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Lecture

BIO120H1 Lecture Notes - Electric Field, Net Force, Resultant Force


Department
Biology
Course Code
BIO120H1
Professor
Jennifer Harris

Page:
of 17
Physics NYB
Solutions to Serway’s end of chapter problems, 9th edition.
ANSWERS TO CONCEPTUAL QUESTIONS
CQ23.1 No. Life would be no different if electrons were positively charged and
protons were negatively charged. Opposite charges would still attract, and like
charges would repel. The naming of positive and negative charge is merely a
convention.
CQ23.2 The dry paper is initially neutral. The comb attracts the paper because its
electric field causes the molecules of the paper to become polarizedthe paper
as a whole cannot be polarized because it is an insulator. Each molecule is
polarized so that its unlike-charged side is closer to the charged comb than its
like-charged side, so the molecule experiences a net attractive force toward the
comb. Once the paper comes in contact with the comb, like charge can be
transferred from the comb to the paper, and if enough of this charge is
transferred, the like-charged paper is then repelled by the like-charged comb.
CQ23.3 The answer depends on whether the person is initially (a) uncharged or (b)
charged.
(a) No. If the person is uncharged, the electric field inside the sphere is zero.
The interior wall of the shell carries no charge. The person is not harmed
by touching this wall.
(b) If the person carries a (small) charge q, the electric field inside the sphere is
no longer zero. Charge q is induced on the inner wall of the sphere. The
person will get a (small) shock when touching the sphere, as all the charge
on his body jumps to the metal.
CQ23.4 All of the constituents of air are nonpolar except for water. The polar water
molecules in the air quite readily “steal” charge from a charged object, as any
physics teacher trying to perform electrostatics demonstrations in humid
weather well knows. As a resultit is difficult to accumulate large amounts of
excess charge on an object in a humid climate. During a North American
winter, the cold, dry air allows accumulation of significant excess charge,
giving the potential (pun intended) for a shocking (pun also intended)
introduction to static electricity sparks.
CQ23.5 No. Object A might have a charge opposite in sign to that of B, but it also might
be neutral. In this latter case, object B causes object A (or the molecules of A if its
material is an insulator) to be polarized, pulling unlike charge to the near face of
A and pushing an equal amount of like charge to the far face. Then the force of
attraction exerted by B on the induced unlike charge on the near side of A is
slightly larger than the force of repulsion exerted by B on the induced like charge
on the far side of A. Therefore, the net force on A is toward B.
CQ23.6 (a) Yes. The positive charges create electric fields that extend in all directions
from those charges. The total field at point A is the vector sum of the
individual fields produced by the charges at that point.
(b) No, because there are no field lines emanating from or converging on
point A.
(c) No. There must be a charged object present to experience a force.
CQ23.7 The charge on the ground is negative because electric field lines produced by
negative charge point toward their source.
CQ23.8 Conducting shoes are worn to avoid the build up of a static charge on them as
the wearer walks. Rubber-soled shoes acquire a charge by friction with the
floor and could discharge with a spark, possibly causing an explosive burning
situation, where the burning is enhanced by the oxygen.
CQ23.9 (a) No. The balloon induces polarization of the molecules in the wall, so that
a layer of positive charge exists near the balloon. This is just like the
situation in Figure 23.4a, except that the signs of the charges are
reversed. The attraction between these charges and the negative charges
on the balloon is stronger than the repulsion between the negative
charges on the balloon and the negative charges in the polarized
molecules (because they are farther from the balloon), so that there is a
net attractive force toward the wall.
(b) Polar water molecules in the air surrounding the balloon are attracted to
the excess electrons on the balloon. The water molecules can pick up and
transfer electrons from the balloon, reducing the charge on the balloon
and eventually causing the attractive force to be insufficient to support
the weight of the balloon.
CQ23.10 (a) Yes. (b) The situation is similar to that of magnetic bar magnets, which can
attract or repel each other depending on their orientation.
CQ23.11 Electrons have been removed from the glass object. Negative charge has been
removed from the initially neutral rod, resulting in a net positive charge on the
rod. The protons cannot be removed from the rod; protons are not mobile
because they are within the nuclei of the atoms of the rod.
SOLUTIONS TO END-OF-CHAPTER PROBLEMS
P23.5 The electric force is given by
P23.10 (a)
(b)
The electric force is
(c) If with q1 = q2 = q and m1 = m2 = m, then
P23.11 The particle at the origin carries a
positive charge of 5.00 nC. The
electric force between this particle
and the 3.00-nC particle located
on the y axis will be
attractive and point toward
the y direction and is shown
with in the diagram, while
the electric force between this particle and the 6.00-nC particle located on the x
axis will be repulsive and point toward the x direction, shown with in the
diagram. The resultant force should point toward the third quadrant, as shown
in the diagram with Although the charge on the x axis is greater in
magnitude, its distance from the origin is three times larger than the 3.00-nC
charge. We expect the resultant force to make a small angle with the y axis and
be approximately equal in magnitude with F3.
From the diagram in ANS. FIG. P23.11, the two forces are perpendicular, and
the components of the resultant force are
ANS. FIG. P23.11