Chapter 17

Electric Forcesand

Electric Fields

Chapter 17 Objectives

• Properties of electric charges• Conductor vs Insulator• Conduction vs Induction• Polarization• Coulomb’s Law• Electric field• Electric field lines

Properties of Electric Charge

• An electric charge is positive because it has lost electrons and other negatively charged particles.– Represented by +– Often drawn in red.

• An electric charge is negative because it has gained electrons and other negatively charged particles.– Represent by –– Often drawn in

black.This idea was first mentioned by Benjamin Franklin (1706-1790)

Behavior of Electric Charges

• Opposite charges attract one another.– Like charges repel.

• Electric charge is conserved.• The transfer of charge occurs because the

negative charge is transferred from one object to another.– So objects either gain or lose negative charge.

• In order to become positive, an object will lose a negative charge.

• Robert Millikan (1886-1953) discovered that charges are quantized, or said to have a fundamental unit of charge.– Meaning the charge is full integer multiples

• + e, + 2e, + 3e, etc.

Conductor v Insulator• A conductor is a

material in which electric charge moves freely.– Electrons are free to

move from atom to atom.

• Metals are good conductors.

• Water is a pretty good conductor.

• An insulator is a material in which electric charge does not move freely.– Electrons do not leave

their respective atoms– Can maintain a charge,

but only at the surface and it does not transfer to other regions of the material.

• Glass, rubber, wood are good insulators.

– Natural fibers are usually good insulators.

• Humans can be good insulators.

Conduction v Induction• Conduction occurs

between objects in contact with each other.

• The object being charged has no way for the charge to escape once it is being charged.

• The object doing the charging loses charge that is gained by the other object.– That way the newly charged

object is left with the same charge of the other object.

• Induction occurs between two objects not in contact with each other.

• The object being charged does have a path for charges to escape.– That is because the object

is grounded, or attached to the unlimited supply of electrons in the Earth.

• Induction lines opposite charges up along the surface of the objects.– This pushes the electrons

toward the grounded surface and the charge flows into the Earth.

Polarization

• The shifting of the centers of charge to favor one side of a molecule or the other is called polarization.

• This often occurs in insulators.• Also occurs in water because of

the unique molecule arrangement between oxygen and hydrogen.

Coulomb’s Law• Charles Coulomb (1736-1806) established

the fundamental laws that govern electric force between two stationary charged sources

1. The electric force is inversely proportional to the square of the separation, r, between the charges.

2. The electric force is proportional to the product of the magnitudes of the charges, |q1| and |q2|

3. It is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.• So you add the sign in after working the computations.

Coulomb the Electrical Newton?

• Coulomb’s Law is the electrical equivalent of Newton’s Universal Law of Gravitation.– Remember that stated there was an attractive force

between all objects depending on mass and separation.

• The set up is the same and must fit the laws stated by Coulomb’s Law

F = kC

(q1)(q2)

r2

Lowercase q stands for chargeMeasured in Coulombs (C)

r is the separation between charges

kC is the Coulomb Constant and is equal to 8.99 x 109 N•m2/C2

e- = -1.60 x 10-19 C e+ = 1.60 x 10-19 C e+ is called a proton

The Electric Field• Charged particles can have a varying effect on each

other in space.– Touching or not touching!

• This effect was best described by Michael Faraday (1791-1867).

• An electric field exists in the region of space around a charged object.– When another charged object enters this region, an electrical

force becomes present between them.

• The direction of the field always points from positive to negative.

• The strength of the field is defined as the magnitude of the electric force divided by the magnitude of its charge.– SI Units: N/C

E = F

q0

q0 is the reference charge, or center of the charge pattern

Electric Field From aSingle Point

• Another way to calculate the electric field generated by single point charge is to ask for the help of Coulomb’s Law– Use this formula if the electric force is unknown.

F = kC

|q| |q0|

r2E =

q0

E =

q0

kC

q

r2

Electric Field Lines

• Remember that the electric field points in a direction from positive to negative.

• An electric field line shows the path and magnitude of the electric field present around a single point charge.

1. Lines always point straight away from charge

2. The number of lines per unit area identify the field strength• The field is larger when the lines are closer

together.

Drawing Field Lines• Follow these rules for

drawing field lines1. Lines must begin at positive

and end at negative.• If there is no positive, start

at infinity.• If no negative, end at

infinity.2. The number of lines drawn

is proportional to the magnitude of the charge.

• More lines, larger the charge.

3. No two field lines cross each other.

• They may connect, but they never cross.

+

+ +

-

Electrostatic Equilibrium• When no net motion of charge occurs within a

conductor, the conductor is said to be at electrostatic equilibrium.

• An isolated conductor (Insulated from ground) has the following properties:

1. Electric field is zero inside the conductor.2. All charge resides entirely on its surface.3. Electric field just outside a charged conductor is

perpendicular to the surface.4. On irregularly shaped conductors, the charge

tends to accumulate at locations of the smallest radius of curvature, or tightest corners.