Do you include a +/- when giving a magnitude?

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2 total messages Started by chem Mon, 27 Sep 1993 05:25

#1810

Author: chem
Date: Mon, 27 Sep 1993 05:25

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Hi. I know this group is for sharing resources but I need
some help from human resources.  Being a novice high school
Physics teacher, I've come across some problems with those
pesky positive and negative signs with Coulomb's Law.
Would I say to the students that two unlike charges experience
an attractive force of -1 N (for example) or +1 N. Can the magnitude
of ANYTHING have a negative sign in front of it? Thanks for any
information. I appreciate it. Marsh Carroll
internet:  chem@uwpg02.uwinnipeg.ca

Re: Do you include a +/- when giving a magnitude?

#1814

Author: Jim Hetrick
Date: Mon, 27 Sep 1993 21:16

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Thus Spake chem:

|> Hi. I know this group is for sharing resources but I need
|> some help from human resources.  Being a novice high school
|> Physics teacher, I've come across some problems with those
|> pesky positive and negative signs with Coulomb's Law.
|> Would I say to the students that two unlike charges experience
|> an attractive force of -1 N (for example) or +1 N. Can the magnitude
|> of ANYTHING have a negative sign in front of it? Thanks for any
|> information. I appreciate it. Marsh Carroll
|> internet:  chem@uwpg02.uwinnipeg.ca

I think the crucial thing to convey about the force is that it is
a *vector* quantity, ie. has a magnitude and a direction. Thus for a
charge of +Q at the origin, a test charge of +q at (x,y,z) feels
the force 

	Qq * r_vec /4 pi r^3 = Qq(x,y,z)/4 pi r^3;  r^2 = x^2 + y^2 + z^2

If q is on the x axis, then 

	F = Qq(x,0,0)/4 pi x^3

which points away from the origin. If q < 0 (-q), then

	F = -Qq(x,0,0)/4 pi x^3 = Qq(-x,0,0)/4 pi x^3

which points toward the origin. The magnitude in each case is Qq/4 pi r^2.
[sorry about units; c = 1 in the above].

Cheers,  Jim



--
<>------------------------------<>---------------------------------<>
      James E. Hetrick            Institute for Theoretical Physics
     hetrick@phys.uva.nl               University of Amsterdam

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