PHYSICS 212
CHAPTER 17
CURRENT AND RESISTANCE
WORKBOOK
ANSWERS
_____________________________________________
STUDENT’S FULL NAME
(By placing your name above and submitting this for credit
you are affirming this to be predominantly your own work.)
_____ / _____ / _____
DATE DUE
INSTRUCTIONS
1. Turn this workbook in on time for credit, even if it is not complete. (No credit
if late.)
2. Complete this workbook neatly. Do not write in ink so that corrections can be
made. (Credit will be lost if this is turned in messy.)
3. Complete the chapter outline section as early as possible. Don’t wait for the due
date to be assigned to start.
4. Complete the sections in sequence.
5. Study and learn definitions of terms, physical quantities, units, principles, and
basic equations before attempting problems.
6. You may work on this with other students but do not copy another student’s
workbook or let a student copy this workbook. Do not copy from other sources
either.
7. Wherever possible, include diagrams in your solutions. Diagrams are required.
8. Keep this workbook after it is graded and returned to you.
9. Using the answer key, redo all questions and problems until you can answer
them all correctly by yourself without help.
10. Use the workbook to learn the general problem-solving strategy rather than
how individual problems are solved.
-1-
PHYSICS 212 CHAPTER 17 OUTLINE
CURRENT AND RESISTANCE
17.1
ELECTRIC CURRENT
Define electric current:
Electric current is the flow of electric charge.
What is meant by the phrase conventional current?
The net flow or effective flow of positive charge.
17.2
A MICROSCOPIC VIEW: CURRENT AND DRIFT SPEED
Describe how charges move in a conductor in electrostatic equilibrium.
Electric charges in electric equilibrium do not move.
Describe how charges move in a conductor when an electric field is present.
Negative charges move in the opposite direction of the electric field in a slow drift
due to the presence of stationary, positive nuclei.
Give an approximate value for the drift speed of electrons in a conducting wire?
10-4 m/s
17.3
CURRENT AND VOLTAGE MEASUREMENTS IN CIRCUITS
What meter is used to measure current?
Ammeter
What meter is used to measure voltage differences?
Voltmeter
-2-
Draw a circuit diagram showing how these two meters would be connected to measure the current
through and the voltage drop across a resistor.
What are the SI units of current and voltage drop?
Current - Amp (A)
17.4
Voltage Drop - Volt (V)
RESISTANCE AND OHM’S LAW
What is resistance?
Resistance is the ability to oppose, reduce and limit the flow of charge.
What is a resistor?
A resistor is a device whose main property is resistance.
How are the quantities I, V, and R related? Write an equation? What is this relation called?
Ohm’s law:
17.5
RESISTIVITY
List and define all the factors that contribute to the value of a resistor’s resistance.
Its length, cross-sectional area, and resistivity.
What are the SI units of resistivity?
-3-
17.6
TEMPERATURE VARIATION OF RESISTANCE
How does resistivity usually vary with temperature?
resistivity in typical materials increases non-linearly with temperature.
Write equation 17.6. List and define each of the variables and constants in this equation also stating
their SI units.
17.7
SUPERCONDUCTORS
What is meant by the phrase critical temperature?
The temperature at which the resistance in certain metals falls to virtually zero.
What happens to some materials when they are cooled below TC?
The resistance is virtually zero.
What happens to a current in a superconducting loop over time?
Nothing, it appears to remain intact.
What application does superconductivity have?
Superconducting electromagnets.
17.8
ELECTRICAL ENERGY AND POWER
Define the physical quantity power?
The rate at which energy is used or transformed.
-4-
How is energy supplied to a circuit?
Energy is supplied by an electromotive force, emf.
How is the rate of energy supplied calculated?
How is energy consumed by a circuit?
Energy is lost by the circuit as current passes through resistance.
How is the rate of energy consumed calculated?
Explain what is meant by the unit Kilowatt@Hour.
This is a unit of energy.
17.9
ELECTRICAL ACTIVITY IN THE HEART
What do the initials EKG stand for?
electrocardiogram
Describe a muscle cell in the resting state by explaining what is meant by the term polarized.
