Thomson's jumping ring: Why net repulsion?

In summary, Lenz's law states that a conducting ring will respond to changes in external magnetic fields by producing its own repelling or attracting field. In Thomson's experiment, which uses an AC coil with a changing field strength, the ring experiences more repulsion than attraction due to the inductance of the ring causing the current to be out of phase with the coil's current. This has been the subject of several research papers.
  • #1
greypilgrim
522
36
Hi.

According to Lenz's law, a conducting ring responds to an increasing external magnetic field with a repelling field and to a decreasing external magnetic field with an attracting one.

Thomson's experiment uses an AC coil, so its field strength changes between increasing and decreasing a 100 times per second (at 50 Hz). Yet there is a net repelling force making the ring jump. Why is there more repulsion than attraction?
 
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  • #2
I haven't looked at it in detail, but it has to do with the inductance of the ring, which makes the current in the ring out of phase with that of the coil enough so that the average force is repulsive. I think several papers have been written on that demo.
 

Related to Thomson's jumping ring: Why net repulsion?

1. What is Thomson's jumping ring experiment?

Thomson's jumping ring experiment is a physics demonstration that was first performed by physicist J.J. Thomson in the late 19th century. It involves a ring made of a non-magnetic metal, such as copper, that is suspended from a string and placed between the poles of a horseshoe magnet. When an electric current is passed through the ring, it jumps up and down, seemingly defying gravity.

2. Why does the ring jump when an electric current is passed through it?

The jumping motion of the ring is due to the interaction between the electric current and the magnetic field created by the horseshoe magnet. When the current flows through the ring, it creates a magnetic field that interacts with the magnetic field of the horseshoe magnet, causing the ring to experience a force and jump.

3. How does Thomson's jumping ring demonstrate the phenomenon of electromagnetic induction?

The jumping ring experiment is a classic example of electromagnetic induction, which is the production of an electric current in a conductor when it is exposed to a changing magnetic field. In this experiment, the changing magnetic field created by the horseshoe magnet induces an electric current in the ring, causing it to jump.

4. Why is the ring repelled by the magnet instead of being attracted to it?

The ring is made of a non-magnetic material, so it is not affected by the magnetic field of the horseshoe magnet. Instead, the electric current passing through the ring creates its own magnetic field, which interacts with the magnet's field and causes the ring to experience a repulsive force.

5. What are the practical applications of Thomson's jumping ring experiment?

Thomson's jumping ring experiment is primarily used as a physics demonstration to illustrate concepts like electromagnetic induction and the interaction between electric and magnetic fields. However, it also has practical applications in devices such as electric motors and generators, where the interaction between electric currents and magnetic fields is used to generate motion or electricity.

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