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Dr. Jie ZouPrinciples of Physics II1 Welcome to PHY 1161: Principles of Physics II

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Dr. Jie ZouPrinciples of Physics II2 Chapter 19 Electric Charges, Forces, and Fields

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Dr. Jie ZouPrinciples of Physics II3 Outline Electric charge Charge separation Polarization Coulomb’s law Insulators, conductors and semiconductors Example of applications: photoconductive material and photocopier

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Dr. Jie ZouPrinciples of Physics II4

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Dr. Jie ZouPrinciples of Physics II5 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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Dr. Jie ZouPrinciples of Physics II6 Electrons, protons and neutrons Electrons Charge = -e, and e = 1.60 x 10 -19 C Mass = m e = 9.11 x 10 -31 kg Protons Charge = +e Mass = m p = 1.673 x 10 -27 kg 2000 m e Neutrons Charge = 0, electrically neutral Mass = m n = 1.675 x 10 -27 kg m p

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Dr. Jie ZouPrinciples of Physics II7 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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Dr. Jie ZouPrinciples of Physics II8 Polarization Induced polarization charge Example: attraction between a charged object and small neutral objects

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Dr. Jie ZouPrinciples of Physics II9 Coulomb’s law Coulomb’s law: the electrostatic force between point charges is give by k = 8.99 x 10 9 N·m 2 /C 2 Direction of the force: Along the line connecting the two charges Like charges repel and opposite charges attract. Newton’s third law applies.

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Dr. Jie ZouPrinciples of Physics II10 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 m e = 9.11 x 10 -31 kg, m p = 1.673 x 10 -27 kg, q 1 = -1.60 x 10 -19 C, q 2 = 1.60 x 10 -19 C, G = 6.67 x 10 -11 N·m 2 /kg 2, and k = 8.99 x 10 9 N·m 2 /C 2 Answer: F e /F g = 2.26 x 10 39. Coulomb’s law: F = k|q 1 ||q 2 |/r 2 and Newton’s law of gravity: F = G m 1 m 2 /r 2 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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Dr. Jie ZouPrinciples of Physics II11 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

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