W1 D3
More on Chapter 14
New Groups are posted on our website.
The Electric Field – W1D3
No QUIZ today. First quiz next Friday
CLICKERS: MONDAY START
Last Time
Coulombs' Law
Vector Notation: <a,b,c>
Superposition
Symmetry
Today – The concept of the Electric Field
Possibly some discussion about the electric dipole
Next Major Topic: Charge and Matter
Note: There is a NEW WebAssign that will appear
sometime today. Topic: E-Field
F
1
q1q2
4 0 r2  r1
r

r
2
 (Unit Vector in the (r2 - r1 )direction)
r
tinu
(r2 - r1 )unit
r2 - r1

r2 - r1
In the figure, what are the horizontal and vertical components of the
resultant electrostatic force on the charge in the lower left corner of the
square if q = 1.5 x 10-7 C and a = 4.0 cm? (Assume the positive
directions are upward and to the right.)
r1  a, 0, 0 
y
2
3
r3
r2
r2  a, a, 0 
r3  0, a, 0 
r1 = r3  a
r2  (a)2  (a)2  (0)2  2a 2  a 2
1
r1
x
Note that r goes from the origin to the point where we are computing
the force. For our example, r=0.
runit
r
=
r
r1  a, 0, 0 
r2  a, a, 0 
r3  0, a, 0 
r1 = r3  a
r2  (a)2  (a)2  (0)2  2a 2  a 2
q  2 x 2qr1unit 2qr2unit 2qr3unit
F



2
2
4 0 
r1
r2
r32
q2

4 0
 4r1unit 2r2unit
r3unit

2 2

2
2
r2
r3
 r1
q2

4 0
 4r1 2r2 2r3 
q2
 3  3  3  `
r2
r3  4 0
 r1






 4r1
2r3 
2r2
 3 
 3  3
2a 2 a 
 a
r1  a, 0, 0 
2q
F
4 0
 2qr1unit qr2unit qr3unit



2
2
2
r
r
r

3
2
1



r2  a, a, 0 
r3  0, a, 0 



2q 2

4 0
 2r1unit r2unit r3unit
 2


2
2
r3
r2
 r1
2q 2

4 0
 2r1 r2 r3  2q 2  2r1
r3 
r2





 3
 3
3 
3
3
3

4
a
a
r
r
r
2
a
2
 1


0
3 
2

2q 2

4 0 a 3


a

0
,
a

0,



0
,

,
a




0
0,
,
a


2



2
2


2q 2

4 0 a 3


a

0
,
a

0,



0
,
,
a



0
0,
,
a
2



2
2


2q 2

4 0 a 2


1

0
1,

0,



0
,
1,



0
0,
2,

  0.6, 0.16, 0  NEWTONS

2 2


A proton is placed a distance d above a table and is then released.
Describe its motion. The room is in a vacuum.


“wt_?”
d
d
Observed
F  qE
F
E
q
E(r)
1
1