Transcript Chapter 15: Electrostatics PowerPoint

AP Physics 1
Chapter 15.1-3
Electric Charge and Force
Chapter 15: Electric Charge, Force,
and Field
15.1
15.2
15.3
Electric Charge
Electrostatic Charging
Electric Force
Homework for Chapter 15
• Read Chapter 15.1-3
• HW 15.A: p.506-507: 7-10, 13, 14, 22,23,25-28, 30.
Warmup: Electric Force
Physics Warmup #102
An electric force is the force that exists between any two charged particles due to
their charge. Objects with opposite charges will attract each other, while objects
with the same charge will repel each other.
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Complete the table for each pair of objects listed.
Pair of Objects
Electric Force Present
Repel or Attract?
a proton and a proton
yes
a neutron and a proton
no
an electron and a neutron
no
a neutron and a neutron
no
an electron and an electron
yes
repel
a proton and an electron
yes
attract
repel
15.1: Electric Charge
electrostatics
- the study of electric forces between charged objects at rest.
electric charge
- the property of an object that determines its electrical
behavior: the electric force it can exert, and the electric force it can experience.
There are two types of charges:
positive
(+) – associated with the proton
negative
(-) – associated with the electron
• The positive and negative signs are arbitrary conventions.
• Both protons and electrons have the same charge, but with opposite signs.
Neutrons have no charge.
law of charges or charge-force law
Like charges repel each other,
and unlike charges attract each other.
-
• The fundamental unit of charge is the magnitude of the charge of an electron:
 e  = 1.60 x 10-19 C
coulomb
(C) - the SI unit of charge
• Charge is “quantized”; it can only occur in integral multiples of e.
The charge of an object is
net charge
charges.
q=ne
(n = 1,2,3…)
- an object has an excess of either positive or negative
law of conservation of charge
-
The net charge of an isolated system remains constant.
Example 15.1: A piece of glass has a net charge of -2.00 C.
a) Are there more protons or electrons?
the
b) What happens if an identically charged piece of glass is placed near
first one?
c) How many fundamental units of excess charge does one piece of the
glass contain?
15.2: Electrostatic Charging
conductor
- a material which
has the ability to conduct or transmit
electric charges
ex: metals
valence electrons are loosely
bound, can move
insulator
- poor electrical
conductors
ex: glass, rubber, plastic
valence electrons are tightly
bound, can’t readily move
semiconductor
- electron
conductivity is intermediate between a
conductor and insulator
ex: transistors, computer
chips
electroscope
- a device that can be used to demonstrate
characteristics of electric charge.
• When a charged object is brought near the bulb of a neutral
electroscope, the leaves separate.
electrostatic charging
- the process by which an insulator or an
insulated conductor receives a net charge.
• Electrostatic charging can be done by four processes.
1) charging by friction
- insulators are rubbed with different materials
and the insulators and the materials acquire equal but opposite charges.
ex: Rubbing a rubber rod with fur will transfer electrons from the fur to the
rubber, and leave the rod with a net negative charge.
ex: After walking across a carpet, you get zapped when touching a metal
doorknob.
2) charging by contact or by conduction
- a charged object
makes a contact with a uncharged object and some of the charge on the
charged object is transferred to the uncharged object.
3) charging by induction
- a charged object is brought near (not
touching) an uncharged object, the uncharged object is then “grounded”,
and the uncharged object acquires an opposite charge than the charged
object.
4) charging by polarization
- the positive and negative charges are
simply separated or realigned within the object and the net charge of the object
is still zero. Charges are merely separated so a portion of the object has excess
positive charges and another has excess negative charges.
When the balloons are charged by
friction and placed in contact with the
wall, an opposite charge is induced on
the wall’s surface, to which the
balloons then stick by the force of
electrostatic attraction.
Water molecules are polar in nature, but some
molecules that are normally not polar can
become induced molecular dipoles in the
presence of a nearby charged object.
A charged balloon easily attracts polar water
molecules.
Check for Understanding
1. A combination of two electrons and three protons would have a net charge
of
a) +1
b) -1
c) +1.6 x 10-19 C
d) -1.6 x 10-19 C
Answer: c
Check for Understanding
2. The directions of the electric forces on two interacting charges are given by
a) the conservation of charge
b) the charge-force law
c) the magnitude of the charges
d) none of these
Answer: b “Like charges repel each other, and unlike charges attract each
other.
