Direction of Induced I in a loop of conductor?

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.

-- John

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

-- Bill Sloman, Nijmegen

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!

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

It's called "boreal" orientation, pretty fundamental stuff...

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.

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

-- Bill Sloman, Nijmegen

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.

probably not.

Bye. Jasen

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