IIT JEE , AIEEE Forum - Physics , Electricity

punterjack

well this is a conceptual doubt regarding the capacitors......

cud you people just give me the basic approach to problem wherin two or more capacitors are first charged to a potential and then one terminal of a capicitor is connected to the other terminal of the other capicitor.......or problems wherin two capicitors are charged and then are connected to each other................a sensible answer within 5 minutes will surely be rated hats........

rooney

When 2 capacitors are charged to a given potential , you come to know of the charge stored in each since you must be given the capacitance of each. So, now you know the charge on each plate of both the capacitors.

Now suppose you join the same charged plates of both the capacitors. Clearly since one plate of 1 capcitor is joined with 1 plate of another capacitor, both these plates form an isolated system. Hence, in steady state, the net sum of charges on both these plates will be same as initial sum of charges on these 2 plates. This gives 1 equation.Also, in steady state, the potential diff across both plates will be same.

Thus, final charges q1 and q2 on both plates have 2 equations. Solve and get the value of q1 and q2.

punterjack

good attempt rooney but i would like it if you would be a bit more clear......

cud u plz give another shot at this one............

rooney

Ok Let me explain using a question.

Suppose there are 2 capacitors A and B with capacitance C₁ and C₂(C₁>C₂).Both are charged to voltage V and the same charged plates are connected to each other . This means that +ve plate of A is in touch with +ve plate of B AND -ve plate of A is in touch with -ve plate of B. Now we have to find the final charge on both the plates.

Let final charge ( in steady state ) on A be Q₁ and on B be Q₂.

Firstly, since net P.D. across a circuit is 0. From formula V=Q/C , we have
V₁ + V₂ = 0
Q₁/C₁ - Q₂/C₂ = 0 ..........(going anticlockwise)....[1]

Now the +ve plate of A and the positive plate of B are an isolated system. So, initial and final sum of charges on both these plates will be same.
So, C₁V + C₂V = Q₁ + Q₂

Only Q₁ and Q₂ are unknown and you have two equations, you can get their value. IF the final polarity of B plate comes out to be opposite, you will get value of Q₂ as negative.

Hope this makes things clearer

Regards,
Rooney

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