Chapter 20

Electric Charges,

and Forces,

Units of Chapter 20

• Electric Charge

• Insulators and Conductors

• Coulomb’s Law

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Structure of Matter• All material objects are

composed of _____. • Atom gain or lose its _______

and become a/an ______.• There are ____ types of _____

such as ______ and _______ _____.

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Structure of atom• An atom consists of a _______

and a vast region of ______ outside the nucleus. ________ are present in the region of space outside the nucleus. They are _________ charged and weakly bound to the atom. _________ are often removed from and added to an atom by normal everyday occurrences.

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Summary of Subatomic Particles

Proton Neutron Electron

In nucleus In nucleus Outside nucleus

Tightly Bound Tightly Bound Weakly Bound

Positive Charge No Charge Negative Charge

Massive Massive Not very massive

Charged versus Uncharged ParticlesPositively Charged Negatively Charged Uncharged

protons > electrons electrons > protons protons = electrons

ELECTROSTATICS:

The study of the behavior of stationary charges.

ELECTRIC CHARGE

There are two types of electric charge. (positive and negative) Unlike charges - attractiveLike charges - repulsive

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What will happen?

• Between positive charged object and neutral object

• Between negative charged object and neutral object

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Methods of charging• By Friction - two objects are rubbed

together • By Induction - without touching • By Conduction - with touching• Grounding (the removal of a charge) - the

process of removing the excess charge on an object by another object of substantial size.

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The total electric charge of the universe is a constant:

Electric charge is conserved.

electric charge cannot be created or destroyed, however, it can be transferred from

one object to another.

CONSERVATION OF ELECTRIC CHARGE

The SI unit of charge is the coulomb (C).

Represented by: - the electron is -e, - the proton is +e.  e = 1.6 x 10-19 C

melectron = 9.11 x 10-31 kg mproton = 1.672 x 10-27 kgmneutron = 1.675 x 10-27 kg.

Charge is quantity

Insulators and Conductors

Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors.

Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.

Insulators and Conductors

If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Check your understanding• After a comb is rubbed on a wool, it is able

to pick up small pieces of paper. Why does the comb lose that ability after a few minutes?

• During a lighting storm, what is it about staying inside a car that makes it safe?

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Electric Force• The electrical force is a push or pull exerted

upon an object as a result of an interaction between electrical charges.

• the electrical force is a non-contact force.• The electrical force is vector quantity

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COULOMB’S LAW Coulomb’s Law states that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (q) and inversely proportional to the square of the distance (r) between their centers. The equation is:

F = electrostatic force (N)q = charge (C)k = 9x109 N. m2/C2

r = separation between charges (m)

The value of k can also be expressed in terms of the permittivity of free space (εo):

9x109 N. m2/C2

k is a proportionality constant (the Coulomb's law constant) The value of this constant is dependent upon the medium that the charged objects are immersed in.

If the charged objects are present in water, the value of k can be reduced by as

much as a factor of 80.

Coulomb’s Law

The forces on the two charges are action-reaction forces.

Coulomb’s Law

If there are multiple point charges, the forces add by superposition.

Coulomb’s Law example problem

Sphere A, with a charge of +6.0 μC, is located near another charged sphere B. It has a charge of – 3.0 μC and is located 4.0 cm to the right of A.

What is the force of sphere A on sphere B?

Coulomb’s Law example problem

A third sphere C, with a + 1.5 μC charge, is added to the configuration. If it is located 5 cm directly below sphere B. What is the net force on sphere B?

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Practice problems

• Page 552 # 9-13• Page 558 # # 26, 27, 31, 36, 40,

43, 48, 49,51 and 60

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Summary

• Electrons have a negative charge, and protons a positive charge, of magnitude

• Unit of charge: Coulomb, C

• Charge is conserved, and quantized in units of e

• Insulators do not allow electrons to move between atoms; conductors allow conduction electrons to flow freely

Summary• The force between electric charges is along the line connecting them

• Like charges repel, opposites attract

• Coulomb’s law gives the magnitude of the force:

• Forces exerted by several charges add as vectors