• Green sheet

• HW assignments (tentative)

• Course overview

See www.physics.edu/becker/physics51

OVERVIEW

C 2009 J. Becker

Electric charge: Chapter 21

•Protons have positive charge

•Electrons have negative charge

•Opposite signs attract

•Similar signs repel

•Electric field – used to calculate force

C 2009 J. Becker

PROTONS are massive and are held inside the nucleus. They do not move from place to place in an object.

ELECTRONS are not as massive and generally can move from one object to another. This is the way electric charge is transferred from one object to another: one object loses electrons and the other gains electrons

C 2009 J. Becker

LITHIUM (Li) ELEMENTAtom: electrically neutral 3 protons and 3 elec. Positive ion: missing one electron so net charge is positiveNegative ion: has added electron so net charge is negative

Protons carry a +e charge

Electrons carry a -e charge

The fundamental unit of electric charge is

e = 1.60 (10)-19 COULOMB

C 2009 J. Becker

CONDUCTORS: materials that have freely moving electrons that respond to an electric field.

INSULATORS: materials that have fixed, immobile electrons that are not easy to move.

C 2009 J. Becker

Two positive charges or two negative charges repel each other. A positive charge and a negative charge attract each other.

Copper is a good conductor of electricity;

Glass and nylon are good insulators

Metal ball is charged negatively as shown in A

B A C

CHARGING METAL SPHERE BY INDUCTION

Charges are free to move in a conductor but are tightly bound in an insulator. The earth (ground) is a large conductor having many free charges.

In an insulator the charges can move slightly (called polarization of the insulator). A piece of paper is attracted to a charged comb because the positive charges are closer to the negatively charged comb (in the upper figure).

CHARGED COMB ATTRACTS PAPER

FORCEbetween two

charges is given by Coulomb’s Law:

| F | = k | Q qo | / r2

We can use our notion of the gravitational field to form the concept of

an ELECTRIC FIELD (E)

Recall force between two masses: F = m g g is the gravitational field (9.8 m/sec2)

| F | = G | M m | / r2

The force between two charges Q and qo is given by: F = qo E

| F | = k | Q qo | / r2C 2009 J. F. Becker

Coulomb’s Law: | F | = k | Q qo | / r2

Rearranged: | F | = | qo [k Q/r2] |

Gives us:F = qo E

where the electric field E is:

| E | = | k Q / r2 |

ELECTRIC FIELD LINES START AND END AT ELECTRIC CHARGES

An electric charge is surrounded by an electric field just as a mass is surrounded by a gravitational field.

ELECTRIC FIELD LINES PERPENDICULAR TO EQUIPOTENTIAL LINES

In Lab #2 you will use a

voltmeter to measure the equipotential

lines (in Volts) in order to

determine the magnitude and direction of the

electric field lines.

Electric field and equipotential lines for a charge near a conductor

Forces on electron beam in a TV tube (CRT)

F = Q E and F = m g (vector equations)

TV tube with electron-deflecting charged

plates (orange)

F = Q E

See www.physics.edu/becker/physics51

Review

C 2009 J. F. Becker