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06 January 2010

Electostatics Introduction

Dang, but it's tough to teach electostatics.  The subject is so, so abstract.  I have no quantitative demonstrations; even getting qualitive demonstrations like hanging a balloon from a wall, or showing the repulsion of identically charged styrofoam balls, to work poses a challenge to me in my classroom.  Nevertheless, the AP curriculum demands that I make my best effort.  And, any student who continues into college physics will appreciate whatever exposure to electrostatics that I can give him.

The key to my approach is to AVOID COULOMB'S LAW as if it had occult powers.  Most textbooks start with the evil Law stating the force between two point charges.  Problem is, if you begin with Coulomb's Law, no matter how carefully you handle the presentation, many students will try to use Coulomb's Law for everything.  Got a charged particle hanging from a string in a known electric field?  F = kQQ/r2.  Got two parallel plates, want to know the electric field between them?  F = kQQ/r2.  Grr.

My solution, which has proven effective for me, is to begin with the definition of an electric FIELD, F = qE.  I spend several days doing nothing but straightforward conceptual questions and simple calculations with this equation.  I hammer over and over, making my students say the words:  Positive charges are forced in the direction of an electric field, negative charges are forced opposite an electric field.  Neither an electric field nor an electric force is ever "negative" -- rather, since both are vectors, we state the magnitude and direction.

The following quiz is given a day or two after we begin the study of electric fields.  Note that only the last question could really be considered at the AP level.  Nevertheless, students regularly bomb this type of quiz until I've given about four similar quizzes.

Next, I try to get the class to understand the difference between a point in space and a charge placed at a point in space.  You think I'm kidding?  See if YOUR class knows the difference.  :-)

1. Write the equation for the force of an electric field.

2. An electric field points right. What is the direction of the electric force on a +3μC charge in this field?

3. An electric field points north. What is the direction of the electric force on an electron in this field?

4. A 500 N/C electric field points left. What is the electric force on a -2 C charge in this field?

5. An electric field points to the right. An electron enters this field moving to the left. In one sentence or less describe the motion of the electron immediately after it enters the field.

6. The charge on an electron is 1.6 x 10-19 C; the mass of a proton is 1.7 x 10-27 kg. A proton is placed in an upward electric field of 200 N/C.
(a) What is the direction of the electric force on the proton?

(b) Which is bigger, the electric force or the gravitational force on the proton?

(c) About how many times bigger is the bigger force?

1 comment:

  1. I like this... to bad I already started on Coulombs Law... I won't make that mistake next year.