Dr. Jie Zou Principles of Physics II 1

Welcome to

PHY 1161: Principles of Physics II

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Chapter 19

Electric Charges, Forces, and Fields

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Outline Electric charge

Charge separation Polarization

Coulomb’s law Insulators, conductors and

semiconductors Example of applications:

photoconductive material and photocopier

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Electric charge

Positive (+) and negative (-) charge

“Like” charges repel and “opposite charges” attract.

SI unit for charge: coulomb (C)

Example: An atom

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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

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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.

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Polarization

Polarization Induced polarization

charge Example: attraction

between a charged object and small neutral objects

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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

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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.

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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