Electromagnetic Induction and Alternating Current 7 Question 5

####5. An LCR circuit is equivalent to a damped pendulum. In an LCR circuit, the capacitor is charged to Q0 and then connected to the L and R as shown.

If a student plots graphs of the square of maximum charge (QMax 2) on the capacitor with time (t) for two different values L1 and L2(L1>L2) of L, then which of the following represents this graph correctly? (plots are schematic and not drawn to scale)

(2015 Main)

(a)

(b)

(c)

(d)

Show Answer

Answer:

Correct Answer: 5. (d)

Solution:

At a general time t, suppose charge on capacitor is q and current in the circuit is i, then applying Kirchhoff’s loop law, we have

Putting

qciRLdidt=0

In the above equation, we have

d2qdt2+RLdqdt+qLC=0

Comparing this equation with the standard differential equation of damped oscillation,

d2Xdt2+bmdXdt+KmX=0

Which has a general solution of amplitude, A=A0ebt2m The general solution of Eq. (i) will be

Qmax=Q0eRt2L or Qmax2=Q02eRtL

Hence, Qmax2 versus time graph is exponentially decreasing graph. Lesser the value of self inductance, faster will be the damping.



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