Re: A rotating disk PARADOX??

From: Bilge (dubious_at_radioactivex.lebesque-al.net)
Date: 06/30/04 

Date: Wed, 30 Jun 2004 04:20:30 -0000

 sal:
>A homopolar generator, as I believe they're called, generates electricity.
>So, it's necessary to put energy into it.
>
>The energy goes in as torque on the rotating shaft attached to the disk.

  That much, I got.

>Since angular momentum is conserved,

  That fact isn't always useful nor even physically relevant to the
question at hand. I think you are trying to get more out of the
picture than is physically contained in the premises.

> that torque must be balanced with an
>opposite torque applied to some other component of the system.

  OK, break the configuration into two pieces. (1) The spinning disk
and solenoid, and (2) the wire with the resistor. The power dissipated
by the resistor must be supplied by the torque acting on the disk.

  By hypothesis, the resistor dissipates the energy needed to keep
the wheel at a constant angular velocity. If we remove the wire and
resistor, the wheel speeds up indefinitely as long as torque is
applied. So, your hypothesis is that there must be torque supplied
by the wire and resistor such that the sum of the torques is zero
at equilibrium.

  If that is true, then it must also be true that the same torque exists
regardless of what causes the current flow. For example, I can get rid of
the solenoid and spinning wheel and connect the two ends of the wire to
battery terminals. Nothing related to the current in the wire or resistor
differs, so it must also be true that the same torque is produced just by
connecting the loop to a battery. So, whatever physical effect is resp-
onsible for conserving angular momentum, the effect has nothing to do with
wheel.

>The only other components in evidence are the magnet which provides the
>fixed field, and the resistor and associated wires which drain off the
>power. One or the other of those _must_ feel a torque, or angular
>momentum for the Earth as a whole is not conserved.
>
>Beyond that, please look at the diagram:
>
>http://physicsinsights.org/images/rotating_disk_with_solenoid.png
 
  I did look at it.
 
>
>and I do not know how to make the question any clearer.
 
  The dipole moment of the loop is an angular momentum.

>Simplicio has asserted that there's a torque on the resistor but when I
>tried to work out what the forces on the resistor would be, they appear to
>be directed toward the surface of the disk (put the resistor immediately
>above the disk, oriented radially, with very short connecting wires, to
>see this -- the B field from the currents in the disk appears to be
>parallel to the surface of the disk).
 
  All of that is irrelevant, since it doesn't matter _what_ the
source of emf is as far as the loop is concerned.

Relevant Pages

  • Re: A rotating disk PARADOX??
    ... A metallic disk is in the field, ... We run the wires from the disk to a resistor, ... > which must exert torque on the disk. ... > angular momentum from the disk to the Earth. ...
    (sci.physics.relativity)
  • Re: A rotating disk PARADOX??
    ... The torques on the disk itself obviously balance. ... disk, and no balancing torque. ... Put the generator in a box. ... > finite resistance of the resistor is the electrical equivalent to the ...
    (sci.physics.relativity)
  • Re: A rotating disk PARADOX??
    ... >>The energy goes in as torque on the rotating shaft attached to the disk. ... and the wire with the resistor. ... > by the resistor must be supplied by the torque acting on the disk. ...
    (sci.physics.relativity)
  • Re: A rotating disk PARADOX??
    ... The energy goes in as torque on the rotating shaft attached to the disk. ... Simplicio has asserted that there's a torque on the resistor but when I ... above the disk, oriented radially, with very short connecting wires, to ...
    (sci.physics.relativity)
  • A rotating disk PARADOX??
    ... We have a solenoid consisting of a single loop of wire, ... We connect wires to the middle and edge of the disk, ... which must exert torque on the disk. ... angular momentum from the disk to the Earth. ...
    (sci.physics.relativity)