UNIT THREE Electricity and Magnetism

1
UNIT THREEElectricity and Magnetism
2
Chapter 12Electrostatic Phenomena
3
What does lightning have in common...
... with hair on a dry winter day?
4
Effects of Electric Charge
Electroscope
5
Effects of Electric Charge
There are two different types of charges,
positive and negative. Like charges repel each
other, and unlike charges attract each other
(Electrostatic Force). Electric charges can flow
from one object to another.
6
Conductors and Insulators

• Different materials conduct electric charge
  differently.

7
Conductors and Insulators

• Conductors
• Materials that charges can readily flow through.
  Examples metals and human bodies
• Insulators
• Materials that do not ordinarily permit charges
  to flow. Examples plastic, glass, ceramics, etc.
  
• Semiconductors
• Materials that are intermediate between a good
  conductor and a good insulator.
• Examples Si, Ge, GaAs, AlAs, etc
• Semiconductors conduction can be manipulated by
  adding small amount of impurities into it
  (doping). This is used in modern technology that
  has profound impacts to our life.

8
Can you charge an object without actually
touching it with another charged object?

• Charging by induction
• First charge a plastic rod, and bring the rod
  near a metal ball mounted on an insulating post.
• Because of electrostatic force, there are
  negative charges buildup on the side opposite the
  rod, and a positive charge on the near side.

9
Can you charge an object without actually
touching it with another charged object?

• Charging by induction (continue)
• Now touch the ball with your finger on the side
  opposite the rod (the negative charge flows from
  the ball to your body, since it is still repelled
  by the negative rod).
• Now remove your finger and then the rod, a net
  positive charge is left on the ball.

10

• Important Roles of Materials Conducting Property
  in Charging by induction.
• Plastic rod
• Metal ball
• Insulating post
• Human Body (your finger)

11
Why are insulators attracted to charged objects?

• Pith balls were attracted to the charged rod
  before they were charged.
• Electrons are not free to move in the insulating
  material of the pith balls, but charges can move
  within each atom or molecule.
• Each atom becomes an electric dipole the center
  of the negative charge is slightly displaced from
  the center of the positive charge. The material
  is polarized.

12

• Since the negatively charged surface is closer to
  the rod than the positively charged surface, it
  experiences a stronger attractive force than a
  repulsive force.
• The overall effect is an attractive force even
  though the total charge is zero.

• After the ball comes in contact with the charged
  rod, some of the charge on the rod is transferred
  to the pith ball. So the pith ball is then
  positively charged like the rod, and repelled by
  the rod.

13

• Polarization explains why small bits of paper or
  plastic foam are attracted to a charged object
  such as a sweater rubbed against some other
  material.
• Induced polarization plays a role in
  electrostatic precipitators used to remove
  particles from smoke in industrial smoke stacks.
• Polarized particles are attracted to charged
  plates in the precipitator, removing them from
  the emitted gases.

14
The Electrostatic Force Coulombs Law

• Coulomb measured how the electrostatic force
  varies with distance and quantity of charge.

15
Coulombs Law

• The electrostatic force between two charged
  objects is proportional to the quantity of each
  of the charges and inversely proportional to the
  square of each distance between the charges.

16
Example Box 12.2 Two positive charges, one 2 ?C
and the other 7 ?C, are separated by a distance
of 20 cm. What is the magnitude of the
electrostatic force that each charge exerts upon
the other?
17

• Coulombs Law compared to Newtons Law of
  Gravity Similar format. Both have the same
  inverse-square dependence on distance.
• If we double the distance, the force falls to
  one-fourth of the original.
• Gravitational force depends on both masses, and
  the electrostatic force depends on both charges.
• Gravity is always attractive and electrostatic
  force could be either positive or negative.
• Gravity is much weaker than the electrostatic
  force.

The search for a unified field theory that would
explain the relationships between all of the
fundamental forces is a major area of research in
modern theoretical physics.
18
Two point charges q13 ?C and q22 ?C are
separated by a distance of 30 cm as shown in the
drawing. A third charge q04 ?C is placed
between the initial two charges 10 cm from q1.
From Coulombs law, the force exerted by q1 on q0
is 10.8 N, and the force exerted by q2 on q0 is
1.8 N. (1) What is the net electrostatic force
acting on the charge q0? (2) What is the
electric field (force per charge) at the location
of the charge q0 due to the other two charges?
Example Box 12.3
19
Electric Field

• The existence of an electric charge in space
  affects the space around it, which then exerts a
  force on another charge.
• The electric field at a given point in space is
  the electric force per unit positive charge that
  would be exerted on a charge if it were placed at
  that point.
• Unit N/C
• It is a vector having the same direction as the
  force on a positive charge placed at that point.

