Lecture02

# Lecture02 - The Coulomb Force 1 Q1Q2 1 Q1Q2 ^ F =F= F= r 2...

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The Coulomb Force Force repulsive Force attractive 1 + + 2 r F 12 + - 2 r F 12 1 ! F = 1 4 !" 0 Q 1 Q 2 r 2 ˆ r ε 0 = permittivity constant 2 2 1 0 4 1 r Q Q F F !" = = ! The force exerted by one point charge on another acts along line joining the charges. The force is repulsive if the charges have the same sign and attractive if the charges have opposite signs.

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How Strong is the Coulomb Force
q F E / ! ! = [N/C] ( ) t z y x E E , , , ! ! ! Electric Field Electric field has units of Newton per Coulomb:

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No ‘self-force’! 2 0 4 1 r q E !" = Point charge does not exert field on itself! r 0, E !
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.

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The Superposition Principle
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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+q -q s x y z Choice of origin: use symmetry Calculating Electric Field Choice of the origin
1.

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