As.rFall ‘12
PHY 122 Homework Solutions #1
Chapter 21 Question 4
A positively charged rod is brought close to a neutral piece of paper,
which it attracts. Draw a diagram showing the separation of charge in
the paper, and explain why attraction occurs.
Solution
Fig 21-9.
The neutral piece of paper is composed of atoms and molecules
which consist of positively charged nuclei surrounded by negatively
charged electrons. The introduction of a positively charged rod will
attract the electrons while maintaining their bond within the atom or
molecule. Since the negatively charged electrons are closer than the
positively charged nuclei the paper will be feel a stronger Coulombic
attractive force than a Coulombic repulsive force.
Chapter 21 Question 6
Contrast the net charge on a conductor to the “free charges” in the
conductor.
Solution
The net charge is difference between the total positive and negative
charge on the conductor. The “free charges” are the sum of all the
total electrons that free to move within the conductor. Thus it is
possible for a conductor to have free charges of electrons but a net
charge of zero.
Chapter 21 Question 7
Figures 21–7 and 21–8 show how a charged rod placed near an
uncharged metal object can attract (or repel) electrons. There are a
great many electrons in the metal, yet only some of them move as
shown. Why not all of them?
Solution
Fig 21-7
Fig 21-8
In Figure 21-7, electrons will move until the attractive force on the
remaining conduction electrons due to the incoming charged rod is
balanced by the repulsive force from electrons that have already
gathered at the left end of the neutral rod. In Figure 21-8, conduction
electrons will be repelled by the incoming rod and will leave the
stationary rod through the ground connection until the repulsive
force on the remaining conduction electrons due to the incoming
charged rod is balanced by the attractive force from the net positive
charge on the stationary rod.
Chapter 21 Question 10
We are not normally aware of the gravitational or electric force
between two ordinary objects. What is the reason in each case? Give
an example where we are aware of each one and why.
Solution
The gravitational force between everyday objects on the surface of
the Earth is extremely small. Consider two objects sitting on the floor
near each other. They are attracted to each other, but the force of
static friction for each is much greater than the gravitational force
each experiences from the other. Even in an absolutely frictionless
environment, the acceleration resulting from the gravitational force
would be so small that it would not be noticeable in a short time
frame. We are aware of the gravitational force between objects if at
least one of them is very massive, as in the case of the Earth and
satellites or the Earth and you. The electric force between two
objects is typically zero or close to zero because ordinary objects are
typically neutral or close to neutral. We are aware of electric forces
between objects when the objects are charged. An example is the
electrostatic force (static cling) between pieces of clothing when you
pull the clothes out of the dryer.
Chapter 21 Question 18
Consider the electric field at the three points indicated by the letters
A, B, and C in Fig. 21–51. First draw an arrow at each point
indicating the direction of the net force that a positive test charge
would experience if placed at that point, then list the letters in order
of decreasing field strength (strongest first).
Solution
At point C, the positive test charge would experience zero net force.
At points A and B, the direction of the force on the positive test
charge would be the same as the direction of the field. This direction
is indicated by the arrows on the field lines. The strongest field is at
point A, followed (in order of decreasing field strength) by B and
then C.
Chapter 21 Problem 3
(I) What is the magnitude of the force a +25μC charge exerts on a
+2.5mC charge 28 cm away?
Chapter 21 Problem 8
A person scuffing her feet on a wool rug on a dry day accumulates a
net charge of -46μC. How many excess electrons does she get, and by
how much does her mass increase?
Chapter 21 Problem 9
(II) What is the total charge of all the electrons in a 15-kg bar of
gold? What is the net charge of the bar? (Gold has 79 electrons per
atom and an atomic mass of 197 u.)
Chapter 21 Problem 11
(II) Two positive point charges are a fixed distance apart. The sum of
their charges is QT? What charge must each have in order to (a)
maximize the electric force between them, and (b) minimize it?
Chapter 21 Problem 20
(III) Two small charged spheres hang from cords of equal length ,
as shown in Fig. 21–55 and make small angles θ1 and θ2 with the
vertical. (a) If Q1=Q, Q2=2Q and m1 = m2=m determine the ratio
θ1/θ2. (b) If Q1=Q, Q2=2Q, m1 =m, and m2=2m determine the ratio
θ1/θ2. (c) Estimate the distance between the spheres for each case.
Chapter 21 Problem 26
(I) What is the electric field at a point when the force on a 1.25μC
charge placed at that point is F = (3.0i – 3.9j) x 10-3 N?