Chapter 32Electrostatics

Electrostatics:the study of electrical charges at

rest

Ion:an atom or molecule with a

charge.

A charged object is created by the separation of charges: 1. an atom is electrically

neutral; it has the same number of protons (positive charges) as

it does electrons (negative charges)

A charged object is created by the separation of charges:

2. objects are charged by adding or removing electrons

A charged object is created by the separation of charges:

3. a positive charge occurs when there are fewer electrons

than protons(Cation)

A charged object is created by the separation of charges:

4. a negative charge occurs when there are more electrons

than protons(Anion)

Law of Conservation of Electric Charge :

The net amount of electric charge produced in any process is zero. If one region or object

acquires a positive charge, then an equal amount of negative

charge will be found in neighboring regions or objects.

During the last century, it became clear that electricity begins inside the atom itself

In a simplified view, the postivively charged nucleus

(containing postively charged protons) of the atom is

surrounded by negatively charged electrons

Electrons move from place to place or as we sometimes say

shell to shell.They can even leave all

together and go to another atom or molecule.

When objects are charged by rubbing, the electrons (which

are free to move) are transferred from one object to

another

Outer electrons are the ones that can move from object to

object. These are called valance electrons.

If you rub a rubber rod with a fur cloth then the electrons leave the fur cloth and go into the

rubber rod.What is the charge of the rod?

-What is the charge of the cloth?

+

Rub a glass rod with silk and the electrons go from the rod

into the silk.What is the charge of the glass

rod?(+)

What is the charge of the silk?(-)

Coulomb’s Lawdescribes the electrostatic force

between two charged objects

(and is a lot like Newton’s law of gravitation)

Charles Coulomb used a torsion balance in the 1780's to investigate electrical forces. He found that if the

charge is doubled on a charged object, the electric force it exerts on another charged object is also doubled. He

found that if the distance between two charged objects was allowed to

increase, the electric force between them decreased with the square of the

distance between them.

-where k is Coulomb’s constant, or

k = 9 x 109 Nm2 /C2 (approximate value) -q is the magnitude of each charge in coulombs -d is the distance of separation in meters -F is the electrostatic force in Newtons. It is

either attractive or repulsive.

SI unit of charge is the Coulomb abbreviated “C”

1 C = 6.24 x 1018 electrons

Coulomb's law calculates the magnitude of the electric force between two charged objects, when the charges are known. The direction of the force is

always along a line joining the two objects. If the two objects have the

same sign, the force on either object is directed away from each other. If the two objects have opposite signs, the

force on either object is directed toward each other.

Similarities between Coulomb's law and the law of universal gravitation:Both are inverse square laws-the force is proportional to the inverse

square of the distance. Both are proportional to the product of a property of each body-mass for

gravity and charge for electricity

The greatest difference between Newton’s and Coulomb’s laws is

that Newton’s law deals with attraction whereas Coulomb’s

deals with attraction and repulsion.

Conductors are materials which permit electrons to flow freely from atom to atom and

molecule to molecule.

Insulators are materials which impede the free flow of

electrons from atom to atom and molecule to molecule.

Semiconductors can be made to behave as conductors or insulators. Silicon is a great

semiconductor.

Superconductors:At temperatures near absolute

zero certain metals acquire infinite conductivity. Once

electrical current is established in a superconductor the

electrons flow indefinitely.

Charge by Friction:when electrons are transferred by friction from one object to

another. Rubbing feet on floor.

Charge by contact:when electrons are transferred from one object to another by direct contact without rubbing.

(Touch a charged rod to a neutral rod it will transfer the charge to the neutral rod.)

Induction:charging an object without

touching the object.

Charge by induction happens in thunderstorms. Cloud bottoms are negatively charged and the

Earth’s surface is positive.

Static buildup in the clouds

The release of the charge