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Electric Field
Physics 102
Professor Lee Carkner
Lecture 11
Force on Charges
Fy =
8.99X109(4)(2)/(22)
=
Fx =
8.99X109(2)(5)/(32)
=
F2 = Fx2 + Fy2
F =
tan q = (Fy/Fx)
q = arctan (Fy/Fx)
q =
q3 = 4 C
2m
q1 =-2 C
3m
q2 = 5 C
Electric Field at Origin
Ey =
8.99X109(4)/(22) =
Ex =
8.99X109(5)/(32) =
q3 = 4 C
2m
3m
E2 = Ex2 + Ey2
E =
Field indicates
direction positive
test charge will
move
q2 = 5 C
To charged objects attract each other with force
F. If the amount of charge on one object is
doubled and the distance between the
objects is also doubled, what is the new
force in terms of the old?
A)
B)
C)
D)
E)
¼F
½F
1F (force is unchanged)
2F
4F
A charge +Q is placed in the center of a square.
When a charge –Q is placed on one corner
of the square the net force between them is
2 N. What is the net force on the center
charge if a charge –Q is placed on all
corners?
A)
B)
C)
D)
E)
0N
2N
4N
8N
You can’t tell from the information given
What information does the electric field give us
about a point in space?
A) The sign of the charges that produced the
field
B) The distance to the nearest charge
C) The magnitude of the electric force at that
point
D) The direction a positive charge would move
from that point
E) The velocity of a charge at that point
Field Lines
We defined the electric field as:
The field fills the entire region around a
charge
We would like to draw the field to give us a
“map” of how the charge will effect other
charges

Field lines indicate how a charge will move
Field Lines and the Field

Direction:

At any point the F vector is tangential to the
field lines
Strength:

Density of lines proportional to field strength
Examples of Fields
Dipole

Called a dipole

Magnetic fields are always dipolar
Dipolar Field
How to Draw Field Lines
Lines determine the force experienced
by a positive charge

Number of lines touching a charge
proportional to the magnitude of the
charge

Lines must begin and end at a charge
or infinity
Today’s PAL
Draw below a point charge of +3q, and to the
right, a point charge of –1q. Draw the electric
field lines between them. Draw at least 10
lines evenly spread out.
At a very large distance away, would a small
positive test charge be attracted or repelled?
If q is one electron’s worth of charge, pick a
point on a line connecting the charges and
compute E there.
Van De Graaff Generator

The positively charged belt attracts the
electrons in the dome making it
positively charged

Conveyer belt to remove electrons
Charge builds up until it is discharged
through the air
Van De Graaff
Why does your hair stand up when you
touch the Van De Graaff generator?

Why do you need to stand on the box?

Conductors and Fields

The charges in the conductor are free to
move and so will react to the field

Charge distributes itself uniformly over
the surface of a conductor
Inside the Conductor
 If we consider both positive and negative charge in
an electric field,

 A charge placed in the middle would feel an equal
force from both sides and not move, thus,
 The field inside the conductor is zero

 A conductor shields the region inside of it
Faraday Cage
If we make the conductor hollow we
can sit inside it an be unaffected by
external fields

Your car is a Faraday cage and is thus a
good place to be in a thunderstorm

Charge Distribution
 How does charge distribute itself over a surface?

 e.g., a sphere

 No component parallel to surface, or else the charges would
move

 Excess charge there may spark into the air
Conducting Ring
No E
Field
Inside
Field Lines
Perpendicular
Charges
Pushed
To
Surface
to Surface
Next Time
Monday, January 8
Read 17.1-17.6
Homework: Ch. 16, P 28, 31, 32, Ch. 17,
P 2, 14