P31_056.PDF

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Chapter 31 - 31.56
56.Let U B ( t )= 2 Li 2 ( t ). We require th e energy at time t to be half of its final value: U ( t )= 2 U B ( t
)= 4 Li f .Thisgives i ( t )= i f / 2. But i ( t )= i f (1
e t/τ L ), so
1
e t/τ L =
1
2
t =
τ L ln 1
1
2
=1 . 23 τ L .
=
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