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PHYS1322-CH22Lec2

# PHYS1322-CH22Lec2 - Lecture 5 Lecture 5 Lecture 5 Yesterday...

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Lecture 5 Lecture 5 R 2 R Lecture 5 Yesterday we introduced electric field and electric field lines Today we will cover some more topics on Electric Fields Dipoles Motion of a charge in an electric field Continuous charge distributions Question 1 Consider a circular ring with total charge + Q . The charge is spread uniformly around the ring, as shown, so there is λ = Q/2 π R charge per unit length. The electric field at the origin is R x y + + + + + + + + + + + + + + + + + + + + + + (a) zero 0 1 2 4 R πλ πε (b) (c) 2 0 1 4 R R π λ πε The key thing to remember here is that the total field at the origin is given by the VECTOR SUM of the contributions from all bits of charge. If the total field were given by the ALGEBRAIC SUM, then (b) would be correct. (exercise for the student). Note that the electric field at the origin produced by one bit of charge is exactly cancelled by that produced by the bit of charge diametrically opposite!! Therefore, the VECTOR SUM of all these contributions is ZERO!! •Lines leave ( + ) charges and return to ( - ) charges •Number of lines leaving/entering charge amount of charge •Tangent of line = direction of E •Local density of field lines local magnitude of E •Field lines cannot cross •Lines from isolated charges go to infinity Rules for Field Lines + - Field Lines From Two Opposite Charges Electric Dipoles Dipoles consist of a positive and negative charge Field lines leave the

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PHYS1322-CH22Lec2 - Lecture 5 Lecture 5 Lecture 5 Yesterday...

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