Activity Based Science Learning >A magnet falling through a conducting tube

Objective: To study the effect of eddy currents when a magnet falls through a conducting tube
Equipment: A metallic tube of length about a meter and inner dia about a centimeter, A strong, short, cylindrical magnet.
Introduction: When a magnet falls through a conducting tube, changing magnetic field is produced in the volume of the tube. Not only field is different at different places in the tube, it is also changing with time at any given place. Taking the long axis of the tube along the z-axis, the field change is largely in z- direction. A field changing in z-direction produces electric field in the circumferential direction. The electric field lines are circular, coaxial with the z-axis. This field drives an electric current in the circumferential direction. The energy is lost in joule heating and this comes from the mechanical energy of the falling magnet. The magnet thus experiences an upward force slowing it down. The magnet takes an extraordinarily long time to fall through the tube.
Procedure: Take a strong, short, cylindrical magnet. These are made of certain magnetic alloys like Niobium-iron-boron alloy. The size should be such that it can easily go through the aluminum tube you will be using. Take a similar looking piece of unmagnetized iron such as a nut or bolt and two or three more small objects made of different materials.
Keep the aluminum tube vertical and hold it in one hand. Drop different objects in the tube at the upper end and ask the students to estimate the time it takes for them to emerge from the other end. If your tube is 1 m long, it will take only a fraction of second and estimates will be difficult to make. But they will have in mind that it is much less than a second.
Now drop the magnet in the same way. Students will be amazed to see that the magnet is not coming out. It takes very long time as compared to other objects. The time depends on the wall thickness of the tube and the strength of the tube. For the tube that I use it is about 7 seconds, more than 25 times longer than the other objects.
Discussion: It is instructive to understand where does the upward force come on the falling magnet. To the advanced students you can discuss the direction of current in the tube. The current goes in circular paths on the tube. Above the magnet it is in one sense and below the magnet it is in the other sense. Suppose the north pole of the magnet is up and the south pole is down. The current above the magnet is anticlockwise as seen from the top and that below is clockwise. The axial component of the magnetic field is outward in the portion above the magnet and inward below the magnet. Use to check that in both cases the force is upwards.
When to do: Class 12, when you teach Faraday’s Law of electromagnetic induction and especially when you teach the induced electric field due to changing magnetic field.
Instrument cost and availability: The strong magnet will cost something like Rs.100. and have to be purchased from specialized stores dealing with strong magnets.


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