CapacitanceHard
Question
A parallel plate capacitor has a dielectric slab in it. The slab just fills the space inside the capacitor. The capacitor is charged by a battery and the battery is disconnected. Now the slab is started to pull out uniformly at t = 0. If at time t, capacitance of the capacitor is C, potential difference across plate is V, and energy stored in it is U, then which of the following graphs are correct ?
Options
A.

B.

C.

D.

Solution
Capacitance of capacitor is = C0 = 

C =
C =
[x + l(L − x)] =
[kL − (k − 1)x] =
[kL − (k − 1) vt]
So, C decreases linearly with time
Charge on capacitor Q = C0V0 =
V0 = constant.
Potential difference across plate is V =
⇒ V ∞ 
V =
Potential energy U =
QV =
C0V0.V

C =
C =
So, C decreases linearly with time
Charge on capacitor Q = C0V0 =
Potential difference across plate is V =
V =
Potential energy U =
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