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ELECTRICITY AND COULOMB’S LAW
Firdiana Sanjaya (4201414050)
Ana Alina (4201414095)
1. There are 2 kinds of charge : positive and negative
2. The charge with the same sign will repel each another
while the charge with different sign will attract each
another
3. Electric charge is quantified
4. Charge is conserved
• Charge can be created and destroyed, but only in positivenegative pairs.
• The Law of conservation of charge states that the net charge
of an isolated system remains constant.
Materials are divided into three categories,
depending on how easily they will allow charge.
These are:
1.
2.
3.
conductors (metals, for example)
semi-conductors (silicon is a good example)
insulators (rubber, wood, plastic for example)
There are three ways that objects can be given a net
charge. They are
1.
2.
3.
Charging by Friction
Charging by Conduction
Charging by Induction
1.
is inversely proportional to the
square of the separation r between
the particles and directed along the
line joining them;
2. is proportional to the product of the
charges q1 and q2 on the two
particles;
3.
is attractive if the charges are of
opposite sign and repulsive if the
charges have the same sign.
We can express coulomb’s law as the equation below
k = 8.99 x 109 Nm2/C2
The law expressed in vector form for the electric
force exerted by a charge q1 on a second charge q2
, written F12, is
• See the different
Where k = 8.99 x 109 N2 m2/C2
G = 6.67 x 10-11 N2 m2/kg2
Similarities between electrostatic and gravitational forces:
1. Both act in a vacuum.
2.
Both are central and conservative.
3. Both obey an inverse-square law
Differences between electrostatic and gravitational forces:
1.
Electrostatic forces are much greater than gravitational forces for natural values of
charge and mass. For instance, the ratio of the electrostatic force to the gravitational
force between two electrons is about 1042.
2.
Gravitational force are attractive for the charge that has the same sign but
electrostatic force will repul in the same condition.
3.
Gravitational forces are always attractive, while electrostatic forces may be either
attractive or repulsive.
ELECTRIC FIELD
• An electric field is said to exist in the region of
space around a charged object—the source
charge.
• The electric field E is analogous to g (acceleration).
• The electric field a distance r away from a point charge
is given by:
•
• If the electric field at a particular point is known, the force a
charge q experiences when it is placed at that point is given by
:
F = qE
• If q is positive, the force is in the same direction as the field; if q
is negative, the force is in the opposite direction as the field.
LEARNING FROM GRAVITY
• F = qE
• ma = qE
• a = qE/m
• g = qE/m
• uniform gravitational field same thing with charges in a uniform
electric field. If you throw a charge into a uniform electric field
(same magnitude and direction everywhere), it would also
follow a parabolic path.
2.4 ELECTRIC FLUX
• Sounds fancy, but it’s not hard
• Electric Flux measures how much an electric field wants to “push through”
or “flow through” some arbitrary surface area
• We care about flux because it makes certain calculations easier.
Flux is a measure of the number of field lines passing through an area. If we
define area as a vector, with its direction perpendicular to the surface
ELECTRIC FLUX: GENERAL DEFINITION
 
 E   E  dA
surface
Electric Flux through a surface depends on three things:
1.
How strong the E-field is at each infinitesimal area.
2.
How big the overall area A is after integration.
3.
The orientation between the E-field and each infinitesimal area.
Case 1
Easiest case:
• The E-field is uniform
• The plane is perpendicular to
the field
E  E A
Electric Flux
Flux depends on how strong the
E-field is and how big the area
is.
Case 2
Junior Varsity case:
• The field is uniform
• The plane is not
perpendicular to the field
 E  E A   E A cos 
Flux depends on how strong the E-field is, how big
the area is, and the orientation of the area with
respect to the field’s direction.
2.5 PERMITTIVITY
Gauss’s Law
 NET
  qenclosed
  E  dA 
surface
0
2.6 Applying Gauss’s Law
Gauss’s Law can be used to
(1) find the Electric field at some position relative to a known charge
distribution, or
(2) to find the charge distribution caused by a known Electric field.
Choose a surface such that…
1.
Symmetry helps: the E-field is constant over the surface (or some
part of the surface)
2.
The E-field is zero over the surface (or some portion of the
surface)
3.
The dot product reduces to EdA (the E-field and the dA vectors
are parallel)
4.
The dot product reduces to zero (the E-field and the dA vectors
are perpendicular)
Choose a surface such that…
1.
Symmetry helps: the E-field is constant over the surface (or some
part of the surface)
2.
The E-field is zero over the surface (or some portion of the
surface)
3.
The dot product reduces to EdA (the E-field and the dA vectors
are parallel)
4.
The dot product reduces to zero (the E-field and the dA vectors
are perpendicular)