# Direction of Induced I in a loop of conductor?

Discussion in 'Electronic Design' started by [email protected], May 26, 2007.

1. ### Guest

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%?

2. ### John O'FlahertyGuest

The current will not go back to zero unless the magnetic field
enclosed by the loop stops changing. The current will always be
proportional to the rate of change of the enclosed magnetic flux. A
change in flux is implied by moving the loop into or out of a static
magnetic field, or by changing the area of the loop, or by the
intensity of the magnetic field changing. An increase in N-S magnetic
flux is equivalent to a decrease in S-N magnetic flux, etc. The
current's direction will be such that the current's induced magnetic
field will tend to maintain the total flux through the loop constant.
So, the direct answer to your question is, the magnetic field must
stop changing for the current to fall to zero. If the field then
starts increasing from its steady level in the same direction, the
current will resume in the same direction.
If the magnetic field was increasing, and then was allowed to decrease
to zero, the current would reverse, then fall to zero. Then,
increasing the magnetic field in the original direction would cause
the current to rise in the original direction.

3. ### Guest

No. The direction of the current flow is totally predictable.

http://en.wikipedia.org/wiki/Right_hand_grip_rule

4. ### Guest

Thankyou for taking the time to post me a link, even if it isn't
relevant The direction is given by Lenz's Law which I completely
forgot about. Doh!

5. ### John LarkinGuest

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.

John

7. ### Guest

The right hand grip rule explicitly gives the the direction of the
current induced in a loop, or a stack of loops aka a solenoid. Lenz's
Law is much less immediately comprehensible.

http://en.wikipedia.org/wiki/Lenz's_law

But I suppose relevance is in the eye of the beholder.

8. ### Paul Hovnanian P.E.Guest

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.

9. ### JasenGuest

probably not.

Bye.
Jasen

10. ### John LarkinGuest

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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