Download Inductor

Inductor
Physical Coil
 A circuit component
designed for inductance is
an inductor.
• Symbol suggests coiled wire
 Ideal inductors act like wires
unless the current changes.
• Emf based on L
 Real inductors have some
resistance.
• Resistivity of length of wire
RL Circuit
 An inductor resists changes in
L
I(t)
current.
• Creates emf
V
R
 A switch can be used to
VL (t )   L
I
t
I
V L
 IR  0
t
complete a circuit with a
resistor.
• One-loop Kirchhoff’s law
• Treat as additional emf source
Inductive Time Constant
 The voltage across an inductor
falls with time.
• Change in current decreases
• Never reaches zero
• Overall current increases
 The period of time for a
decrease by a factor of e is the
time constant t.
I R
 I
t L
I I

t t
t  L/ R
Exponential Decay
 The emf of an inductor can be
represented by an exponential
curve.
• Depends on battery voltage
VL (t )  V0 e  t /t
Inductor Current
 Inductor and resistor voltages
sum to battery voltage.
• Kirchhoff’s voltage law
 Circuit current can be
determined from Ohm’s law
VR (t )  V0 (1  e t /t )
VR V0
t / t
I (t ) 
 (1  e )
R
R
Disconnection
 Opening a switch with current
L
I(t)
V
flowing is a change in current.
R
 The inductor will create an emf
to resist the change.
VL (t )   L
I
t
I
V L
 IR  0
t
 For a large current the emf can
create a spark.
Safe Discharge
 A pair of opposite switches can
provide a safe path for current
in an inductor.
• A and B always in opposite
position
SA
I(t)
V
SB
 When switch A is closed the
battery powers the circuit.
 When switch B is closed the
inductor current flows through
the switch and resistor.
I
L
 IR  0
t
L
R