Chapter 23 Electric Charge and Matters

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Chapter 23 Electric Charge and Matters

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Lightning
What causes lightning?
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Charge distribution in clouds
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The first electrical machine
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Electrostatic generators
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The Van de Graaff generator
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Static charge
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Electric Charge
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Origin of electric
In 1600, William Gilbert, physician to Queen
Elizabeth I, was the first one to coin the word
electric, deriving from elektron, which is
Greek for amber.
As early as 600 B.C., Thales of Miletus noted
that when the mineral amber rubbed by wool or
fur, it could attract small pieces of straw or
feathers.
In the first century B.C., the poet Lucretius
noted that lodestone could attract iron and did
not need to be rubbed.
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Charge
Charge is a property of matter that causes it to
produce and experience electrical and magnetic
effects.
The subject of the electrical effects of charges
at rest is called electrostatics.
The fact that like charge repel and unlike charge
attract led Charles du Fay to surmise in 1733
that there are two kinds of charges.
Benjamin Franklin, around 1750, proposed that a
single fluid flows from one body to another
during rubbing. He called the body that gains
fluid positively charged while the one loses
fluid negatively charged. He mistakenly believed
that glass gains the fluid during rubbing, thus
positively charged.
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Checkpoint 1
A
C
D
C
B
B
D
A
A
D
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Modern view
Matter is made of atoms, which consists of a tiny
nucleus of positive charge surrounded by
electrons of negative charge. In the lowest
energy state, atoms are neutral, i.e. with equal
amount of negative and positive charge. Different
atoms have different electron affinity. Thus the
rubbing action between two materials might cause
electrons to transfer from one to the other.
The SI unit of charge is the coulomb (C), which
is defined in terms of electrical currents. It
turns out that a coulomb is a large mount of
charge. A typical charge acquired by a rubbed
body is 10-8 C.
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Quantization charge
In 1909, R.A. Millikan measured the electrical
charge through the drag force on tiny oil
droplets and found that charge is quantized. The
smallest charge is approximately
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Charge and mass
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Conservation of charge
In an isolated system, the total charge is
constant.
Examples

• Franklins experiment
• (a) glass rod rubbed with silk.
• (b) person A received positive charge from the
  glass rod and person B received negative charge
  from the silk.
• (c) spark seen when either the knuckle of A or B
  was brought close to that of a neutral third
  person C.
• (d) spark not seen in the same situation as in
  (c) if A and B had touched each other beforehand.
  
• (e) Franklin concluded that equal amount of
  positive charge and negative charge were
  generated on rod and silk by rubbing and then
  transferred to A nd B respectively.
• 2. Na Cl- ? NaCl
• 3. n ? p e ?

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Conductors and insulators
As early as 1729, an amateur named Stephen Gray
discovered that most materials can be classified
into two groups conductors and insulators.
Conductors, such as metals and ionic solutions,
allow charge to flow freely while insulators,
such as wood, silk, and glass, do not.
Note that a third class of materials, called
semiconductors such as silicon, germanium, and
gallium arsenide, behave like insulators when
they are pure and behave like metals when certain
impurities are added in.
Charge relaxation time is about 10-12 s for
copper, 2 s for glass, 4 ? 103 s for amber, and
1010 s for polystyrene.
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Charging by induction
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Charging by induction
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Coulombs law
Charles A. Coulomb (1736-1806)
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Coulombs law
q1
q1
q2
q2
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Coulombs law
Force on q
unit vector from Q to q
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Coulombs law in Gaussian system
F in units of dyne, or g cm/s2
R in units of cm
q in units of statcoulomb
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The magnitudes of electrostatic and gravitational
force in a hydrogen atom
The electron and proton in a hydrogen atom are
0.53 ? 10-10 m apart. Compare the electrostatic
and gravitational force between them.
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Exercise
Designing experiments to prove that the following
expressions for the force between two point
charges are incorrect.
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Superposition of forces on electrical charges
the force acting on the point charge 1 due to
the presence of the point charge n.
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Two shell theorems

1. A shell of uniform charge interacts with a point
   charge outside the shell as if all the shells
   charge were concentrated at its center.
2. There is no net force on a point charge inside
   the shell due to the shells uniform charge.

Note that the shell theorems are results of the
fact
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Spherical conductors
Question Why there is no charge inside a
spherical conductors and the net charge is
distributed uniformly on the surface?
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Some examples
Three point charges lie on the x-axis shown
below. Both q1 15.0 µC and q2 6.0 µC are
fixed in positions. Determine the position of q3
such that its net force is zero.
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Some examples
Two identical small charge sphere, each having a
mass of 3.0 ? 10-2 kg and a charge of q. The
length of each string is 0.15 m, and the angle ?
is 5.0º. Find the magnitude of the charge q.
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Change in equilibrium position and frequency for
small oscillation amplitudes
q
q
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Change in equilibrium position ?
Change in oscillation frequency ?? for small
amplitude ? from
Restoring force
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Home work
Question (??) 1, 2, 19
Exercise (???) 1, 7, 12
Problem (??) 11, 19, 25, 26