INDUCTORS AND INDUCTANCE

PHY110W MAGNETISM INDUCTANCE
RL CIRCUITS
Initially, an inductor acts to oppose changes in the
current through it. A long time later, it acts like
ordinary conducting wire.
Applying Kirchhoff's loop rule to
the adjacent circuit as the
constant emf E is switched on …
E
increasing i
R
L
E L
E
− iR − L di = dt
0
solving which for i we get
i
⎛
=
E
1 −
R
⎜
⎝
e
−Rt L
⎞
⎟
⎠
(31-42)
which can be written as
i
⎛
=
E
−
R ⎜
⎝
−t
τ
L
1 e
⎞
⎟
⎠
(31-43)
where τ L is the inductive time constant (τ L = L / R )
At time t = 0, i = 0. But as t → ∞, i → E / R .
−1
At time t = τ L i =
E
( 1− e ) = 0,63
E .
R
R
Graphically:
i
E
/ R
0.63 E / R
τ L
t
33