Transcript Document

Welcome to Physics 152
©Hyde-Wright, ODU
© Walker, Prentice Hall, 2007
Topics to be covered
• Electricity and Magnetism (Chapters 19-23)
• Light and Optics (Chapters 25-28)
• Modern Physics (Chapter 29-32)
Walker, Chapter 19
2
Chapter 19
Electric Charges, Forces, and Fields
Fundamental Forces in Physics
•
•
•
•
Gravity
Electromagnetism
Weak Interaction
Strong Interaction
All of physics is based on these four forces
Walker, Chapter 19
3
Energy in our World
• Nuclear Fusion in sun E=mc2
 H H H H  He + n + n + Energy
 Thermal Energy at surface converted to visible
light energy
• Light Energy  Chemical Energy
(photosynthesis)
• Plants  Fossil Fuels
Fusion
Radiation
 Fuel for cars (motion)
 Fuel for power plants
• Plants  Food
 Energy for thought, motion of muscles, etc...
Walker, Chapter 19
4
Electrostatic Phenomena
• Rubbing things makes an electrostatic charge
Walker, Chapter 19
5
Electrical Charge
Effect of Electric charge have been known since ~600 B.C.
Greeks experiment: amber rubbed against fur - charge
(Greek word for amber is elektron.)
Glass rubbed against paper towel + charge
The SI unit of electrical charge is the Coulomb (C ).
Walker, Chapter 19
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The Structure of an Atom
The atom consists of a positively
charged nucleus, orbited by
negatively charged electrons.
The nucleus contains protons
(positive) and neutrons (neutral).
Walker, Chapter 19
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The Electron
One of the fundamental particles found in nature is the
electron.
• The electron mass is 9.11  10-31 kg.
• The electron charge (-e) is -1.6  10-19 C.
 The symbol e is the magnitude of the electron’s charge
Walker, Chapter 19
8
The Proton
The proton is not a fundamental particle.
The proton mass is 1.67  10-27 kg.
• The proton is 2000 times heavier than the electron, so the
vast majority of an atom’s mass resides in the nucleus.
• The proton charge (+e) is +1.6  10-19 C.
• The proton charge and electron charge are known to be equal
and opposite.
Walker, Chapter 19
9
• An object may contain both positive and negative
charges. If the object possesses a net charge it is
said to be charged. If the object possesses no net
charge it is said to be neutral.
• An atom is normally neutral, because it possesses
an equal number of electrons and protons.
However, if one or more electrons are removed
from or added to an atom, an ion is formed, which
is charged.
• Charge is always conserved: charge may be
transferred but it is never created or destroyed.
 However, charges can be created and destroyed in
positive and negative pairs, so that the net charge in the
universe does not change.
Walker, Chapter 19
10
Electrical Forces
Two charged objects will exert forces on one another.
• Unlike charges attract one another.
+

• Like charges repel one another.
+
+
• The force decreases with the square of the distance between the
charges


Walker, Chapter 19
11
Polarization
An object is polarized
when its charges are
rearranged so that
there is a net charge
separation. Charged
objects can be
attracted to neutral
objects because of
polarization.
charged
Walker, Chapter 19
neutral & polarized
12
Insulators and Conductors
Materials are classified by how easily charged
particles can “flow” through them.
• If charges flow freely, the material is a
conductor (metals, for example)
• If charges are unable to move freely, the
material is an insulator (glass, for example)
• Some materials have properties in between
insulators and conductors, these are called
semiconductors.
Walker, Chapter 19
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Charge Transfer
Charge is usually transferred because electrons move
from one place to another.
But sometimes the flow of both positively or
negatively charged ions (atoms or molecules) is
important (cells, batteries…).
The earth can be viewed as an infinite (conducting)
reservoir of electrons. An object in electrical
contact with the earth is said to be grounded.
Walker, Chapter 19
14
Coulomb’s Law
The magnitude of the force between two point objects
separated by a distance r with charges q1 and q2 is given by
Coulomb’s Law:
F k
q1q2
r
2
q1 and q2 are the values
(+ or ) of the two
charges
where k = 8.99…  109 Nm2/C2
The direction of the force on one charge is either toward
(negative) or away (positive) from the other charge.
Walker, Chapter 19
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Force: vector, magnitude, component

| q1 || q2 | • Magnitude (strictly positive)
| F | k
2
r
Walker, Chapter 19
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Force: vector, magnitude, component

| q1 || q2 |
| F | k
r2
• Magnitude (strictly positive)
F12  Force on charge q1 from charge q2
F21  Force on charge q2 from charge q1
F21  - F12 : Newton's Third Law: Action-Reaction
q1
F12
F21
Walker, Chapter 19
+
q2
17
Walker Problem 14, pg. 657
Given that q = +12 mC and d = 16 cm, (a) find the direction
and magnitude of the net electrostatic force exerted on
the point charge q2. (b) How would your answers to part
(a) change if the distance d were tripled?
Walker, Chapter 19
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Walker, Chapter 19
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b) Tripling the separations:
Walker, Chapter 19
20
Multiple Charges
• If there are more than two
charges present, the net
force on any one charge is
given by the vector sum of
the forces on that charge
from all surrounding
charges. This is an
example of the Principle
of Superposition.

