Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: induced emf again



I don't think so, Ludwik.
With the currents in the same direction, the driving emfs will also be in
the same direction. In the moving bar (stationary magnet) case you measure
the PD created by the terminal charges - this will be a rise in the
direction of VxB and the emf. If instead you move the magnet (staionary
wires) so as to get a current in that same direction, you will measure an
emf in that same direction - it will now be due to a CurlE=-dB/dt effect and
will be distributed around the loop but the direction will still be in the
direction of the current! I don't see what you are proving.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Ludwik Kowalski" <kowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, May 06, 2002 9:38 PM
Subject: Re: induced emf again


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