Electrostatics Chapter 23. Week12. What’s Happening. Clicker use will start on Friday (maybe). We will use them today informally. There will NOT be a quiz this week. There WILL be a quiz a week from Friday. WebAssigns are now active. Get to work!!.
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Electrostatics Chapter 23
Week12
If lightening had actually traveled down the kite string, old Ben Franklin would have been toast!
Probably never happened, but good story!
Pivot
motion
Rubber rod
Pivot
Rubber rod
Rubber rubbed with
skin of dead rabbit
Pivot
Rubber rubbed with
skin of dead rabbit
We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE.
We will try to understand what charge is and how it behaves.
We add to the properties of materials:
Mass
Charge
Pivot
Glass rubbed with wool
Glass rubbed with wool
Pivot
Rubber rubbed with
skin of dead rabbit
Glass rubbed with wool
An object without either a (+) or () charge is referred to as being NEUTRAL.
Separation
Volunteer Please
++++++++++
++++++++
Easily Removed

+
Pivot
Rubber rubbed with
skin of dead rabbit
Metal Rod
Positive charge attracts negative charges.
Rod becomes “polarized.
Negative end is closer to positive charge
Distance effect causes attraction.
Balloon Physics
What happens when two surfaces touch or rub?
Bonding!
The Triboelectric Series
When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged.MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE
No! No!
No! No!
Force is a VECTOR!!
Force is a Vector !
The Unit of Charge is called
THE COULOMB
Smallest Charge: e ( a positive number)
1.6 x 1019 Coul.
electron charge = e
Proton charge = +e
Three point charges are located at the corners of an equilateral triangle as shown in Figure P23.7. Calculate the resultant electric force on the 7.00μC charge.
Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal, insulating rod, extending from the origin to the point x = d. As shown in Figure P23.10, a third small charged bead is free to slide on the rod. At what position is the third bead in equilibrium? Can it be in stable equilibrium?
The Electric Field
M
m
mg
Mg
Ming the
merciless
this guy is
MEAN!
The Force may not be with you ….
The
FORCE FIELD
Force
Big!
Force
o
Position
F
Q
A Charge
r
F
q
The spot where we want
to know the Electric Field
Superposition and Symmetry
Concept – Charge per
unit length m
dq= mds
Why?
q
dEy
dE
q
r
x
dx
L
setup
A line of charge
m=charge/length
(standard integral)
1/r dependence
Let's look at it...
Define surface charge density
s=charge/unitarea
dq=sdA
(z2+r2)1/2
dA=2prdr
dq=s x dA = 2psrdr
(z2+r2)1/2
q
(z2+r2)1/2
dq
thk=dr
dq=rdV=r x surface area x thickness
=r x 4pr2 x dr
(Note: I left off the unit vectors in the last
equation set, but be aware that they should
be there.)
REMEMBER THIS !