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Today 3/10
Plates if charge
E-Field
Potential
“Plates of Charge”
Due Thursday, 3/13
Lab: “Electric Deflection of Electrons”
HW:
++++++++++++++++++
Looking from farther away
We call
this an
“infinite
plate.”
+ + + + +
Field lines for a positive plate
 is the charge on each square meter of the plate.
Near the plate the field lines must (by symmetry)
point directly away from the plate.
How does the field strength at the X’s compare?
Same field line density,
(+)
same strength!
Field lines for a plate
(+)
Field lines for a negative plate
(-)
Can’t tell if the field is caused by a pos plate below or a neg
plate above!
Size of the E-field
For plates:
0 = 8.8x10-12 C2/Nm2
For point charges:
k = 9.0x109 Nm2/C2

E
2 0
Note: no dependence on distance
kq s
E 2
r
Two Parallel Plates
(+)
(-)
Two Parallel Plates
(+)
(-)
Two Parallel Plates
Enet
(+)
(-)
Two Parallel Plates
Enet
Enet
(+)
(-)
Enet = 0
Two Parallel Plates
Enet
Enet
(+)
Enet = 0
(-)
How big is E?
Enet
(+)
(-)
Enet = 0
How big is E?

E 
2 0
(+)

E 
2 0
E net
Enet = 0
(-)


0
What do the field lines look like?
(-)
(+)
E = 0 outside!
Points A and B are located as shown between two
large uniformly charged parallel plates. Which of
the following correctly describes what happens to
the electric fields at A and B if the plate on the right
is removed?
a. The field at A drops more
+Q
-Q
than the field at B.
b. The field at B drops more
A B
than the field at A.
c. The fields at A and B
remain that same.
d. The fields at A and B drop
by the same amount.
Charged conducting
plate
0 = Q/A
A = Area
of one side
What’s wrong with
this picture?
Charged conducting
plate
L = 0/2
R = 0/2
Free charge
always goes to
surface of
conductor.
Charged conducting
plate
L = 0/2
EL = L/20
EL = 0/40
What is the
electric field inside
the conductor?
R = 0/2
Charged conducting
plate
L = 0/2
R = 0/2
ER = R/20
ER = 0/40
What is the
electric field inside
the conductor?
Charged conducting
plate
L = 0/2
R = 0/2
The electric field is
zero everywhere
inside the
conductor.
What is the
electric field inside
the conductor?