PHY2061
R. D. Field
Department of Physics
chp36_3.doc
University of Florida
RL Circuits
"BuildingUp" Phase:
Connecting the switch to
position A
corresponds to the
"building up" phase of
an RL circuit
.
Summing all the potential
changes in going around the loop gives
ε
−
−
=
IR
L
dI
dt
0
,
where
I(t)
is a function of time.
If the switch is closed (
position A
) at t=0
and
I(0)=0
(
assuming the current is zero at t=0
) then
d I
d t
I
R
= −
−
1
τ
ε
,
where I have define
τ
=L/R
.
Dividing by (I
ε
/R) and multiplying by dt and integrating gives
(
)
dI
I
R
dt
I
t
−
= −
∫
∫
ε
τ
/
0
0
1
, which implies
ln
/
/
I
R
R
t
−
−
= −
ε
ε
τ
.
Solving for
I(t)
gives
(
)
I t
R
e
t
( )
/
=
−
−
ε
τ
1
.
The potential change
across the inductor is
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 Spring '08
 FRY
 Physics, Inductor, RC circuit, DI, RL circuit

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