Where does the angular momentum go in Feynman's disk paradox?

In summary, this conversation discusses the transfer of angular momentum through a solenoid and disk system. By disconnecting the battery and decreasing the current, there is a decrease in the flux of the magnetic field through the disk, resulting in an induced emf according to Faraday's law. The angular momentum of the field is transferred to the disk, causing it to rotate counterclockwise, and this is similar to the classic rotating disk problem. The decrease in current also results in a counterclockwise swirl of the electric field inside the wire, which is transmitted through space. Feynman offers an explanation for this phenomenon.
  • #1
FallenApple
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There is a solenoid on top of a disk with charges attached to it. and there is a clockwise current through the solenoid.

E&M(rotating_disk).png


So by disconnecting the battery, the current will lower, and hence the flux of the magnetic field though the disk will lower, and according to faraday's law, there will be an induced emf in space.

So this is an explanation that the angular momentum of the field is transferred to the disk. It makes sense. I can imagine the e field swirling around the disk, causing it to rotate counterclockwise. (the induced current must be counterclockwise to oppose to decrease in flux)

But where was the angular momentum before? Was it from the decrease of clockwise swirl of the E field inside wire of the solenoid? When the current was being decreased, wouldn't there be a counterclockwise swirl of the E field to reduce the current?

This seems very analogous to the classic rotating disk falling on another stationary disk problem. Except here it isn't internal friction that mediates the transfer of angular momentum. So it seems like the counterclockwise E field's swirl inside the wire during the I reduction was transmitted though space.
 
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  • #2
Have you tried googling around for the solution to this problem? Feynman has even supplied an answer himself.
 
  • #3
https://www.researchgate.net/profile/Gabriel_Lombardi/publication/243489921_Feynman%27s_disk_paradox/links/5646523408ae9f9c13e7503e.pdf
 
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1. What is Feynman's disk paradox?

Feynman's disk paradox is a thought experiment proposed by physicist Richard Feynman to demonstrate the concept of conservation of angular momentum. It involves a spinning disk that is divided into two halves, with the two halves spinning in opposite directions.

2. Where does the angular momentum go in Feynman's disk paradox?

In Feynman's disk paradox, the total angular momentum of the system remains constant. However, the angular momentum of each half of the disk is opposite in direction and cancels out, resulting in a net angular momentum of zero.

3. How does Feynman's disk paradox relate to conservation of angular momentum?

Feynman's disk paradox serves as a demonstration of the law of conservation of angular momentum, which states that in a closed system, the total angular momentum remains constant. In this paradox, the angular momentum is transferred between the two halves of the disk, but the total remains unchanged.

4. Can Feynman's disk paradox be observed in real life?

Yes, the principles of Feynman's disk paradox can be observed in real life situations. For example, a spinning ice skater can slow down their spin by extending their arms and transferring their angular momentum to the air molecules around them.

5. What are the implications of Feynman's disk paradox in the field of physics?

Feynman's disk paradox highlights the importance of conservation laws in physics, particularly the conservation of angular momentum. It also demonstrates the counterintuitive nature of quantum mechanics and the need for complex mathematical explanations to understand seemingly simple phenomena.

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