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Question: Answered & Verified by Expert

A parallel plate capacitor C with plates of unit area and separation d is filled with a liquid of dielectric constant K=2, the level of liquid isd3,initially. 
Suppose, the liquid level decreases at a constant speed v, the time constant as a function of time is

PhysicsCapacitanceBITSATBITSAT 2018
Options:
  • A 6ε0R5d+3vt
  • B (15d+9vt)ε0R2d3-3dvt-gv2t2
  • C 6ε0R5d-3vt
  • D (15d-9vt)ε0R2d3+3dvt-9v2t2
Solution:
1215 Upvotes Verified Answer
The correct answer is: 6ε0R5d+3vt

We have, the time constant, τ=RC'    i

Now, C'=C1C2C1+C2

=Aε0d-xKAε0xAε0d-x+KAε0x

  C'=KAε0x+K(d-x)

Putting the value of C' in Equation. (i), we get

τ=RKAε0d3-vt+kd-d3+vtx=d3-vt

Given, A=1 and K=2

  τ=3×2ε0×Rd-3vt+6d-2d+6vt=6Rε05d+3vt

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