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I am just studying for my physics final exam. I came across this example and am not satisfied with the answer.

enter image description here

My answer is that Z is South and the current would flow from Y to X because of the right hand solenoid rule.

But the model answer is that it flows from X to Y. Why?

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  • $\begingroup$ Well when moving the north pole of the magnet away from the coil, the magnetic flux through the coil is going to decrease. By Lenz's law, the induced EMF is going to induce a magnetic field that opposes this change in magnetic flux. Thus, the induced magnetic field will be pointing to the right, meaning the induced EMF will be traveling from X to Y (by the right hand rule). $\endgroup$
    – Muster Mark
    Commented Oct 28, 2014 at 7:52
  • $\begingroup$ Did you ever figure this out to your satisfaction? $\endgroup$
    – DanielSank
    Commented Nov 11, 2014 at 16:07

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Lenz's law tells you that the coil will act to try to keep the magnetic flux constant. You're removing a magnet bar with the north pole on the right, so the coil will try to produce a magnetic field to cancel that removal by making a $B$ as if the bar were sitting inside the coil.

As you correctly predicted, this means the end of the coil near the bar is the $S$ end. Now think about the current flow in the coil. If you curl the fingers of your right hand and stick out your thumb, the right hand rule says that current flowing out of your fingertips produces a magnetic field pointing along your thumb. Since we said the $B$ field has the $N$ end to the right, your thumb points to the right. This puts the current flowing from top to bottom on the illustration. If you just follow that with your eye, you find the current goes from $X$ to $Y$.

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    $\begingroup$ Well, maybe I get the 3D picture wrong, but the right hand rule gives me current flowing up in the back, and down in the front. That looks to me like going from $Y$ to $X$. (I agree that the field due to induced current should have North right) $\endgroup$
    – mikuszefski
    Commented Oct 28, 2014 at 14:21
  • $\begingroup$ @mikuszefski: Look closely at the picture. I agree that the current is going down in the front but I'm pretty sure that's $X$ to $Y$. $\endgroup$
    – DanielSank
    Commented Oct 29, 2014 at 5:56
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    $\begingroup$ I am looking for quite some time now, but I can't see it that way. The $X$ cable is connected to an end which is frontside, the $Y$ cable to an end that goes backside, is it not? Uff, getting dizzy starring at the image. $\endgroup$
    – mikuszefski
    Commented Oct 29, 2014 at 10:54
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    $\begingroup$ Yes, you are right: $X$ cable connected to front side. So if current runs downward in front it is running into $X$. $\endgroup$
    – DanielSank
    Commented Oct 29, 2014 at 14:00

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