
Views
3K
Replies
7
Views
436
  • Last Post
3
Replies
67
Views
1K
Replies
10
Views
691
  • Last Post
Replies
12
Views
4K
  • Last Post
Replies
4
Views
11K
  • Last Post
Replies
21
Views
3K
Top