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A Suggested Method for Determining Whether the "Faraday emf" or Spin-Induced Electric Field is the More Correct View in the Case of a Spinning Faraday Disc

G.R.Dixon, 1/31/2006

Author's Note: the RB-100 magnet, available from www.magnetshop.com, might make a good Disc-Shaped permanent magnet/conducting disc (or Faraday Disc) combo.  I have ordered one.

In a previous discussion, two paradigms for the emf in a circuit incorporating a spinning Faraday disc were considered. The first paradigm focuses upon the so-called "Faraday emf." According to this view the current-producing emf in Fig. 1 is caused by magnetic forces on disc conduction electrons. There are no electrical forces. The second paradigm proposes that the spinning magnet engenders an electric field with radial components, and that each conduction electron in the entire circuit experiences an electric force. In the disc part of the circuit, however, the electric force on each conduction electron is hypothetically equal but oppositely directed to the magnetic force. The net Lorentz force on a disc conduction electron is theoretically zero, and the emf between disc center and periphery is accordingly zero. This leaves the electric forces, on conduction electrons in the (stationary) closing wire and shaft unopposed and there is again a net emf around the circuit.

Figure 1

Spinning Faraday Disc and Hypothetical Spin-Induced Electric Field

In this article a method is suggested for deciding which of the two paradigms is correct. Fig. 2 depicts a conducting rod attached to the disc’s surface. (A second rod is included for rotational balance.) The conducting rod is insulated from the conducting disc.

Figure 2

Faraday Disc with Attached Radial Conducting Rod

If we consider the "Faraday emf" paradigm, we expect the conducting rod to be electrically polarized when the magnet/disc is spun. For the indicated w and magnetic polarity, the peripheral end of the rod should be positively charged and the axial end should be negative.

If the spin-induced electric field view is correct, then the rod’s conduction electrons theoretically experience no net Lorentz force when |w|>0. There should accordingly be no electric polarization of the rod when the magnet/disc is spun.

Of course when |w| is reduced back to zero, any rod polarization should dissipate. How might we determine whether or not electric polarization has occurred after |w| has been returned to zero? Fig. 3 suggests an answer. The rod is divided into two rods, with a bridging diode between the two segments. The diode acts as a conduction electron check valve. Any electric polarization that occurs when |w|>0 should persist after |w| has been returned to zero.

Figure 3

Rod With Diode

Once the rod in Fig. 3 is returned to a state of rest, it should be a simple matter to jumper the diode with a voltmeter and to determine whether the peripheral half of the rod bears a residual, positive charge. If it does, then the "Faraday emf" view would seem to be indicated. If it does not, then the spin-induced radial electric field hypothesis (replete with the related idea that conduction electrons experience no net Lorentz force when the magnet/disc/rod are rotated) would seem to be corroborated.