Transcript Properties of Electric Charges

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 other
6) 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 = k e 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