Download A Brief History of Planetary Science

Electric Field
Physics 102
Professor Lee Carkner
Lecture 10
Force on Charges
Fy =
8.99X109(4)(2)/(22)
= 1.8X1010N
Fx =
8.99X109(2)(5)/(32)
= 1X1010 N
F2 = Fx2 + Fy2
F = 2.1X1010 N
tan q = (Fy/Fx)
q = arctan (Fy/Fx)
q = 61 degrees
q3 = 4 C
2m
q1 =-2 C
q
3m
q2 = 5 C
Electric Field at Origin
Ey = 8.99X109(4)/(22) =
9X109 N /C
Ex = 8.99X109(5)/(32) =
5X109 N/C
E2 = Ex2 + Ey2
E = 1X109 N/C
Field indicates
direction positive test
charge will move
tan q = (Ey/Ex)
q = 61 degrees (below
X axis)
q3 = 4 C
2m
q
3m
q2 = 5 C
Electric Field

E = k q/r2
Where k = 8.99X109 (N m2/C2)

Note that,
F = Eq0
mC = millicoulombs =
mC = microcoulombs =
We can draw the field lines to get a map of
the field
Examples of Fields
Field Lines and the Field
What is the force on a small positive test
charge?
Direction:

At any point, the F vector is tangential to the
field lines
Strength:

Density of lines proportional to field strength
Dipole

Called a dipole
Electric field is
strongest in the space
between them
Magnetic fields are
always dipolar
How to Draw Field Lines

Lines point from positive to negative

More charge, more lines
Lines must begin and end at a charge
or infinity
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

 The positive will go to one surface and the negative will go
to the other side

 The field inside the conductor is zero
 For any point inside, the forces from all the charges
cancel out
 A conductor shields the region inside of it
Faraday Cage

Called a Faraday cage
Your car is a Faraday cage and is thus a
good place to be in a thunderstorm

Conducting Ring
No E
Field
Inside
Field Lines
Perpendicular
Charges
Pushed
To
Surface
to Surface
Charge Distribution
How does charge distribute itself over a
surface?
A uniform surface will have a uniform
distribution

The charge arrangement will make the E field
perpendicular to the surface

Charge accumulates at points (for a nonuniform surface)

Next Time
Read 17.1-17.6
Homework: Ch. 16, P 28, 31, 32, Ch. 17,
P 2, 14
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
A fixed negative charge sits on a table. Directly
above the charge.
A) The field points down and a negative
charge would move up
B) The field points down and a negative
charge would move down
C) The field points up and a negative charge
would move up
D) The field points up and a negative charge
would move down
E) The field would be zero and a negative
charge would not move