I think this question caters to the student who wants his math to do all
of his physics for him (ie., plug and chug into a formula). We should
insist that math is our language - it does not know conceptual physics- we
must speak physics through the math. We must always be ready to step in
and let the math be our servant - not vice-versa. (Consider the multiple
math solutions which are not all physical -eg, quadratic equation
solutions.)
In this case, the math of "Ohm's Law" does not tell the whole story -
words must be added to make it fully useful. V=IR is only a magnitude
statement- absolute values are implied. It must be added that the
potential difference,V, is a drop in the direction of the conventional
current - and stress that this is so because this is the direction of the
driving electric field - the force on positive charge carriers. If this
is understood, the question will not be asked.
This lesson of managing the math to say the correct physics is best
learned by the exercise of programming a dumb computer (no
conceptualizing - only plug & chug) to solve your physics problems. Doing
our own math does not relieve us of this conceptual thinking process.
Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net http://www.velocity.net/~trebor
----- Original Message -----
From: "Robert Cohen" <Robert.Cohen@PO-BOX.ESU.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, February 11, 2003 10:04 AM
Subject: Re: Ohm's 'law' & minus sign
| It seems to me that the typical relationship between
| V, I and R is a relationship between scalars and, as such,
| does not need a minus sign. Is this correct?
|
| On the other hand, can we start with E =3D rho j (which is
| a vector relationship) and replace E with - Del V,
| rho =3D AR/L, and I =3D JA, to get a vector equation
| with Del V, L, R and I? In that case, the vector
| equation would necessarily have a minus sign, no?
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