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Today 2/24
Read 18.6 about E-Fields
note we put E-field first and Force second
E-Field Lines Chapter 18.7 Today
Lab: “Electrostatics”
Read 18.4 in text for background
HW:
None assigned
Electrostatics Lab: This should be Fun!
Tools:
•electroscope
•“proof plane”
•metal cup
•ground wire
•charging rod and
glass
delicate gold foil
proof plane
Proof Plane: What happens…
(Start with the proof plane uncharged)
...when the metal
touches a charged
insulator?
some rubs off
...when the metal
touches an
uncharged plate?
nothing
...when the metal
touches a charged
plate?
…when the metal touches the
center of the plate???
proof plane
charge moves through
the conductors
Two identical positive charges, A and B, are arranged as
shown. The distance from point C to A is twice the
distance from point C to B. Which of the following best
represents the electric field at point C?
A
a
d
b
c
B
C
e
Two identical positive charges, A and B, are arranged as
shown. The distance from point C to A is twice the
distance from point C to B. Which of the following best
represents the electric field at point C?
A
a
d
b
c
B
C
e
Field Lines
If we want to draw a picture of the E-Field everywhere at once
we must fill space with X’s and arrows.
Pepe le Pew:
Where is Pepe?
Where is Pepe?
Where is Pepe?
PU!
The closer we get
to the source, the
closer together
the lines of
STINK and the
bigger the PU.
PU!
What if it’s a HOT DAY?
More STINK
means more
lines.
Field Lines
Red “E-Field lines” display the
information more clearly.
If we want to draw a picture of the E-Field everywhere at once
we must fill space with X’s and arrows.
Field Lines
Red “E-Field lines” display the
information more clearly.
If we want to draw a picture of the E-Field everywhere at once
we must fill space with X’s and arrows.
Electric Field Lines:
direction of E-field vector at any point is parallel to the
lines near that point
magnitude of E-field vector at any point is
proportional to line density (line spacing)
number of lines leaving or entering a charge is
proportional to the size of the charge (more charge,
more lines)
away from positive charge and toward negative charge
What direction would the
force point on a small positive
charge at 1,2,3?
Compare the size of the force
exerted on a small charge at
a. 1 and 2
b. 2 and 3
The test charge is doubled
when at 2
a. how does the force
change?
b. how does the electric
field change?
What direction would the
force point on a small positive
charge at 1,2,3?
Compare the size of the force
exerted on a small charge at
a. 1 and 2 1 larger
b. 2 and 3
The test charge is doubled
when at 2
a. how does the force
change?
b. how does the electric
field change?
What direction would the
force point on a small positive
charge at 1,2,3?
Compare the size of the force
exerted on a small charge at
a. 1 and 2 1 larger
b. 2 and 3 3 larger
The test charge is doubled
when at 2
a. how does the force
change? 2x
b. how does the electric
field change? Stays
the
same