# Lec3 - Physics 8B Professor Catherine Bordel Lecture 3 ASUC...

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Physics 8B Professor Catherine Bordel 09/01/10 Lecture 3 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley. Do not share, copy, or illegally distribute (electronically or otherwise) these notes. Our student-run program depends on your individual subscription for its continued existence. These notes are copyrighted by the University of California and are for your personal use only. D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. LECTURE I made a new request for the demo. One of the reasons why it didn’t work last time is that I used the rods as an intermediate tool to transmit the charge, but it’s actually much more convincing if you do it directly. If you use an intermediate tool you lose part of the charge. Let’s start with the demo, and then we’re going to study a quick example regarding the superposition principle. For electric force or electric fields, you can add up the individual contributions to find the resulting force or field. Demo: Two Styrofoam balls are free to rotate on a rod. Oppositely charged balls are shown to attract, like charged balls repel. If the balls are completely uncharged, there should be no attraction or repulsion. Humidity in the air affects the demo. Superposition Principle Let’s take a distribution of point charges. Two points A and B have point charges of equal charge, -q, that are equidistant from origin. Point C is the same distance from origin and has charge +q. We want to calculate ) 0 ( E v , the electric field at the origin from the contributions of point charges A, B, and C. ) 0 ( ) 0 ( ) 0 ( ) 0 ( C B A E E E E v v v v + + = The unit vector always points away from the charge creating the field. We can use the vector conventions from math, i ˆ in the x direction and j ˆ in the y direction. Remember that r r kq E ˆ 2 = v . 0 ˆ ) 0 ( ˆ ) 0 ( ˆ ˆ ˆ 2 2 = + = = = = B A B A B A E E i l kq E i l kq E r i r v v v v Student : Should that be negative? No, because the unit vector for B points in the opposite direction, so it is i ˆ . The contributions from point A and B cancel out, so the contribution only comes from point C.

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Lec3 - Physics 8B Professor Catherine Bordel Lecture 3 ASUC...

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