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Lecture 3 - Why Why Are we spending so much time on these...

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Why? Are we spending so much time on these abstract things called electric fields? Because E-fields are real. They cause charges to move, currents to flow, and your toaster and brain cells to work. And because the basic rules for electromagnetism are rules about electric fields and magnetic fields. Why so much calculus in studio and no calculus on HW? Because the basic rules for electromagnetism can only be expressed in terms of calculus. And despite the good job are math department has done, applying calculus to physics problems the first time benefits from being in a room where you can get a lot of help. There will be 1 or 2 “calculus” problems on quiz 1!
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Electrical effects of dust The van de Graaf generator attracts small neutral The van de Graaf generator attracts small neutral objects (dust, pollen, water droplets) to it when charged. What would happen to one of these objects if it touched the generator. A. Nothing much; it would bounce off at the same speed it contacted the generator contacted the generator. B. It would stick to the generator. C. It would be repelled from the generator at high speed.
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The principle of superposition The electric field at any point in space r is the sum of the electric fi ld h ld b d d b ll h l i h i h fields at r that would be produced by all the electric charges in the universe. = N i r r 1 1 r r Thi i t ll d h f E fi ld l l ti I = × × × = i i i i i i r r r r Q r r E 1 2 0 | | | | 4 ) , ( r r r r r r r πε This is actually good enough for any E-field calculation. In many situations however, there are so many charges that are spread out so smoothly that we can think of the charge in coming in a continuous charge density. This density can come in the form of a charge per unit length, usually called λ ; a charge per unit area, called σ , or a charge per unit volume, called ρ .
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Effective point charge
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