Why do magnets fall more slowly through a copper tube?
This never ceases to blow my mind.
I love things like this! However, I do feel the need to go into a bit more detail, and to correct the very last bit, where they call it Lenz’s Law. Lenz’s Law is certainly relevant, as it dictates which direction the induced current will flow in, but the phenomenon that induces the current is called Faraday’s Law, which in general states that the electric field along a closed path is proportional to the rate at which the strength of the magnetic field penetrating an area bound by that path is changing. In this instance, the “closed path” is the circumference of the copper tube, which is a very good conductor; and in the presence of an electric field, a good conductor will generate a current that flows in the same direction as the electric field.
Now, here’s the the thing about nature: it is, in general, conservative, meaning that if something is changing, it’s going to resist that change, and in this case, that resistance takes the form of Lenz’s Law. As stated in the original post, the changing magnetic field induces a current, and the current induces its own magnetic field. Lenz’s Law essentially states that current induced by a changing magnetic field will flow in the direction that creates a magnetic field which opposes the change in the original magnetic field. So if the field is getting stronger, the current will produce a field that points in the opposite direction of the original, and if the field is getting weaker, the current will produce a field that points in the same direction.
This is essentially what we’re seeing here. If we look at a chunk of the tube that’s below the magnet, we see that, because the magnet is getting closer, the field through that chunk is getting stronger; thus, that chunk of the tube will produce a magnetic field that opposes the magnet’s field, pushing it upward. It’s essentially saying, “Whoa bitch, back the fuck up I will cut you I swear to Maxwell.”
However, that’s only half of the phenomenon! We also have to consider a chunk of the tube that’s above the magnet. In this case, the magnet is getting further away and so the field through that chunk is decreasing, so it produces a magnetic field that’s in the same direction as the magnet’s, pulling it upward. So it’s like, “Wait, no, come back, I’m sorry! I love you! I know what I said was hurtful before but I didn’t realize how much I wanted you in my life! Come back, please!”
But of course, the Earth’s gravitational field is stronger than the induced magnetic fields, and so the magnet continues to fall, into the chunks of copper tube that want it to back off, and out of the chunks of copper tube that wish it’d come back.