Download Class 8 , Physics 260 Electric Charge, Electric Field (3) 1. For a

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