A bar magnet is brought near to a similarly sized bar of unmagnetised iron. The bar magnet induces a polarity in the iron. Which end of the bar of iron 𝑋 or 𝑌 will be the north pole of the induced magnet?
So we know we’ve got a bar magnet here on the left. And we’ve got a bar of unmagnetised iron on the right. We also know that the bar magnet induces a polarity in the iron bar. What we need to do is to find out which end of the iron bar, 𝑋 or 𝑌, will be the north pole of the induced magnet.
First of all, what does it mean to induce a polarity? Well, if we’ve got some unmagnetised iron, in this case in the shape of a bar, and we bring it close to a permanent magnet, then the unmagnetised iron also temporarily becomes a magnet itself. This is what it means when we induce a polarity, returning the unmagnetised iron into a temporary magnet.
So in this question, all we’re trying to do is to work out which end of the initially unmagnetised iron bar will become the north pole when the polarity is induced. To do this, we can first think about a slightly different experiment.
Instead of bringing an iron bar close to the permanent magnet, let’s think about bringing some iron filings close to the permanent magnet. When we bring these iron filings close to the permanent magnet, we see that they’re attracted to the magnet regardless of which end is brought close to them. That is to say, they’re attracted both to the north pole and to the south pole.
Now these iron filings are made from the same stuff as the iron bar. That means that a polarity is being induced in the iron filings same as for the bar of iron. But if these iron filings are always attracted to the permanent magnet, then that tells us something about the induced polarity.
When we induce the polarity in these iron filings, the poles have to be aligned so that the north pole of the iron filing attracts the south pole of the permanent magnet, and vice versa. And the same must be true for the bar of iron.
When we induce a polarity, it will be attracted to the magnet regardless of which end of the permanent magnet we bring close to it. So in this situation, when we bring the north pole close to the bar of iron, the bar of iron obviously will be attracted, which means that 𝑋 must be the south pole. And therefore 𝑌 must be the north pole.
And so our final answer is that the 𝑌 end of the unmagnetised iron bar will be the north pole. So to recap quickly, when we induce a polarity in unmagnetised iron, the poles that end up being generated in the temporarily magnetised iron are always aligned such that the iron is attracted to the permanent magnet.