Is My Physics Teacher's Explanation of Dropping Magnets Through Pipes Accurate?

[Miracles]

My physics teacher explained to our class the traditional "dropping a magnet through a tube" experiment. As we all know, it slows down and takes longer than usual to fall through. However, he gave us an explanation that I am unsure about. I seem to remember reading about a different explanation and a different pattern of induced currents from another book, but I don't remember it that well. His explanation is as follow:

Suppose we drop a magnet with the south pole facing downward:
|---|
|---| <---position A
| N |
| S |
|---| <---position B
|---|
At position A, to create a field that prevents the magnet from falling, a field similar to a magnet with north pointing up, south pointing down is created. Hence, by right hand screw rule, the current is counterclockwise, looking from the top of the tube.
At position B, for the same reason, a field with south pointing up and north pointing down is created. Hence, the current is clockwise looking from the top.

I can't find anything wrong with the explanation, but it sounds somewhat suspicious that the current directions at two different positions would be in opposite directions. Not to mention that as the magnet moves downward, the clockwise and counterclockwise currents move downwards too.

Can anyone confirm if this explanation is really valid please?

 

[Andrew Mason]

Can anyone confirm if this explanation is really valid please?

It is. The direction of the induced current depends on the direction of the magnetic field and the direction of motion. The direction of motion does not change but the direction of the field does (ie. N pole changes to S pole) between the magnet entering the tube and the field as it is leaving.

AM

 

[thepatientmental]

The explanation given by your physics teacher is indeed valid. When a magnet is dropped through a tube, it creates a changing magnetic field. This changing magnetic field induces an electric current in the tube, which in turn creates a magnetic field that opposes the motion of the magnet. This is known as Lenz's law.

At position A, the magnet is approaching the bottom of the tube, so the induced current creates a magnetic field that repels the magnet and slows it down. At position B, the magnet is moving away from the bottom of the tube, so the induced current creates a magnetic field that attracts the magnet and slows it down. This is why the currents at these two positions are in opposite directions.

As for the current directions moving downwards with the magnet, this is due to the fact that the magnet is constantly creating a changing magnetic field as it moves, which in turn induces a current in the tube. The direction of this current is always opposite to the direction of the changing magnetic field, which in this case is downwards.

It is also worth noting that the strength of the induced currents and the resulting magnetic fields depends on the speed at which the magnet is dropped. The faster the magnet is dropped, the stronger the induced currents and the more resistance the magnet will experience as it falls through the tube.

Overall, your teacher's explanation is accurate and in line with the principles of electromagnetism. If you are still unsure or have any further questions, I would recommend discussing it with your teacher or doing some additional research to solidify your understanding.