20
Two point charges, 3 ?C and 2 ?C, are separated
by a distance of 30 cm. A third charge q0 is
placed between them as shown. The force exerted
by q1 on q0 is 10.8 N, and the force exerted by
q2 on q0 is 1.8 N.(1) What is the net
electrostatic force acting on q0?(2) What is the
electric field at the location of the charge q0
due to the other two charges?
21
Three positive charges are located along a line
as shown. The 0.10-C charge at point A is 2 m to
the left of the 0.02-C charge at point B, and the
0.04C charge at point C is 1 m to the right of
point B.(1) What is the magnitude of the force
exerted on the 0.02-C charge by the 0.10-C
charge?(2) What is the magnitude of the force
exerted on the 0.02-C charge by the 0.04-C
charge?(3) What is the net force exerted on the
0.02-C charge by the other two charges?(4) If we
regard the 0.02-C charge as a test charge used to
probe the strength of the electric field produced
by the other two charges, what are the magnitude
and direction of the electric field at point B?
Example SP1
22

• Electric Field can be used to find the force on
  any other charge placed at that point

Example E 9. A uniform electric field is
directed upward and has a magnitude of 20N/C.
What are the magnitude and direction of the force
on a charge of -5C placed in the filed?
23
Electric Field Lines

• Maxwell was the major contributor to the electric
  field concept, and Faraday developed the idea of
  field lines as a means of visualizing both the
  direction and strength of the field.

• The direction of the electric field lines
  indicates the direction of the electrostatic
  force if a positive charge is placed at that
  point.
• The density of the electric field lines indicates
  the strength of the electric field.

• The electric field lines associated with a
  positive charge radiate in all directions from
  the charge.

24

• Electric Field Lines of a negative charge

25
Two charges, of equal magnitude but opposite
sign, lie along a line as shown. What are the
directions of the electric field at points A, B,
C, and D?
If we change the negative charge in above problem
to a positive charge of the same magnitude, what
are the directions of the electric field at
points A, B, C and D?
26

• An electric dipole is two charges of equal
  magnitude but opposite sign, separated by a small
  distance.
• Electric field lines originate on positive
  charges and end on negative charges.

27
Three equal positive charges are located at the
corners of a square, as in the diagram. Using
arrows, indicate the direction of the electric
field at point A and B on the diagram.
Example Q23
28
Suppose that two equal positive charges lie near
one another, as shown in the diagram. (a) Using
small arrows, indicate the direction of the
electric field at the labeled points on the
diagram. (b) By drawing an equal number of filed
lines emerging from each charge, sketch the
electric field lines for this distribution of
charge
Example SP2
29
Suppose that four equal positive charges are
located at the corners of a square, as in the
diagram. (a) Using small arrows, indicate the
direction of the electric field at each of the
labeled points. (b) Would the magnitude of the
electric field be equal to zero at any of the
labeled point? Explain.
Example SP3
30
Finding the Change in Potential Energy of a
Charge

• Two parallel metal plates containing equal but
  opposite charges produce a uniform electric field
  between the plates.
• This arrangement is an example of a capacitor, a
  device to store charge.
• A positive test charge placed in the uniform
  electric field will experience an electrostatic
  force in the direction of the electric field.
• An external force F, equal in magnitude to the
  electrostatic force qE, will move the charge q a
  distance d in the uniform field.

31

• The work done by the external force w Fd
  qEd.
• The increase in potential energy of the charge
  ?PE w qEd.
• This is analogous to what happens when a mass m
  is lifted against the gravitational force.

32
Electric Potential

• Electric potential is defined as the potential
  energy per unit of positive charge that would
  exist at some point in space if a charge were
  present there. It is also called voltage, and its
  unit is volt.

• It is the change in electrostatic potential that
  is meaningful.

33

• Electric potential is related to electrostatic
  potential energy in much the same way as electric
  field is related to electrostatic force.
• The change in electric potential is equal to the
  change in electrostatic potential energy per unit
  of positive test charge
• Electric potential and potential energy are
  closely related, but they are NOT the same.
• If the charge q is negative, its potential energy
  will decrease when it is moved in the direction
  of increasing electric potential.

34
Two plates are oppositely charged so that they
have a uniform electric field of 1000 N/C between
them, as shown. A particle with a charge of
0.005 C is moved from the bottom (negative)
plate to the top plate (d3cm). (1) What is the
change in potential energy of the charge?(2)
What is the change in electric potential from the
bottom to the top plate?
Example Box 12.4
35

• The potential energy of a positive charge
  increases when we move it against the field.
• For a uniform electric field, there is a simple
  relationship between the magnitude of the
  electric field and the change in electric
  potential ?V Ed.

• For non-uniform fields, the the electric
  potential always increases most rapidly in the
  direction opposite to the electric field.
• For a positive point charge, the electric
  potential increases as we move closer to the
  charge.

36
What is lightning?