Today 224

1
Today 2/24

• Read 18.6 about E-Fieldsnote 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

2
Electrostatics Lab This should be Fun!

• Tools
• electroscope
• proof plane
• metal cup
• ground wire
• charging rod and glass

delicate gold foil
proof plane
3
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?
proof plane
charge moves through the conductors
when the metal touches the center of the
plate???
4
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?
5
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?
6
Field Lines
If we want to draw a picture of the E-Field
everywhere at once we must fill space with Xs
and arrows.
7
Pepe le Pew
8
Where is Pepe?
9
Where is Pepe?
10
Where is Pepe?
The closer we get to the source, the closer
together the lines of STINK and the bigger the PU.
11
What if its a HOT DAY?
More STINK means more lines.
12
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 Xs
and arrows.
13
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 Xs
and arrows.
14
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

15
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?
16
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?
1 larger
17
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?
1 larger
3 larger
2x
Stays the same