Properties of Electric Charges

• Glass on silk (+ve), plastic on wool (-ve) when rubbed together produce a static electric charge.

• Benjamin Franklin demonstrated that there were two types of electric charges, +ve and -ve

• Like charges repel unlike attract.

• Electric charge is not created but conserved.

Properties cont.

• +ve charge located in the nucleus 10-15m in radius, -ve charge located on the outer edge at10-11m

• In a gram of matter about about 1023 +ve and -ve particles. Most matter is neutral.

• As the nucleus is held firmly, electric charge is obtained by the gain or loss of electrons.

Robert Millikan 1886-1953

• In 1909 Millikan discovered that an electric charge is a multiple of a fundamental unit e (quantized) where e = 1.60219x10-19 C

• where C = coulomb

Insulators and Conductors

• Substances are classified by their ability to conduct electricity. In conductors electric charges move freely in response to an electric force. When a conductor is charged, the charge is readily distributed throughout the material. Ex. Copper. Silver.

• When charging an insulator the charge remains localized. Ex. Glass, plastic

Semiconductors

• Semiconductors have properties between an insulator and a conductor. They are used extensively in electronics

• Ex. Silicon and germanium

Charging by Conduction

• If a -ve charged object is placed near a neutral object the neutral object’s -ve charges are repelled leaving the object with +ve and -ve locations. Upon contact with each other the -ve charges will move onto the neutral object neutralizing the +ve locations

• This is called charging by conduction.

Charging by Induction

• If a -ve charge is placed near a neutral object that objects -ve charges are repelled. If a grounding wire is now connected to the neutral object the -ve charges will pass to the ground leaving the once neutral object with a net +ve charge. No contact was made between the two objects.

• This is charging by induction.

Charles Coulomb 1736-1806

• 1785 Coulomb established a fundamental law of electric force between two stationary charged objects

• F = ke q1 q2

r2

q1 = charge one q2 = charge two

r = the distance b/n the centers of charges

ke = Coulomb’s constant 8.9875x109N.m2/C2

Coulomb’s Law cont.

• The law is applicable to point charges of spherical distribution

• Electrical force is a vector quantity. Several forces acting collectively on an object are computed individually and then added as vectors

Electric Field

• Like gravitational force, electrical force acts through space. (physical contact is unnecessary). An electrical field exists in space around a charged object. This field exerts a force on any charged object in its field.

Electrical Field cont.

• Consider a large charge Q exerting a force F on a small charge qo. The electrical field E is given by E = F/qo N/C

• Given that F = ke q1 qo

r2

Then E = ke q1 q2 x1/qo

r2

E = ke q1

r2

Electrical Field Lines

• Introduced by Faraday. electrical field lines indicate the direction of electrical field vectors

1) The electrical field vector E is tangent to the electrical field lines at any point.

2) The number of lines per unit area is proportional to the electrical field strength. E is large when the lines are close together

Electrical Field Lines cont.

3) Lines for a point charge begin at the positive and end at the negative charge.

4) The number of lines drawn are close together at the source and separate away from the source

5) No two field lines can cross each other6) Symmetrical lines between two charges

forms an electrical dipole

Coulomb’s Law

• 1785 Coulomb established a fundamental law of electric force between two stationary charged objects.

• F = ke q1 qo

r2

q1 = charge 1 qo = charge 2

r = the distance between the two objects

ke = Coulomb’s constant 8.9875x109

N.m2/c2

Cont.

• The law is applicable to point charges of spherical distribution.

• Electric force is a vector quantity. Several forces acting collectively on an object are computed individually and then added as vectors. ie. Superposition Principle

Electric Fields

• Like gravitational force, electric force acts through space. Physical contact is unnecessary. An electric field exists in space around a charged object. This field exerts a force on any charged object in its field.

• Consider a large charge Q exerting an electrical force F on a small charge qo, the electrical field E is given by E = F/qo N/C

Cont.

• Given that F = ke q1 qo

r2

And E = F/qo then E = keq1/r2

Electrical Field Lines

• Electrical field lines indicate the direction of electric field vectors. ( introduced by Faraday).

• The electric field vector is tangent to the electric field lines at any point.

• The number of lines per unit area is proportional to the electric field strength in a given region. E is large when the lines are close together.

Field Lines cont.• Lines for a point charge begin at the +ve

charge and end at the –ve charge.• The lines drawn are closer together at the

source and separate away from the charge• No two field lines can cross each other• The charge symmetrical lines between two

opposite charges forma a configuration called a dipole.

• If +ve charge = 2q and the –ve is q half the lines end at infinity.

Conductors in Electrostatic Equilibrium

• Conductors ( copper) have electrons that are free to move about the material. (metal Lattice). When no net motion of charge occurs within the conductor it is in electrostatic equilibrium. Particular properties of this situation are, (cont next page)

Cont.

• The electric field is zero everywhere inside the conductor.

• Any excess charge on an isolated conductor resides entirely on the surface.

• The electric field just outside the conductor is perpendicular to the surface.

• On an irregularly shaped conductor the charge accumulates at sharp points.

Electric Flux

• Electric flux is volume per unit area.• Consider an electrical field that is uniform in both

magnitude and direction. Electric field lines exist perpendicular the the surface area. The number of lines is proportional to the magnitude of the electric field per unit surface area.

• фE = EA N.m2/C if the surface is not

perpendicular to the field фE = EAcosθ

Flux cont.

• The flux has a maximum value when θ =0 and a minimum value when θ = 90

• As electric flux is a vector, lines passing into a surface are negative and lines passing out are positive.

• Flux is a measure of how much electric field passes through a surface.

Gauss’s Law

• Karl Gauss (1777-1855) provided a technique for calculating flux. The law relate the electric flux through a closed surface to the total charge inside that surface. A closed surface (sphere) has only an inside and outside.

• Consider a point charge q surrounded by a spherical surface of radius r. The magnitude of the electric flux is given by,

Gauss’s Law cont.

• E = keq/r2

• The electric flux everywhere through the surface is ф = EA

• Then ф = keq/r2(4πr2)• ф = keq4π• The constant Єo (permittivity of free space) = 1/4πke = 8.85x10-12 C2/N.m2

• Ф = q/ Єo