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Here are random comments on subjects that seem to be confusing to
Physics 112 students.
2. Please explain the use of superposition.
1. I don't understand the difference between "Electric Field" and "Force."
See the last few slides in lecture 2, as well as the first few in lecture 3. (In the following, I'm leaving off the vector signs on forces and fields.)
Here's an analogy with gravity: if you're asked to figure out the gravitational force on an object at the Earth's surface (how much it weights), you know to calculate F = mg. The acceleration of gravity at the earth's surface (this is g) depends on the earth's mass and radius. You don't need to know anything about the object you're about to weigh if you only want to answer the question "What's the value of g?". You do, however, need to know the Earth's mass and radius (as well as Newton's gravitational constant G) to calculate it from scratch. Once you know/calculate/are told the value of g (this is the strength of the Earth's gravitational field) you can calculate the gravitational force on anything you want using the formula F = mg.
The gravitational field is the force per kilogram exerted on an object. You need to use information about the thing that generates the field (the eearth does this) in order to calculate the field, but you don't need to know anything about the object you might put into the field unless you're then asked to calculate the force, which is mass times field. Mass is the "charge" associated with the gravitational field.
If you're asked to figure out the Coulomb force on an object, you need to calculate F = qE. The electric field (this is E) depends on the arrangement of all the other charges. You don't need to know anything about the object that will feel this Coulomb force if you only want to answer the question "What's the value of E?". You do, however, need to know the positions and charges of everything else (as well as the appropriate numerical constant) to calculate it from scratch. Once you know/calculate/are told the value of E (this is the electric field) you can calculate the Coulomb force on anything you want using the formula F = qE.
The electric field is the force
per Coulomb exerted on a charged object.
You need to use information about the stuff that generates the field (the
other
charges do this) in order to calculate the field, but you don't need to
know anything about the object you might put into the field
unless you're then asked to calculate the force,
which is charge times field.
2. Please explain the use of superposition.
Subject: Superposition (Re:
1.1 and 1.2)
Date: Tue,
25 Jan 2000 21:19:08 -0600
From: "Mark Lippmann"
<lippmann@uiuc.edu>
Newsgroups: uiuc.class.physics112
Superposition:
1. Pick a charge, any charge.
2. Using coulomb's law, determine the magnitude
of the E-field for that
charge at the point in question.
3. Separate the magnitude into x and y components.
Now start worrying about
positives and negatives.
4. Looking only at the components of your current
charge, stick a positive
test charge at the point in question.
5. If the test charge gets sent in the negative
direction (based on
opposites attract, etc.), give that component
a negative sign. If the test
charge gets sent in the positive direction, leave
the sign alone.
6. Make two lists, x and y. Stick your two components
in the appropriate
lists.
7. Go on to the next charge and repeat the above
steps.
8. When you're out of charges, simply add up
each list, making sure you've
got the signs correct. Your added lists are the
x and y components of the
total (superposed) E-field at that point. This
works for forces also. I
think the confusing part is realizing you need
to separate stuff into
components before you even think about signs.
At least, that's the way I do
it.
Good luck.
Mark
==============================================================================
I seem to be in desperate need of some sort
of an explanation. I seem to
keep running up against the same problems all
over this homework. Could
someone please explain the use of superposition.
I understand the concept,
but the application is killing me. Anything
at all would be helpful.
Thanks.
University of Illinois Physics 112 FAQ
