Re: induced emf again

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

In the previous message I suggested an experiment to
test Feynman's idea that the so-called "flux rule" covers
two distinct phenomena. I wrote:

> Use a voltmeter to find out about the direction of E
> lines inside the rod, use a galvanometer to determine
> the direction of the current. ... The experiment  should
> be performed twice: (a) when the rod is sliding in a
> constant magnetic field and (b) when the rod is
> stationary but the magnetic field is changing. In
> the first case the direction of E inside the rod will be
> opposite to the direction of conventional current, in
> the second the two directions will coincide.

It turns out that using a galvanometer (to determine
the direction of the current) makes determinations
of the direction of E (in the cross bar) very difficult.
This has to do with the galvanometer's resistance
of about 100 ohms. The direction of I can be
determined with a compass needle.

Here are some practical considerations:

1) We have copper wire whose diameter is 5 mm.
The length of the U-shaped rail will be 5+5+5=15 cm;
the effective length of the cross bar will be 5 cm. This
means that the total R will be 0.0173 ohms.

2) Suppose B=0.5 T (a neodymium magnet) and that
dt=0.25 second. With the d(AREA)=25 cm^2 the
emf will be 0.01 volts. The current will be 1.15 A.

3) The voltage across the bar will 25% of the total
or 2.5 mV; its polarity can be measured with the
oscilloscope. Note that the voltage on the cross
bar would be negligible is q 100 ohm galvanometer
was inserted into the loop. The measured polarity
(for a fixed direction of electric current) is expected
to be different for two ways of changing the flux
(sliding the bar versus moving the magnet). Any comments?
Ludwik Kowalski