Announcements

Homework for tomorrow…
Ch. 26: CQ 4, Probs. 12 & 14
25.65: a) E1=(8.5 x 103 N/C) ihat-(4.0 x 103 N/C) jhat, E2=(1.0 x 104 N/C) ihat,
E3=(8.5 x 103 N/C) ihat+(4.0 x 103 N/C) jhat
b) Yes, as forces obey principle of superposition and F~E,
E-fields must obey principle of superposition. c) Enet=(2.7 x 104 N/C) ihat
CQ2: a) 1 line to left of right charge b) 3 lines to right of right charge
26.4: E = 7.6 x 103 N/C, 90o below horizontal
26.6: a) 2 nC
b) 180 N/C
Office hours…
MW 12:30-1:30 pm
TR 9-10 am
F 1-2 pm
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Tutorial Learning Center (TLC) hours:
MW 8-10 am, 12-6 pm
T 8-1 pm, 2-6 pm
R 8-12 pm, 2-6 pm
F 8-10 am, 2-5 pm
Chapter 26
The Electric Field
(The E-Fields of Rings, Disks, & Planes)

Linear & surface charge densities

E-field of a rod of length L &
charge Q in the bisecting plane..

E-field of an infinite line charge..
L
+ + + + + + ++ ++++ + + +++++
Last time…
r
P
Quiz Question 1
Two protons, A and B, are in an E-field.
Which proton has the larger acceleration?
1.
2.
3.
Proton A
Proton B
Both protons have the same acceleration
i.e. 26.4
E-field of a ring of charge..
A thin ring of radius R is uniformly charged with total charge Q.
Find the E-field at a point on the axis of the ring.
i.e. 26.4
E-field of a ring of charge..
A thin ring of radius R is uniformly charged with total charge Q.
Find the E-field at a point on the axis of the ring.


Q: What about for –z?
Q: What about for –Q?
Quiz Question 2
Positive charge, +Q, is uniformly distributed on the upper
half of a semicircular rod and negative charge, –Q, is
uniformly distributed on the lower half.
What is the direction of the electric field at point P, the center
of the semicircle?
+Q
1.
2.
3.
4.
5.
Up.
Down.
Left.
Right.
Up and to the left.
P
-Q
E-field of a disk of charge..
A disk of radius R is uniformly charged with total charge Q.
Find the E-field at a point on the axis of the disk.
E-field of a disk of charge..
A disk of radius R is uniformly charged with total charge Q.
Find the E-field at a point on the axis of the disk.
Notice:
 For z<0, same magnitude
but opposite direction
E-field of a plane of charge..
A plane is uniformly charged with uniform surface charge
density η. Find the E-field…
∞
+ + + + + + + + +++ + + + + + + + + + + + + + + + + +++ + + + + + + + +
∞
E-field of a plane of charge..
A plane is uniformly charged with uniform surface charge
density η. Find the E-field…
∞
+ + + + + + + + +++ + + + + + + + + + + + + + + + + +++ + + + + + + + +
∞
Notice:
 A constant!
 Zero height dependence?