Re: Inductors and Current

["Spagna Jr., George" <gspagna@RMC.EDU>]

Tina asks:
-----Original Message-----
> From: Tina Fanetti [mailto:FanettT@QUEST.WITCC.CC.IA.US]
> Sent: Wednesday, May 01, 2002 12:27 PM


> Let's say you have a battery, resistor and a coil in a circuit.  The
> current in the circuit has reached a constant value.  The coil won't
> have inductance because the current is not changing and inductors
> need a changing current to work.

> The next part asks if the coil affects the value of the current in
> the circuit.  I believe that is would. As the current changes it
> adds more resistance but as the current is steady doesn't an
> inductor act like an ideal wire?

> Okay so it wouldn't change the current.  Right?

Tina - confusion about AC circuits is not unusual.  You've stated bits and
pieces of what I would take as the "correct" reasoning, but some of your own
statements may be leading you away from your own intuitive understanding.

The coil has (self) inductance, irrespective of whether current is present
in the circuit, since the underlying property of inductance derives from the
geometry of the coil.  What changes as the current changes is the "induced
voltage."  You're right, if the current is steady, the potential drop
measured across the inductor should vanish ... hence an "ideal wire."
Attempt to increase the current, the induced voltage is in the opposite
sense, hence is sometimes called a "back emf."  Attempt to reduce the
current (in particular, do so suddenly by opening the switch) and the
induced voltage again opposes the change, and we get a "forward emf."  This
is the cause of the spark sometimes seen at a switch with the contact is
broken.

So, at steady current, the coil might as well not be in the circuit, but try
to change the current and it most certainly will have an effect.

*********************************************
"If A equals success, then the formula is:
              A = X + Y + Z
          X is work. Y is play.
        Z is keep your mouth shut."
                       - Albert Einstein
********************************************
George Spagna
Department of Physics
Randolph-Macon College
P.O. Box 5005
Ashland, VA 23005-5505
phone: (804) 752-7344
FAX (804) 752-4724
e-mail: gspagna@rmc.edu
http://www.rmc.edu/~gspagna/




Tina (who has now confused herself)

Tina Fanetti
Physics Instructor
Western Iowa Technical Community College
4647 Stone Ave
Sioux City IA 51102
712-274-8733 ext 1429