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Ch 21 Electric Charge and Electric Field © 2005 Pearson Education

21.1 Electric Charge © 2005 Pearson Education Initially, rods neither attract nor repel

Two plastic rods rubbed with fur repel each other © 2005 Pearson Education

Initially, rods neither attract nor repel

Two glass rods rubbed with silk repel Each other © 2005 Pearson Education

Plastic rod rubber with fur Glass rod rubbed with silk

Silk attracts glass Rod form (d) © 2005 Pearson Education Fur attracts plastic rod from (b)

© 2005 Pearson Education Operation of laser printer

© 2005 Pearson Education Electric charge and the structure of matter

Mass of individual particles

21.2 Conductors, Insulators, and Induced Charges © 2005 Pearson Education

21.3 Coulomb’s Law Coulomb’s law: force between two point charges © 2005 Pearson Education

Coulomb’s law The magnitude of the electric force between two point charge is directly proportional to the product of the charges and inversely proportional to the square of the distance between them The magnitude of the electric force between two point charge is directly proportional to the product of the charges and inversely proportional to the square of the distance between them

21.4 Electric Field and Electric Forces © 2005 Pearson Education How does charged body A exert a force on charged body B?

© 2005 Pearson Education

definition of electric field as electric force per unit charge © 2005 Pearson Education

Unit vector point from source S to field point P

© 2005 Pearson Education At each point P, the electric field set up by an isolated positive point charge q points directly away from the charge in the same direction as r

Electric field of a point charge © 2005 Pearson Education At each point P, the electric field set up by an isolated negative point charge q points directly toward from the charge in the opposite direction from r

Example 21.5 What is the magnitude of the electric field at a field point 2.0m from a point charge q=4.0nC? What is the magnitude of the electric field at a field point 2.0m from a point charge q=4.0nC? ANS: ANS:

21.6 Electric Field Lines © 2005 Pearson Education The direction of the electric field at any point is tangent to the field line through that point

© 2005 Pearson Education

21.7 Electric Dipoles © 2005 Pearson Education

Electric dipole is a pair charges with equal magnitude and opposite sign separated by a distance d. Electric dipole is a pair charges with equal magnitude and opposite sign separated by a distance d. Electric dipole moment Electric dipole moment © 2005 Pearson Education

magnitude of the torque on an electric dipole torque on an electric dipole, in vector form potential energy for a dipole in an electric field © 2005 Pearson Education

Conductors are materials that permit electric charge to move easily within them. Insulators permit charge to move much less readily. Most metals are good conductors; most nonmetals are insulators.

© 2005 Pearson Education

Field lines provide a graphical representation of electric fields. At any point on a field line, the tangent to the line is in the direction of E-field at that point. The number of lines per unit area (perpendicular to their direction) is proportional to the magnitude of E-field at the point

© 2005 Pearson Education

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