• Green sheet

• Online HW assignments

• Practice Problems

• Course overview

See course website www.physics.sjsu.edu/Becker/physics51

OVERVIEW

C 2012 J. Becker

PHYSICS 51 GREEN SHEET

Front side: Course outline

Back side: Reading assignment and problems

3 slides printed on a page

Chapter 21A Electric Field and Coulomb’s Law

• Electric charge (sec. 21.1)• Conductors, insulators,

and induced charge (sec. 21.2)• Coulomb’s Law (sec. 21.3)• Electric field lines (sec. 21.6)

C 2012 J. Becker

Learning Goals - we will learn:

• The nature of electric charge.• How objects become electrically charged.• How to use Coulomb’s Law to calculate the electric force between charges.• How to calculate the electric field caused by electric charges.• How to use the idea of electric field lines to visualize electric fields.

Electric charge

•Protons have positive charge

•Electrons have negative charge

•Opposite signs attract

•Similar signs repel

•Electric field – used to calculate force between charges

C 2012 J. Becker

Photocopiers are amazing devices. They use electric charge to hold fine

dust (toner) in patterns until the pattern may be transferred to paper

and made permanent with heat.

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

A positive charge and a negative charge attract each other.

Two positive charges (or two negative charges)

repel each other.

Figure 21.5

Electric forces in action

Figure 21.1a

Figure 21.1b

Figure 21.1c

Figure 21.6a

Copper is a good conductor of electricity;

Glass and nylon are good insulators

Figure 21.6b

Figure 21.6c

CHARGING A 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.

POLARIZED insulator

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 A PIECE OF PAPER

e - PAPER DISPLAY (Kindle, i - PAD, etc.)wikipedia.org/wiki/Electronic_paper

Scheme of an electrophoretic display.

e - PAPER DISPLAY (Kindle, i - PAD, etc.)wikipedia.org/wiki/Electronic_paper

Each capsule contains an oily solution containing black dye (the electronic ink), with numerous

white titanium dioxide (white) particles suspended within. The white particles are

slightly negatively charged. Applying a negative charge to the surface electrode repels the

particles to the bottom of local capsules, forcing the black dye to the surface and giving the

capsule, or pixel, a black appearance. Reversing the voltage has the opposite effect - the particles

are forced to the surface, giving the pixel a white appearance.

An uncharged conductor can attract the charge imparted to paint droplets. Excess charges can flow to or from

“ground”

-e

Car door

Figure 21.2

The imaging drum is aluminum coated with selenium, which changes from an insulator to a conductor when illuminated with light.

LASER PRINTER USES CHARGED TONER

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 | / r2

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 and equipotential lines are perpendicular to each other

In Lab #2 a voltmeter is

used to measure the equipotential

lines (in Volts) in order to

determine the magnitude and direction of the

electric field lines.

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.sjsu.edu/Becker/physics51

Review

Vectors Review (See Chapter 1)

Used extensively throughout course

INTRODUCTION: see Ch. 1

C 2012 J. Becker

Vectors are quantities that have both magnitude and direction.

An example of a vector quantity is velocity. A velocity has both magnitude (speed) and direction, say 60 miles per hour in a DIRECTION due west.

(A scalar quantity is different; it has only magnitude – mass, time, temperature, etc.)

A vector may be composed of its x- and y-components as shown.

2 2 2

cos

sinx

y

x y

A A

A A

A A A

Note: The dot product of two vectors is a scalar quantity.

cos x x y y z zA B AB A B A B A B

The scalar (or dot) product of two vectors is defined as

sinA B AB

The vector (or cross) product of two vectors is a vector where the direction of the vector product is given by the right-hand rule.

The MAGNITUDE of the vector product is given by:

Figure 21.14

PROFESSIONAL FORMAT