Charge redistribution on capacitors

[Jahnavi]

Homework Statement

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 ?

 

[cnh1995]

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.

 

[Jahnavi]

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

You are forgetting energy dissipated as heat .

 

[cnh1995]

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 statement 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.

 

[Jahnavi]

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 statement incorrect.

It doesn't make a difference whether a resistance is shown in the circuit or not .

Heat is dissipated in the wires .

 

[cnh1995]

Heat is dissipated in the wires .

And the wires are shown to be ideal (although you can assume them to have some resistance).

 

[Jahnavi]

Anyways , I am finding all four options correct .

Which option do you find incorrect ?

Answer given is 3)

 

[cnh1995]

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!).