Welcome to Physics 152 © Hyde-Wright, ODU © Walker, Prentice Hall, 2007.
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Transcript of Welcome to Physics 152 © Hyde-Wright, ODU © Walker, Prentice Hall, 2007.
Walker, Chapter 19 2
Topics to be covered
• Electricity and Magnetism (Chapters 19-23)
• Light and Optics (Chapters 25-28)
• Modern Physics (Chapter 29-32)
Walker, Chapter 19 3
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 4
Energy in our World
• Nuclear Fusion in sun E=mc2
H H H H He + Energy Thermal Energy at surface converted to visible
light energy
• Light Energy Chemical Energy (photosynthesis)• Plants Fossil Fuels
Fuel for cars (motion) Fuel for power plants
• Plants Food Energy for thought, motion of muscles, etc...
Fusion
Radiation
Walker, Chapter 19 6
Electrical ChargeEffect 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 7
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 8
The ElectronOne 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 9
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 10
• 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 11
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
Electrical Forces
+
++
Walker, Chapter 19 12
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.
neutral & polarized
charged
Walker, Chapter 19 13
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 14
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 15
Coulomb’s LawThe magnitude of the force between two point objects
separated by a distance r with charges q1 and q2 is given by Coulomb’s Law:
where k = 8.99… 109 Nm2/C2
The direction of the force on one charge is either toward (negative) or away (positive) from the other charge.
221
r
qqkF
q1 and q2 are the values (+ or ) of the two charges
Walker, Chapter 19 16
Force: vector, magnitude, component
• Magnitude (strictly positive)
221 ||||
||r
qqkF
Walker, Chapter 19 17
12 1 2
21 2 1
21 12
Force on charge from charge
Force on charge from charge
: Newton's Third Law: Action-Reaction
F q q
F q q
F - F
12F 21F1q 2q
Force: vector, magnitude, component
• Magnitude (strictly positive)221 ||||
||r
qqkF
- +
Walker, Chapter 19 18
Walker Problem 14, pg. 657Given that q = +12 C 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 21
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 22
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 C and d = 47 cm.
Walker, Chapter 19 24
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
• 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
0
FE
q
Walker, Chapter 19 25
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
Q
q0
Walker, Chapter 19 27
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.
+
q0 q0
E
E
Walker, Chapter 19 28
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 C 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 C?
Note: In problems like this we do not need to know what charges created the electric field. E = 1000 N/C
q
Walker, Chapter 19 29
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
.20
r
qqkF
.2r
qkE
Since the definition of the electric field is
,0q
FE
the magnitude of the electric field from a point charge is given by
Walker, Chapter 19 30
Walker Problem 28, pg. 642
What is the magnitude of the electric field produced by a charge of magnitude 10.0 C at a distance of (a) 1.00 m and (b) 2.00 m?
2r
QkE
k = 8.99 ·109 N m2/C2
Walker, Chapter 19 31
SuperpositionJust 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 cb
Walker, Chapter 19 32
Walker Problem 76, pg. 661An object of mass m = 3.7 g and charge q = +44 C 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 34
Electric Field LinesIn 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 35
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 36
Electric Field Lines
Note that twice as many field lines originate from the +2q charge than the +q or –q charges.
Walker, Chapter 19 37
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?
Walker, Chapter 19 38
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 39
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 40
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 41
Electric FluxWe define electric flux as
the product of the surface area A times the component Ecos of the electric field perpendicular to the surface.
In general, = EAcos
aEA
b = 0
(c) = EAcos
is the angle between the electric field and the line perpendicular to the surface.
Walker, Chapter 19 42
Gauss’s LawConsider an arbitrary (imaginary) closed surface (called a
Gaussian surface) enclosing a total charge q. The electric flux through the surface is
0q
22124
10 m/NC1085.8
k
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.
Walker, Chapter 19 43
ExampleThree point charges are arranged as shown. q1 = +4 C, q2
= -6 C and q3 = -4 C. Find the electric flux through the three Gaussian surfaces labeled a, b and c.
q1
q3
q2
acb