This is my homework. I am not asking you to complete it for me - I just need a push in the right direction so I can complete the rest. The question is translated from German, so it reads a bit awkward. I have read the homework questions guidelines and am not trying to be rude. My specific questions are after the actual assignment. I have added the assignment so you (hopefully) understand the context.

A square conductor loop of length $l$ and mass $m$ is located between two square pole shoes of a magnet of the surface area $l^2$ between which the homogeneous $\vec B$ field prevails. Outside the space directly between the shoes there is no field. $x$ indicates the position of the loop as shown in the figure. From the sketched initial position at $x = 0$ in which the conductor loop initially rests, the latter falls downwards under the influence of the gravity.

Calculate how long it takes to leave between the pole shoes in the following task points.

A) In which direction a current flows in the conductor loop during the fall

B) Specify in which of the four sides a voltage $U$ is induced during the fall time. Calculate the magnitude of the voltage and the current $I$ flowing in the loop with the resistance $R$.

This is the figure: https://www.dropbox.com/s/sh31whsvuwndten/Screenshot%202017-05-29%2017.08.09.png?dl=0

My specific questions:

In which direction does the vortex field rotate? In which sides does a current get induced and why?

  • $\begingroup$ Can you include a link of the figure? $\endgroup$
    – BLAZE
    Commented May 29, 2017 at 19:57
  • $\begingroup$ "as shown in the figure" located where? Until you add this we can't really answer your question. $\endgroup$
    – BLAZE
    Commented May 29, 2017 at 20:17
  • $\begingroup$ Whoops. I forgot to add it. I will do so now. $\endgroup$ Commented May 29, 2017 at 21:05
  • $\begingroup$ @BLAZE Forgot to tag. I have added the photo now. Thank you. $\endgroup$ Commented May 29, 2017 at 21:25

1 Answer 1


As the loop enters the region with magnetic field, there is a change in flux in the loop. The current will flow in a direction to oppose this change of flux (Lenz's Law).

The current depends on the EMF and the resistance of the loop; the EMF is given by Faraday's Law.

The result will be that the loop is trying to resist falling - in other words the current will cause an upward force. Note that once the loop is fully inside the homogeneous field region and starts to fall "out", the flux is getting smaller and the direction of current will reverse.

Can you take it from here?

  • $\begingroup$ Thank you. I am familiar with that part of it. Also, I was careless in my original question; I forgot to add the figure. It is there now. Do you know to which side the electrons go, and why? That would help me. $\endgroup$ Commented May 29, 2017 at 21:35
  • $\begingroup$ Which way does the current have to flow to oppose the flux that appears in the loop when it enters the magnet? Use the cork screw / left hand rule... (or whatever rule you learnt). $\endgroup$
    – Floris
    Commented May 29, 2017 at 23:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.