Re: Direction of Induced I in a loop of conductor?

On Sat, 26 May 2007 20:15:32 -0700, "Paul Hovnanian P.E."
<paul@xxxxxxxxxxxxx> wrote:

John Larkin wrote:

On 26 May 2007 06:19:38 -0700, jmc8197@xxxxxxxxxxxx wrote:

Suppose a loop of conductor is intitially carrying no current and is
placed in a changing magnetic field for time T, and the induced
current is allowed to go back to zero. If this is repeated with the
magnetic field changing in the same way, will the induced I circulate
clockwise 50% of the time, anti-clockwise 50%?

It depends on whether the loop has resistance or if it's a
superconductor. If it has resistance and the loop's L/R time constant
is fast compared to the rate of change of the external field, the loop
current will be proportional to the rate of change of the field.

If there's no resistance in the loop, the current will be proportional
to the instantaneous value of the appplied field, and follow its sign.

In between, it's in between.

To be precise, a changing magnetic field induces an emf in a conductor.
The resulting current flow depends on that plus the impedance in the
circuit.

In the case of a ring exposed to an external magnetic field, the
current in the ring generates a local field that fights the external
field, so the emf is a complex function of the result. In the case of
a superconductive loop, the induced emf is obviously zero, the current
is finite, and the field created by the ring exactly cancels the
applied field. And "changing" no longer applies: the current is
determined by the static applied field, with some constant of
integration lurking maybe. In other words, the L/R time constant of
the ring is infinite.

John



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