p33 027

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27. The assumption stated at the end of the problem is equivalent to setting φ = 0 in Eq. 33-25. Since the

maximum energy in the capacitor (each cycle) is given by q

2

max

/2C, where q

max

is the maximum charge

(during a given cycle), then we seek the time for which

q

2

max

2C

=

1

2

Q

2

2C

=

⇒ q

max

=

Q

2

.

Now q

max

(referred to as the exponentially decaying amplitude in

§33-5) is related to Q (and the other

parameters of the circuit) by

q

max

= Qe

−Rt/2L

=

ln



q

max

Q



=

Rt

2L

.

Setting q

max

= Q/

2, we solve for t:

t =

2L

R

ln



q

max

Q



=

2L

R

ln



1

2



=

L

R

ln 2 .

The identities ln(1/

2) =

ln

2 =

1
2

ln 2 were used to obtain the final form of the result.


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