Download 12 Electrostatic Phenomena Suggestions for presentation Answers

Many demonstrations are possible. Useful references
include the following:
Physics Demonstration Experiments, Harry F. Meiners,
editor. (Ronald Press, New York 1970)
Electrostatics, by A. D. Moore. (Doubleday, New York,
Anchor Book Edition, 1968)
As well, there are wonderful labs in computerized form
and interactive sites on the Internet give great demonstrations.
Although we are taking a fundamental approach in this
chapter, it is worth mentioning to students that there are some
useful applications of electrostatic principles in Xerography,
electrostatic precipitators and spray painting.
12 Electrostatic Phenomena
1 Effects of Electric Charge
2 Conductors and Insulators
3 The Electrostatic Force: Coulomb’s Law
4 Electric Field
5 Electric Potential
Everyday Phenomenon: Lightning
The fundamental concepts of electricity--two kinds of
charges and attractive and repulsive forces between charges-are introduced through analysis of simple experiments with
pith balls and electrified rods. Charging by induction versus
charging by contact is discussed in terms of experiments with
a charged rod and an electroscope. The force law is
introduced with a discussion of Coulomb's experiment. Following the mathematical statement of Coulomb's Law are the
concepts of the electric field, lines of force, electric potential
energy and electric potential.
Answers to Questions
Q1
Q2
Suggestions for presentation
In keeping with the experimental approach of this chapter,
it is highly desirable to perform the experiments discussed
using pith balls, glass rod and rayon or nylon cloth, hard rubber
or plastic rod and fur or wool, and an electroscope. Instructors
with large classes may prefer to use helium-filled balloons on
strings as upside-down pith balls on strings. This could be
sufficiently amusing to help students retain some of the
concepts introduced in this chapter.
First demonstrate the two kinds of charges produced on
the two different rods by making contact with two pith balls.
The balls that are repelled from the charge rods after contact
attract each other. It is worth showing that effect of the
intimate contact of the cloth and the rod in rubbing just
produces a separation of charge. Contact of the hard rubber
rod with the electroscope will cause the leaves to diverge.
Contact of the fur that rubbed the rod with the electroscope will
cause the leaves to fall back together.
High humidity is the death knell for demonstrations of
electrostatics. If you have long periods of high humidity the
surface of the rods and cloths will adsorb moisture. A drying
box will help. Even on a day when the charged glass or hard
rubber has little effect on the pith ball or the electroscope, an
electrophorus may work. An electrophorus consists of simply
a flat piece of lucite and a flat aluminum disk with an insulating
handle. After rubbing the plastic with fur or wool, place the
aluminum disk on it and touch the metal with your hand to
charge it by induction. Now you will have a substantially larger
charge to demonstrate with than from rubbing a plastic rod.
The demonstration device is inexpensive (Cenco 78676.)
Having used an electroscope that shows a deflection
when contacted by a charged rod, now show charging of the
electroscope by induction.
Exciting demonstrations are possible with a Van de Graff
generator. Students like to stand on an insulated platform with
one hand on the spherical dome while it is charged up and
have a hair-raising experience. One can also use a dowel stick
with paper streamers attached to simulate this. A cup with
plastic popcorn packing pieces placed on the dome will give a
nice snowstorm effect as the field builds up.
Q3
Q4
Q5
Q6
Q7
Q8
Q9
Q10
33
They acquire different types of charge. An
experiment to test this could be; rub a piece of
plastic with a silk rag. Now hold each of these items,
one at a time, near a light object (such as a pith ball)
with known charge. Note the behavior of the light
object in each case.
a. Each pith ball will acquire a negative charge. In
charging by contact, a pith ball will share some
of negative charge produced on the rod by
rubbing.
b. Since both pith balls have the same sign of
charge, they will repel one another.
The nylon cloth will acquire a negative charge, which
means it has an excess of electrons after rubbing the
glass rod. Since the glass becomes positive and it is
only the negatively charged electrons that are
transferable, after rubbing, some electrons which
were originally on the glass will attach themselves to
the nylon.
a. The pith ball touched to the glass rod will acquire
a positive charge and the one touched to the
cloth will gain a negative charge. Objects
charged by contact acquire the charge on the
contacting body.
b. Since they have opposite signs of charge, the
two pith balls will attract one another.
No. They gain like charges. This is clear from the fact
that the leaves spread apart rather than close up
petal-like as they would do if their charges were
opposite in sign.
The leaves move closer together. This is because
the rod has acquired opposite charges from the
rubbing process. The leaves acquired the same
charge as the rod had and therefore, opposite the
charge of the fur.
The comb is expected to acquire a negative charge,
like the case of rubbing a plastic rod with fur.
The single fluid model gave indications of surplus or
dearth of the fluid. A surplus carried a positive
amount of the fluid; a dearth implied a negative
amount. Rather than deal with the invisible fluid
itself, one could easily deal with the consequence of
the fluid’s presence or lack in the object; that is, the
positive charge (indicating a surplus of fluid) or a
negative charge (indicating a lack of fluid) or a
neutral charge (indicating neither too much nor too
little fluid).
