Lecture_02

# Lecture_02 - How Strong is the Coulomb Force Electric Field...

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How Strong is the Coulomb Force

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q F E / = [N/C] ( ) t z y x E E , , , Electric Field Electric field has units of Newton per Coulomb:
No ‘self-force’! 2 0 4 1 r q E πε = Point charge does not exert field on itself! r 0, E →∞

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The net electric field at a location in space is a vector sum of the individual electric fields contributed by all charged particles located elsewhere. The Superposition Principle The electric field contributed by a charged particle is unaffected by the presence of other charged particles.
The Superposition Principle

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The E of a Uniformly Charged Sphere Can calculate using principle of superposition: r r Q E sphere ˆ 4 1 2 0 πε = for r>R (outside) 0 = sphere E for r<R (inside) Recall this every night before bed!
Electric dipole: Two equally but oppositely charged point-like objects What is the E field far from the dipole (r>s)? +q -q s Example of electric dipole: HCl molecule The Superposition Principle The electric field of a dipole:

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Another kind of dipole
+q -q s x y z Choice of origin: use symmetry Calculating Electric Field Choice of the origin

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1. E along the x -axis x x x E E E , , , 1 + + = = 1 4 πε 0 q r s 2 ( ) 2 + 1 4 0 q r + s 2 ( ) 2 E 1, x = 1 4 0 qr 2 + qrs + qs 2 / 4 qr 2 + qrs qs 2 / 4 r s 2 ( ) 2 r + s 2 ( ) 2 E 1, x = 1 4 0 2 qrs r s 2 ( ) 2 r + s 2 ( ) 2
2 2 0 , 1 2 2 2 4

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