A magnetic field which opposes a falling magnet is created by an induced current.An induced current is built into a conductive tube with a falling magnet. Absent from a long solenoid, unless each turn is shorted to the next.
On Sunday, September 18, 2022 at 12:09:40 PM CDT, Antti Savinainen via Phys-l <phys-l@mail.phys-l.org> wrote:
Hi,
Nowadays I seldom get a question that I cannot answer from my students.
The reason is that after nearly 30 years of teaching physics, I've heard
a lot of questions.
The recent question I couldn't anwer is about a falling magnet, induced
emf, and eddy currents. If a solenoid is long, zero voltage is observed
when the magnet is fully inside the solenoid:
On the other hand, the effect of eddy currents is clearly observable
with naked eye if a neodymium magnet and an aluminium tube is used.
This is easy to explain using Faraday's law.
So far, so good?
A student asked why there are no eddy currents in the long solenoid when
the magnet is fully inside, since the situation looks quite similar to
the aluminium tube case. Of course, one clear difference is that the
long coil has a much larger resistance than the aluminium tube which
would limit the effect of eddy currents on the falling magnet if there
were an induced emf in the first place. However, there is a change in
magnetic flux through an individual coil turn in the solenoid (quite
similarly to the aluminium tube) when the magnet passes by, although
there is no net change of magnetic flux through the solenoid as a whole.
Perhaps I'm missing something important here. How would you answer the
student?