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Class 8 , Physics 260 Electric Charge, Electric Field (3) 1. For a uniformly charged disk with charge/unit area σ, and radius R, with the x axis perpendicular to the disk and passing through the center of the circle, an integration finds the electric field to be 1 σ 1 − r Ex = 2 R 20 1+ 2 x When R −→ ∞, then Ex = σ 20 independent of x. 2. Electric Field Lines –a method for visualization of the electric field. Rules for drawing: (a) E-lines come out of + charge and go the − charge or to ∞. (b) The number of lines out of (+) Q is ∝ Q. (c) The tangent to and E-line is parallel to E at that point. (d) The number of lines/⊥ unit area is ∝ E (e) In electrostatics, no E-line may close on itself, with no charge. 3. An electric dipole which has equal positive and negative charges separated by a distance d has a diple moment p given by p = qd where d points from the negative to the positive charge. 4. In a uniform external electric field, a dipole experiences a torque given by τ =p×E 5. In a uniform external electric field, a dipole has potential energy U = −p · E 2