Conceptual Physics

Chapter Assumptions

For any problems in this chapter that involve a household electrical outlet, assume that the voltage supplied is fixed at 120 V. While

this is not true for actual outlets, whose voltage varies and periodically exceeds 120 V, the simplification will not affect the accuracy

of your answer.

Chapter 25 Problems

Conceptual Problems

C.1 The three parts of the illustration show positive

and negative charges moving in the indicated

directions, at the rates shown. (a) Which

scenario depicts the greatest current? (b)

Which scenario depicts the smallest current?

(a) 1 2 3

(b) 1 2 3

C.2 3 amperes of current flow through a wire. (a) What net charge flows past a point in the wire each second? (b) What is the net

charge on the wire?

(a) C

(b) C

C.3 A current of 2 amperes flows to the left in a stationary wire. If the wire also starts moving to the left, what happens to the

current as measured by a stationary observer? Why?

i. The current increases

ii. The current decreases

iii. The current stays the same

C.4 Two like charges of one coulomb each, separated by one meter, repel each other with a force of 8.99×10^9 N, equivalent to

the weight of 10 battleships. A flashlight bulb may have a current of 0.40 A flowing through it, which is equivalent to one

coulomb of charge flowing through every 2.5 seconds. But a flashlight circuit is clearly not subject to tremendous electrostatic

forces -- or even small ones. Why not?

C.5 A section in the textbook that explains drift speed displays a diagram that shows the zigzag path followed by a black electron

as it pursues an average path to the right through a section of conducting wire, but collides with many yellow atoms in the

wire on its way. Suppose that the black circle in the diagram were a steel ball instead of an electron, and that the yellow

circles were bumpers in a pinball game instead of atoms. That is, suppose this were a diagram of a pinball machine in play. In

a pinball game, what field is acting analogously to the uniform electric field that propels electrons through a wire?

C.6 The section in the textbook that explains drift speed displays a diagram that shows the zigzag path followed by an electron as

it pursues an average path to the right through a section of conducting wire, but collides with many atoms in the wire on its

way. The diagram correctly shows the zigs and zags as being curved rather than straight line segments. (a) Why is this? (b)

What is the shape of the curved line segments?

(a)

(b)

C.7 A resistor has 2.0 A of current flowing through it when the potential difference across it is 6.5 V. When the potential difference

is increased to 19.5 V, the current increases to 5.5 A. What kind of material is this?

Ohmic Non-ohmic

C.8 Two wires are made from materials, such as copper and aluminum, that have different resistivities. The wires have the same

length. Is it possible for them to have the same resistance? Explain.

Yes No

C.9 Would you expect there to be any materials that have both high resistivity and high conductivity? Explain.

Yes No

C.10Equations are given in the textbook that allow you to calculate how the resistivity of materials and resistance of objects vary

with the temperature. The discussion of the equations does not specify whether the temperatures in the equations should be

measured in kelvin or in degrees Celsius. Does it matter? If it matters, deduce which should be used. If it doesn't matter, tell

(^470) Copyright 2007 Kinetic Books Co. Chapter 25 Problems