Slow fall

Electricity and Magnetism Level 2

The above video shows a copper tube in which a magnet is dropped. Why does the magnet fall so slowly in the copper tube?

Copper repels magnets Air resistance slows down the magnet An opposing magnetic field is generated in the tube

3 solutions

Kushal Patankar
Mar 1, 2017

As the magnet falls under gravity through the copper tube a current is induced in the tube to keep the magnetic flux through the tube constant. Magnetic field so produced due to the current, opposes the motion of magnet while going inside the tube and also while coming outside. Since the copper tube has finite resistance, magnetic field induced is not enough to prevent the movement but it certainly slows the magnet down.

Animation below give the idea of how things work, you can assume tube to be a lot of thin rings arranged together.

Image Credit - MIT OCW

Pro level problem related to this : https://brilliant.org/problems/magnet-falling-in-a-copper-tube/

neelesh vij - 4 years, 2 months ago

Thanks for sharing this. Ronak's problems are really great.

Agnishom Chattopadhyay - 4 years, 2 months ago

What if instead of using a circular copper tube , we use a hollow equilateral triangle shaped copper tube ? Will the effect be still the same or in this case the cylindrcal magnet will fall faster ?

Aniruddha Bagchi - 4 years, 3 months ago

Effect surely will be same. It will fall in similar fashion as chage in flux is same in the tube.

Kushal Patankar - 4 years, 3 months ago

What if the circular copper tube is kept the same but the cylindrical magnet's base diameter is reduced to half of the original base diameter ? Will the effect be still the same or what ?

Aniruddha Bagchi - 4 years, 3 months ago

@Aniruddha Bagchi – This is an interesting question. If the magnet's radius is reduced, then its magnetic field strength will also decrease, and the resistive force acting on it would also decrease. On the other hand, its mass will also decrease. We cannot say directly if it slow down faster or slower.

In either case, we can be sure that the magnet will experience an opposing force due to currents generated in the copper tube.

Pranshu Gaba - 4 years, 2 months ago

@Pranshu Gaba – What if the shape of the cylindrical coil is altered , maybe a square or triangle crossection coil ?

Aniruddha Bagchi - 4 years, 2 months ago

@Aniruddha Bagchi – Your questions are quite fundametal ( annoying ). You will get your answers here , you just need to find them. 😊.

Kushal Patankar - 4 years, 2 months ago

@Kushal Patankar – Sorry for being too curious ☺. Thanks for the weblink.

Aniruddha Bagchi - 4 years, 2 months ago

@Pranshu Gaba – it maybe not possible due to friction of magnet ... it could be touched to coiled copper tube .There must not be a contact between the coil and magnet.

A Former Brilliant Member - 4 years, 2 months ago

The magnetic field slows it down

Noa Dobzinski - 4 years, 3 months ago

When magnet falls through copper tube there copper produce magnetic flux with opposite magnetic flux called magnetic induction.and due to produced magnetic flux repels each other due to gravity it falls with same speed until the resistance between copper tube and magnet reduced.

A Former Brilliant Member - 4 years, 2 months ago

Why is such a current induced?

Agnishom Chattopadhyay - 4 years, 3 months ago

For the current induced in tube is because of free electrons fluctuation causes to produce electricity.you might be heard about moving magnet inside the surrounded copper coil. Every element has an sufficient configurated electrons in there orbit and if there is more than sufficient i.e. outermost orbit electron is more than complete octate.So it called as free electron which can be donated by atoms that the copper has free electrons in there outermost orbit and that are mobile which are moveable.A magnet also same kind of electrons efficient example which has free electrons causes to produce electricity and electricity produce magnetic flux.Whenever magnet came in contact with other element it the fluctuation of mobile electron cause induced current which is very small amount called eddy current.It produce magnetic flux so copper itself act as magnet when it come in contact with other magnet. The falling magnet is moving cause to produce induced current through tube.

A Former Brilliant Member - 4 years, 2 months ago

Joy Patel
Mar 28, 2017

The answer lies in Lenz law. When the magnet falls down then a certain amount of electricity is generated.The coil tries to oppose the increasing magnetic feild by creating an opposing magnetic feild.This slows down the magnet.

Cheers!

Vijay Simha
Mar 16, 2017

This can be created on a larger scale and serve to entertain people in amusement parks. A one-hundred and fifty foot high copper cylinder with a 40 foot diameter hole through it and a corresponding magnet over which people stand and slowly descend down into a large pool of water.

That's right, this phenomenon of braking due to eddy currents is actually used in roller coasters and railroad cars to slow them. Its main advantage is that it causes very less wear and tear in the moving parts.

Pranshu Gaba - 4 years, 2 months ago

Slow fall

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