No. Glass is an insulator not a conductor. The
electroscope will not discharge.
It will discharge. People are good conductors.
Q11
Q12
Q13
Q14
Q15
Q16
Q17
Q18
Q19
Charging by induction results in like charge.
Therefore the charge on the ball will be negative, as
is the charge of the plastic rod.
No. The separated charges will return to their
undisturbed positions. There will not be any nearby
charge imbalance to hold them in separation.
Yes. The bits of paper become polarized by the
charged rod being brought close, with the sign of the
charge on the side of the bit of paper near the rod
being opposite to the sign of charge on the rod.
The uncharged pith balls were attracted to the
charges rod because unbalanced forces were
induced on the balls by the rod. When contact is
made, the balls gain charge of the same sign as the
rod.
Yes. The repulsive force between two charges
produces a torque about the thin wire that supports
the beam causing the wire to twist. The twisting of
the wire develops an elastic restoring torque that
returns the beam to its original position when one
charge is removed.
Start with four insulated spheres, one with at least
16 units of charge, the others uncharged. Touch the
charged sphere to a second sphere. Both of these
spheres now carry a charge of 8 units. Touch one of
these to an uncharged sphere. Now there are three
charged spheres, one with eight units and two with
four units of charge each. Repeat, touching one of
the four unit charge carries to the last uncharged
sphere. Now there are four charged spheres with
units of charge eight, four, two, and two
respectively.
No. Since the Coulomb force varies inversely as the
square of the distance between the charges, doubling
the distance reduces the force by a factor of 1/4.
The force between the two charges will be
quadrupled. The Coulomb force between the charges
varies as the product of the two charges. Doubling
each gives a four-fold increase.
No. The force between charges can be either
attractive or repulsive but the gravitational force
between matter is always attractive.
Q24
Q25
Q26
Q27
Q28
Q29
Q20
Q30
Q31
Q21
Q22
Yes. All that is required is that there exists a charge
somewhere in the space so that it can exert a force
on a positive test charge brought to a given point.
The field at A is zero.
Q32
Q23
34
No. The electric field lines that we can imagine
emanating from the charge diverge with increasing
distance indicating a field that decreases in strength
with distance.
Potential energy decreases. A positive charge that is
free to move will go from higher to lower potential
energy, that is toward the negative charge.
The potential energy increases. Potential energy is
the work against a conservative force. To move the
negative charge at constant velocity closer to the
negative charge requires a force opposite the
direction the charge is moved.
The potential energy of the negative charge
increases. The force on the charge will be opposite
the direction to the field, so moving the charge in the
direction of the field requires positive work against the
field.
Electric potential is defined with respect to position
of a unit positive charge. Potential energy of a
positive charge decreases with approach to
negative charge. Hence electric potential decreases
as we move closer to a negative charge.
They are related concepts. We can consider the
potential energy that a given charge would
experience at a point in an electric field. The electric
potential is the potential energy experienced by a unit
positive charge at a point, so it is a quantity that is
determined by the field, independent of the test
charge (provided it is small enough not to distort the
field).
The electric potential at each point is the same.
Identical test charges placed at either point would
require the same force to hold it in place against the
present electric field.
No, higher. Electric field lines go from high to low
potential. A negatively charged particle which is free
to move will experience a force opposite the direction
of the field, so it will move from lower to higher
electric potential, unless you define electric potential
to be greater for any charge moved against the force.
No. A negatively charged particle is attracted to the
region of higher potential because that region
contains (more) positive charge unless, of course,
you are the only conductor around. But at least you
would be insulated from the ground so that you would
not discharge violently.
SP2
Answers to Exercises
+13
E1
E2
E3
E4
E5
E6
3.0 x 10 electrons
+9 C
+4 C each
32 N
3N
a. 14.4 N
b. see diagram below
E7
a. 1.8 N
b. see diagram below
E8
9.2 x 10 N along a straight line from the proton to
the electron.
100 N downward
6
3.0 x 10 N/C downward
6
2.67 x 10 N/C east
0.26 N to the left
12.5 J
-24 J
20,000 V
a. 0.2 J
b. Decreases
E9
E10
E11
E12
E13
E14
E15
E16
a.
b.
-8
SP3
a.
Answers to Synthesis Problems
SP1
a.
b.
c.
d.
e.
6
4.5 x 10 N
6
7.2 x 10 N
6
2.7 x10 N to the left
8
1.35 x 10 N/C to the left
6
8.1 x 10 N to the right
b.
35
SP4
a.
b.
SP5
a.
b.
c.
d.
Yes, at point A. Charges at opposite ends of a
diagonal would exert equal and opposite forces
on a unit test charge placed at the center.
-0.12 J
Toward the top plate
Toward the top plate
4
3.33 x 10 N/C
36