Check for Understanding
3. A rubber rod is rubbed with fur (fur will lose electrons to a rubber rod). The fur
is then quickly brought near the bulb of an uncharged electroscope. What is
the sign of the charge on the leaves of the electroscope?
a) positive
b) negative
c) neutral
d) unable to tell
Answer: a. When positively charged fur is brought near an electroscope, the
leaves are charged by induction, so the charges on the leaves are positive.
Check for Understanding
4. When a positively charged rod is brought near the bulb of a negatively
charged electroscope, the leaves
a) separate further
b) collapse
c) remain unchanged
d) unable to tell
Answer: b. Negative charge is drawn to the top, and the leaves collapse.
Warmup: Just Charge It
Physics Warmup #100
Most objects are electrically neutral. They contain equal amounts of positive and
negative charges. However, many objects often become charged as they gain or
lose electrons. One way this may happen is through charging by friction, in which
two neutral objects rub against each other. The resulting transfer of electrons
leaves one object negatively charged and the other positively charged.
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Describe a common event in which objects become charged by friction.
Answer: example – walking across a carpet; clothes tumbling in a dryer.
15.3: Electric Force
• On your gold sheet, Coulomb’s law constant k is expanded to:
k = 1 where 0 = 8.85 x 10-12 C2/N·m2 (vacuum permittivity)
40
0 is a physical constant given on your blue sheet.
• The mutual electrostatic forces on two point charges are equal and
opposite.
• For a configuration of two or more point charges, the force on a particular charge
is the vector sum of the forces on it due to all the other charges.
• Like the universal law of gravitation, Fg = Gm1m2 ,
r2
Coulomb’s law in a inverse square law because force is proportional to 1
r2
• Electrical forces are MUCH stronger than gravitational forces.
• Electric charge comes in two types, positive and negative. Mass does not; it only
attracts.
Example 15.2: Two charges are separated by a distance d and exert mutual
attractive forces F1 on each other. What are the mutual attractive forces if the
charges are separated by a distance of 3d?
Example 15.3: A +4.0 coulomb charge is at the origin and a +9.0 coulomb charge
is at x = 4.0 m. Where can a third charge q3 be placed on the x axis so the net
force on it is zero?
Example 15.4: Consider three point charges located at the corners of a triangle as
shown in the figure on the right. If q1 = 6.0 nC, q2 = -1.0 nC, and q3 = 5.0 nC, what
is the net force on q3?
Check for Understanding:
1. The magnitude of the electric force between two point charges is given by
a) the charge-force law
b) conservation of charge
c) Coulomb’s law
d) both a and b
Answer: c
Check for Understanding:
2. Compared to that of the electric force, the strength of the gravitational force
between two protons is
a) about the same
b) somewhat large
c) very much larger
d) very much smaller
Answer: d
Check for Understanding:
3. An electron is a certain distance from a proton. How would the electric force be
affected if the electron were moved twice the distance away from the proton?
a) 2 times greater
b) 4 times greater
c) half as much
d) one quarter as much
Answer: d
Check for Understanding:
4. Two identical point charges are at a fixed distance from one another. How would
the electric force be affected if one of the charges was doubled and the other
halved?
a) 2 times greater
b) same
c) half as much
d) one quarter as much
Answer: b
HW 15.A: p.506: 7-10, 13, 14, 22,23,25-28, 30.
Warmup: It’s a Wrap
Physics Warmup #101
Clear food wraps cling to whatever surface they are pressed against (even
themselves). When stretched tightly over a container and pressed against the
sides, they keep their tension and secure the container. The secret behind these
plastic wraps is static electricity. Some of the static charge is created when the roll
is manufactured. Since plastic is not a good conductor, the charge remains for a
long time. More static charge is created when the plastic is pulled off the roll. Pull
faster and it will cling even better.
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You might have noticed that these clear plastic wraps do not work as well in humid
conditions and hardly work at all when the container has moisture on it. Explain
why this is so.
Answer: Moisture drains the static charge.
Summary of Chapter 15 Formulas