Figure E.2:
An RLC filter,
input
, output
.
 |

An
inductor can be made physically using a coil of wire, and it
stores magnetic flux when a current flows through it. Figure
E.2
shows a circuit in which a resistor

is in series with the
parallel
combination of a capacitor

and inductor

.
The defining equation of an inductor

is
 |
(E.3) |
where

denotes the inductor's stored magnetic flux at time

,

is the
inductance in
Henrys (H), and

is the
current through the inductor coil in
Amperes (A), where
an Ampere is a Coulomb (of electric charge) per second.
Differentiating with respect to time gives
 |
(E.4) |
where

is the voltage across the inductor in
volts. Again, the current

is taken to be positive when flowing
from plus to minus through the inductor.
Taking the
Laplace transform of both sides gives
by the
differentiation theorem for Laplace transforms.
Assuming a zero initial current in the inductor at time 0, we have
Thus, the
driving-point impedance of the inductor is

.
Like the capacitor, it can be analyzed in steady state (
initial
conditions neglected) as a simple resistor with value
Ohms.
Mechanical Equivalent of an Inductor is a Mass
The mechanical analog of an inductor is a
mass. The voltage

across an inductor

corresponds to the
force 
used to
accelerate a mass

. The current

through in the inductor
corresponds to the
velocity

of the mass. Thus,
Eq.

(
E.4) corresponds to
Newton's second law for an
ideal mass:
where

denotes the
acceleration of the mass

.
From the defining equation

for an inductor [Eq.

(
E.3)], we
see that the stored magnetic flux in an inductor is analogous to mass
times velocity, or
momentum. In other words, magnetic flux may
be regarded as electric-charge momentum.
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