Rec03-Electric Fields from Charge Distributions Fa20.docx -...

PHYS 212 Rec 03
ELECTRIC FIELDS FROM CHARGE
DISTRIBUTIONS
Section #_21R_____________ Date: 09/07/2020
HYSICS OBJECTIVES AND READINGS
Physics objectives: The complete list of PHYS 212 course objectives is on Canvas.
This recitation focus on
1. Electric Charges, Fields and Forces, specifically
c. relating electric force on a charge and the electric field experienced by the
charge
d. determining the electric field created by point charges or symmetric
distributions of electric charge
Readings: Knight Chap 23
Useful Equations
F
1 on 2
=
K q
1
q
2
r
2
^
r
⃗
E
on q
=
⃗
F
onq
q
Electric field of a point charge:
⃗
E = K
q
r
2
^ r
Differential field element for an extended object:
d
⃗
E =
K d q
r
2
^ r
With K =
1
4 π ϵ
0
= 8.99 × 10
9
N m
2
C
2

PHYS 212 RECITATION
Useful integrals ( C is a constant)
∫ sin x dx =− cos x + C ∫ cos x dx = sin x + C
2

PHYS 212 RECITATION
EXERCISE 1: ELECTRIC FIELD FROM POINT CHARGES
The figure below shows four point charges located on a circle of radius R . You need
to determine the electric field at the circle’s center.
In the figure, sketch four vectors that
represent the contributions to the
electric field at the center of the
circle due to each of each of the four
individual point charges on the circle.
Where should the vectors begin?
Label
1. the field vectors by charge
subscript, e.g.,
⃗
E
1
,
⃗
E
2
,
⃗
E
3
,
⃗
E
4
Also indicate the direction unit
vectors:
^ r
1
, ^ r
2
, ^ r
3
, ^ r
4
. You may
draw the
^ r
vectors starting
.