Physics 212 Lecture 3, Slide 1

1
Physics 212 Lecture 3
Today's Concepts Electric Flux and Field
Lines Gauss Law
2
Index
Preflight 1
Net Torque
Net Force
Guess the Field
Point Charges
Preflight 2
Cube
Non-Uniform E (1)
Preflight 3
Game Hit The Cake
Non-Uniform E (2)
Music
Extra 2
Extra 3
Extra 4
Extra 5
Preflight 4
3
Preflight 1
Preflight 1
D
E
4
The cosine makes sure that the line count is the
same for both surfaces even though one surface is
bigger.
BACK
D
E
5
Preflight 2
6
Preflight 2

• EdS increases because the strength of the
  field increases as the charge gets closer to the
  element dS.
• As the positive moves from 1 to 2 to 3, the
  density of the field lines passing through ds
• decreases so the flux decreases.
• The electric flux through dS doesn't change
  because the number of lines passing through dS
  remains unchanged. It's the same number of lines
  leaving the shell.

D
E
7
1
2
The total flux is the same in both cases (just
the total number of lines)
The flux through the right (left) hemisphere is
smaller (bigger) for case 2.
8
The total flux is zero since as many lines enter
as leave the surface.
BACK
9
Preflight 3
B
D
A
C
10
Preflight 3
B
A
C
E
D
F
11
Preflight 3
B
D
A
C
BACK
12
Which of the following field line pictures best
represents the electric field from two charges
that have the same sign but different magnitudes?
A
B
C
D
BACK
13
Cube in Constant Field
y
Cube of side a Label faces 1 x 0 2 z a 3
x a
3
-q
2
x
Define Fn Flux through Face n F
Flux through Cube
2q
z
14
Cube in Constant Field Q
y
Cube of side a Label faces 1 x 0 2 z a 3
x a
3
2
x
Define Fn Flux through Face n F
Flux through Cube
z
How does Flux change?
BACK
15
Point Charges
-q
2q
What charges are inside the blue sphere?
16
Point Charges
-q
2q
BACK
17
Net Force
A dipole is made by attaching two opposite
charges to the ends of a rod
The net force on the dipole is A) Zero B) To
the right C) To the left
BACK
18
Net Torque
A dipole is made by attaching two opposite
charges to the ends of a rod
The net torque on the dipole is A) Zero B)
Clockwise C) Counter-clockwise
19
Since the field is uniform and the magnitudes
of the charges are equal, the rightward force on
the positive (top) charge is equal to the
leftward force on the negative charge.
The net force is zero
The net torque turns the dipole CW
BACK
20
Non-Uniform E (1)
A dipole is made by attaching two opposite
charges to the ends of a rod
The net force on the dipole is A) Zero B) To the
right C) To the left
21
The electric field is weaker toward the right
(field lines are further apart).
The leftward force on the negative charge is
bigger than the rightward force on the positive
charge
The net force on the dipole is to the left.
BACK
22
Non-Uniform E (2)
Suppose the dipole is initially oriented as
shown. It is released and is free to move and to
turn.
Which of the following statements best
describes its motion A) It remains motionless B)
It turns CW but does not move left or right C) It
turns CW and moves left D) It turns CW, moves
upward, and moves left
23
1
2
3
4
BACK
D) It turns CW, moves upward, and moves left
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Music

• Who is the Artist?
• Billy Joel
• Boz Scaggs
• Mark Knopfler
• Donald Fagen
• Michael McDonald

BACK
25
Ad Lib 2
BACK
26
Ad Lib 3
BACK
27
Ad Lib 4
BACK
28
Ad Lib 5
BACK