+
+
What is the direction of the net
force on each charge (roughly)?
Walker, Chapter 19
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Walker Problem 20, pg. 658
Find the direction and
magnitude of the net
electrostatic force exerted
on the point charge q2 in
the Figure. Let q = +1.8
mC and d = 47 cm.
Walker, Chapter 19
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Walker, Chapter 19
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Electric Field
• If a test charge q0 experiences a force F at a given location r,
the magnitude of the electric field at that location is defined
by
F
E
q0
• The electric field can also be thought of as a disturbance in
space caused by nearby charges.
• The electrostatic force experienced by a charge is the
interaction between the charge and the electric field at that
position.
• The SI units of electric field are Newtons/Coulomb = N/C
Walker, Chapter 19
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Electric Force F(r) from charge Q acting on a test charge q0 at
various locations r : F=kQq0/r2
Electric Field E(r)= F/ q0
q0
Q
Walker, Chapter 19
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Electric Field E(r) from charge Q at various locations r:
E=kQ/r2
r
Q
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Electric Field Direction
The direction of the electric field is defined to be the
direction of the force that would be experienced if
the test charge is positive. Because the field has a
direction, it must be a vector.
E
q0
q0
E

+
Walker, Chapter 19
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Electric Field (cont.)
The electric field is the force per charge at a given
location. If you know the electric field, then the force
on a charge can easily be found using
F = qE
Example: A charge q of +8 mC experiences a uniform
electric field of 1000 N/C to the right. (a) What is the
force on the charge? (b) What would the force be if
the charge were –8 mC?
q
Note: In problems like this we do not need to know
what charges created the electric field.
Walker, Chapter 19
E = 1000 N/C
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Electric Field of a Point Charge
From Coulomb’s Law, the magnitude of the force
experienced by a test charge q0 a distance r from a
charge q is
qq
F k
r
0.
2
Since the definition of the electric field is
F
E ,
q0
the magnitude of the electric field from a point charge is
given by
q
Ek
r
.
2
Walker, Chapter 19
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Walker Problem 28, pg. 642
What is the magnitude of the electric field produced by a
charge of magnitude 10.0 mC at a distance of (a) 1.00
m and (b) 2.00 m?
Q
Ek 2
r
k = 8.99 ·109 N m2/C2
Walker, Chapter 19
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Superposition
Just like with forces, electric fields must be
added as vectors. The electric field from
several charges is the vector sum of the
electric field from each charge.
Example: Consider two identical negative charges
as shown. At which lettered point is the
magnitude of the electric field greatest? Least?
a
b

c
Walker, Chapter 19

31
Walker Problem 76, pg. 661
An object of mass m = 3.7 g and charge q = +44 mC is
attached to a string and placed in a uniform electric
field that is inclined at an angle of 30.0° with the
horizontal. The object is in static equilibrium when the
string is horizontal. Find (a) the magnitude of the
electric field and (b) the tension in the string.
Walker, Chapter 19
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Walker, Chapter 19
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Electric Field Lines
In order to visualize the electric field in space it is convenient
to draw Electric field-lines (see Fig. 19-13). The field
lines are directional [curved] lines that everywhere point in
the direction of the electric field at that point.
+

+
Dipole
Walker, Chapter 19
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Field Line Properties
• The electric field is tangent to the field line at any point
in space.
• The strength of the electric field is proportional to the
density of field lines.
• The field lines always begin on positive charges or at
infinity and end on negative charges or at infinity.
• No two field lines can ever cross.
• The number of field lines leaving a positive charge or
approaching a negative charge is proportional to the
magnitude of the charge.
Walker, Chapter 19
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Electric Field Lines
Note that twice as many field lines originate from
the +2q charge than the +q or –q charges.
Walker, Chapter 19
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1. The net charge inside
the grey ellipse is:
a) Positive
b) Zero
c) Negative
Hint: Are there more
Electric Field lines
entering, or leaving
the gray ellipse, or
is it equal?
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2. The net charge inside the grey
ellipse is:
a)
Positive
b)
Zero
c)
Negative
Hint: Are there more Electric
Field lines entering, or
leaving the gray ellipse, or
is it equal?
Walker, Chapter 19
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Electrostatic Equilibrium
Recall that charges within a conductor are free
to move around easily.
If the charges within a conductor are not in
motion, then the system is said to be in
electrostatic equilibrium.
Walker, Chapter 19
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Properties of Electrostatic
Equilibrium
• In the presence of electrostatic forces, the charges
on the conductor move around until the following
static conditions are achieved:
 The electric field is zero everywhere inside a conductor.
 The excess charge on a conductor resides entirely on its
surfaces.
 The electric field just outside a charged conductor is
perpendicular to its surface.
• On irregularly shaped objects, the charge
accumulates at sharp points, and the electric field
is most intense at sharp points.
Walker, Chapter 19
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Electric Flux
We define electric flux F as
the product of the surface
area A times the
component Ecosq of the
electric field
perpendicular to the
surface.
In general, F = EAcosq.
(a) F  EA
(b) F = 0
(c) F = EAcosq
q is the angle between the electric field and
the line perpendicular to the surface.
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Gauss’s Law
Consider an arbitrary (imaginary) closed surface (called a
Gaussian surface) enclosing a total charge q. The
electric flux through the surface is
q
F
0
 0  41k  8.851012 C2 /N  m 2
This integral property is a consequence of
the 1/r2 Coulomb Law, and is valid for
any irregular surface, no matter how
complicated the electric field produced
by internal or external charges.
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Example
Three point charges are arranged as shown. q1 = +4 mC,
q2 = -6 mC and q3 = -4 mC. Find the electric flux
through the three Gaussian surfaces labeled a, b and c.
a
b q
1
c
q3
q2
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• Two charges Q1 and Q2 are separated by a distance of
0.010 m. The Electrostatic force of Q1 on Q2 is 2.0e-5 N.
• At what distance of separation between Q1 and Q2 would
the force be 1.0e-5N?
 a) 0.02 m
 d) 0.007m
b) 0.014 m
e) 0.005 m
Walker, Chapter 19
c) 0.01 m
44