dr. jie zouprinciples of physics ii1 welcome to phy 1161: principles of physics ii
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Dr. Jie Zou Principles of Physics II 3
Outline Electric charge
Charge separation Polarization
Coulomb’s law Insulators, conductors and
semiconductors Example of applications:
photoconductive material and photocopier
Dr. Jie Zou Principles of Physics II 5
Electric charge
Positive (+) and negative (-) charge
“Like” charges repel and “opposite charges” attract.
SI unit for charge: coulomb (C)
Example: An atom
Dr. Jie Zou Principles of Physics II 6
Electrons, protons and neutrons
Electrons Charge = -e, and e = 1.60 x 10-19 C Mass = me = 9.11 x 10-31 kg
Protons Charge = +e Mass = mp = 1.673 x 10-27 kg 2000 me
Neutrons Charge = 0, electrically neutral Mass = mn = 1.675 x 10-27 kg mp
Dr. Jie Zou Principles of Physics II 7
Charge separation Charge separation:
Electrons can be separated from the atoms and transferred from one object to another, for example, through the rubbing process.
The atoms that lose electrons become positive ions, and those that receive extra electrons are negative ions.
Example: rubbing a piece of amber with fur Some electrons are transferred from the fur to the
amber. Amber is “-” charged and fur is “+” charged.
Conservation of electric charge The total electric charge in the universe is constant.
Dr. Jie Zou Principles of Physics II 8
Polarization
Polarization Induced polarization
charge Example: attraction
between a charged object and small neutral objects
Dr. Jie Zou Principles of Physics II 9
Coulomb’s law Coulomb’s law: the
electrostatic force between point charges is give by
k = 8.99 x 109 N·m2/C2
Direction of the force: Along the line connecting the
two charges Like charges repel and
opposite charges attract. Newton’s third law applies.
221
r
qqkF
Dr. Jie Zou Principles of Physics II 10
Example: Compare the electric and gravitational forces
Compare the electric and gravitational forces between a proton and an electron in a hydrogen atom. Data known:
r = 5.29 x 10-11 m me = 9.11 x 10-31 kg, mp
= 1.673 x 10-27 kg, q1 = -1.60 x 10-19 C, q2 =
1.60 x 10-19 C, G = 6.67 x 10-11
N·m2/kg2, and k = 8.99 x 109 N·m2/C2
Answer: Fe/Fg = 2.26 x 1039.
Coulomb’s law: F = k|q1||q2|/r2 and Newton’s law of gravity: F = G m1m2/r2
Similarities: The force decreases as the square of
the distance between the two objects.
Both forces depend on a product of intrinsic quantities.
Differences: Force of gravity: attractive; electric
force: attractive or repulsive. In astronomy: force of gravity
dominates; electrical forces play hardly any role.
In atomic systems: force of gravity plays essentially no role.
Dr. Jie Zou Principles of Physics II 11
Insulators, conductors and semiconductors Insulators: Materials in which charges
are not free to move. Conductors: Materials in which charges
can move about more or less freely. Semiconductors: Intermediate
between insulators and conductors. Real world applications:
Photoconductive material and photocopier