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Re: induced emf again



Yes the net force is zero, but if there is resistance one of the forces is
a retarding force. Consider the limit when this retarding force becomes
irresistible (the rod is an insulator). Then no charges move or
accumulate; no E is established; yet qVxB still operates - balanced
completely by the retarding force.

(I have to leave now - will return this evening)

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Sunday, May 05, 2002 11:28 AM
Subject: Re: induced emf again


| Bob Sciamanda wrote [as corrected]:
| >
| > To answer directly: yes, qVxB = qE holds only for zero current (or
even
| > with current if the rod has zero resistance).
|
| I wouldn't have said that. When E is equal to v × B, that
| is the condition for zero net force on the electrons. Any
| steady straight-line current meets this condition, whether
| the current is zero or not, and whether the resistance is
| zero or not.
|
| =================
|
| Please, let's try to stick to the known physics (Maxwell
| equations, Lorentz force law, et cetera).
|
| There are one or two correct ways to analyze this problem.
| We know what they are.
|
| There are innumerably many incorrect ways to analyze this
| problem. It's not worth discussing each of them in detail.
|
| As an analogy, consider the problem of getting a hot saucepan
| off the stove. It is recommended to grab it by the handle.
| There are lots and lots of other places you _could_ grab it,
| but you'll get burned. After you've been burned enough times,
| you'll learn to grab it by the handle. (There are other, more
| complicated approaches that might work, but simply grabbing it
| at some random place is really not worth trying.)
|
| Don't sit on the cactus. There are better places to sit.
|
| Don't sleep in the middle of the highway. There are better
| places to sleep.
|