Understanding RL Circuits (Lab 9)

PHYS 212 Lab Understanding RL Circuits 2
where B is the magnetic flux through the inductor. This magnetic flux is the flux created
through the inductor by the current which passes through it (this is why inductance is more
properly called “self-inductance”). The inductors we encounter are often made of many turns of
wire and so it can be useful to note that the magnetic flux could be looked at as the magnetic flux
created by one turn of wire ( B-one ) multiplied by the number of turns of wire (N) making the
inductor. In other words, the more turns of wire an inductor has, the larger its inductance will be.
The inductance can also be increased by placing a ferromagnetic material within the center of the
inductor. The magnetic field created by the inductor in the current will magnetize this core; as
the inductor’s magnetic field changes so will the magnetization of the core. This in turn will
correspond to a larger change in flux.
When an inductor is connected in series with a resistor of resistance R and a power supply or
battery with EMF , the inductor will oppose any change in flux (by trying to keep the current at
its initial value, zero). This will cause the voltage across the inductor to spike. The voltage
across the inductor then decays with time. As the inductor opposes the change in current, the
current will increase rapidly at first, then the rate at which the current changes will decrease as it
eventually reaches a final steady-state value. This process is described by the equations
t
i t 1 e
(Eq. 3)
R
and
V
L
t
t
e
(Eq. 4)
As with an RC circuit, the quantity t is called the time constant of the circuit. For an RL circuit,
the time constant is given by
L
(Eq. 5)
R
If an inductor has a current through it and then is removed from the battery in such a way that it
is still connected to a resistance R, the inductor will still oppose any change in flux (by trying to
keep the current at its initial value and direction, whatever those are). The current through the
inductor will decrease rapidly but then the rate will slow as it approaches zero. This process is
described by the equations
t
it
e
(Eq. 4)
R
and
V
L
t
t
e
(Eq. 5)