CHAPTER 21 DISCUSSION QUESTIONS
Q 21.8 Good conductors of electricity, such as metals, are typically good conductors of heat; insulators, such as
wood, are typically poor conductors of heat. Explain why there is a relationship between conduction of electricity
and conduction of heat in these materials.
Q 21.12 If you walk across a nylon rug and then touch a large metal object such as a doorknob, you may get a
spark and a shock. Why does this tend to happen more on dry days than on humid days? (Hint: See Fig. 21.30.)
Why are you less likely to get a shock if you touch a small metal object, such as a paper clip?
Q 21.14 When two point charges of equal mass and charge are released on a frictionless
table, each has an initial acceleration (magnitude) a0. If instead you keep one fixed and
release the other one, what will be its initial acceleration: a0, 2a0, or a0/2? Explain.
Q 21.22 The electric fields at point P due to the positive charges q1 and q2 are shown in Fig.
Q21.22. Does the fact that they cross each other violate the statement in Section 21.6 that
electric field lines never cross? Explain
CHAPTER 21 PROBLEMS
P 21.58 Two charges are placed on the x-axis: one, of 2.50 mC, at the origin
and the other, of -3.50 mC, at x =0.600 m (Fig. P21.58). Find the position on
the x-axis where the net force on a small charge +q would be zero.
P 21.63 A small 13.0 g plastic ball is tied to a very light 27.2 cm string that is attached to the vertical
wall of a room (Fig. P21.63). A uniform horizontal electric field exists in this room. When the ball has
been given an excess charge of -1.10 mC, you observe that it remains suspended, with the string
making an angle of 17.4° with the wall. Find the magnitude and direction of the electric field in
the room.
P 21.74 Two tiny spheres of mass 6.80 mg carry charges of equal magnitude, 72.0 nC, but
opposite sign. They are tied to the same ceiling hook by light strings of length 0.530 m.
When a horizontal uniform electric field E that is directed to the left is turned on, the
spheres hang at rest with the angle u between the strings equal to 50.0° (Fig. P21.74).
(a) Which ball (the one on the right or the one on the left) has positive charge?
(b) What is the magnitude E of the field
P 21.75 Consider a model of a hydrogen atom in which an electron is in a circular orbit of radius r =5.29X10^-11 m
around a stationary proton. What is the speed of the electron in its orbit?
