I Lenz's Law in action with different length magnets

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1. Oct 21, 2016

magnetics

Hi,
I was doing an experiment with Neodymium magnets and a long copper pipe (3cm internal diameter) to see Lenz's Law in action. At first I used a long bar magnet (about 15cm long) and there was little resistance to the magnet falling through the pipe. It was only slightly slower than a non-magnetic object. I then used a bar magnet of the same material and diameter that was only 3cm long and this magnet fell about 5 times slower. It took around 5sec to fall through the pipe compared to around one second for the 15cm magnet.

Can someone please describe why there is such a difference in the rate of falling (including using maths)?

It must have to do with the amount of electrical current each magnet generates within the conductor (copper pipe) and the extra weight of the longer magnet since F = mg.

Thank you.

2. Oct 21, 2016

drvrm

<The fall of a strong magnet inside a conducting pipe is damped by the gradually increasing and opposing magnetic force that

the pipe wall exerts on the magnet. If the pipe is long enough, the magnet eventually reaches a constant terminal

speed. The braking force on the magnet arises from circular eddy currents, also known as Foucault currents. These eddy

currents are generated in the pipe by the e.m,f, induced in the pipe by the time-varying magnetic flux that the falling magnet

produces.>

For details see <http://www2.fisica.unlp.edu.ar/materias/FEIII/OLD/2012/AJP000193.pdf> [Broken]

Last edited by a moderator: May 8, 2017
3. Oct 21, 2016

lychette

I would say that the extra weight of the long magnet was the major factor for the difference