A cell is said to be polarized when the concentrations of ions inside and outside the
cell membrane establish a charge distribution and a voltage across the cell
membrane.
What does a depolarization wave do?
A depolarization wave changes the ion concentrations and alters the electric field.
A normal EKG shows a series of pulses. List these pulses and state what happens to cause these
pulses.
P- just before atrial contraction
Q, R, & S- just before ventricles contract
T- when the ventricles begin to recover
-5-
PHYSICS 212
CHAPTER 17 MULTIPLE CHOICE QUESTIONS
CURRENT AND RESISTANCE
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
C____ 1. The current in an electron beam in a cathode-ray tube is measured to be 70 :A. How many electrons hit the
screen in 5.0 s? (e = 1.6 ´ 10-19 C)
A. 2.2 ´ 1011 electrons
D. 8.8 ´ 1018 electrons
B. 8.8 ´ 1013 electrons
E. 2.2 ´ 1020 electrons
15
C. 2.2 ´ 10 electrons
C____ 2. A wire carries a steady current of 0.1 A over a period of 20 s. What total charge passes through the wire in
this time interval?
A. 200 C
D. 0.005 C
B. 20 C
E. 0.002 C
C. 2 C
B____ 3. If the current in a wire is tripled, what effect does this have on the electron drift velocity in the wire?
A. It stays the same.
D. It increases by a factor of nine.
B. It triples.
E. It decreases by a factor of nine.
C. It decreases by a factor of three.
E____ 4. The size of the electric current in an electrical conductor is a function of which of the following?
A. velocity of charge carriers
D. conductor length
B. conductor cross sectional area
E. All of the above choices are valid.
C. density of charge carriers
B____ 5. When an electric current exists within a conducting wire, which of the following statements describes the
condition of any accompanying electric field?
A. must be zero
D. must be perpendicular to current flow
B. must be parallel to current flow
E. None of the above choices is valid.
C. must be anti-parallel (opposite direction)
to current flow
C____ 6. Materials having resistance changes as voltage or current varies are called:
A. ohmic.
D. deohmic.
B. inohmic.
E. ohmless.
C. nonohmic.
B____ 7. You measure a 25.0-V potential difference across a 5.00-W resistor. What is the current flowing through it?
A. 125 A
D. 1.00 A
B. 5.00 A
E. 0.125 A
C. 4.00 A
-6-
B____ 8. The unit of electric current, the ampere, is equivalent to which of the following?
A. V×W
D. V/s
B. V/W
E. W/V
C. W×m
A____ 9. The unit of electric resistance, the ohm, is equivalent to which of the following?
A. V/A
D. A/m
B. V×m
E. A/V
C. A/s
E___ 10. If a certain resistor obeys Ohm's law, its resistance will change:
A. as the voltage across the resistor changes.
B. as the current through the resistor changes.
C. as the energy given off by the electrons in their collisions changes.
D. as the electric field inside the resistor changes.
E. none of the above, since resistance is a constant for the given resistor.
A___ 11. The resistivity of a material is doubled when heated a certain amount. What happens to the resistance of a
resistor made of this material when heated the same amount.
A. It doubles.
D. It stays the same.
B. It quadruples.
E. It squares.
C. It halves.
C___ 12. The temperature coefficient of resistivity is a quantity that is:
A. always positive.
D. represented by the symbol s.
B. always non-negative.
E. always negative.
C. sometimes negative.
B___ 13. The temperature coefficient of resistivity for a "perfect" ohmic material would be:
A. positive and constant.
D. positive and uniformly increasing.
B. zero.
E. positive and uniformly decreasing.
C. negative.
C___ 14. Superconductivity was discovered by:
A. Volta.
B. Ohm.
C. Onnes.
D. Bednorz and Müller.
E. Joule and Lentz.
D___ 15. A superconducting wire's chief characteristic is which of the following?
A. an extremely great length
D. no resistance
B. a large cross sectional area
E. a great number of charge carriers
C. an extremely high temperature
C___ 16. When a superconductor's temperature drops below the critical temperature, its resistance:
A. equals that of a semiconductor of equal
D. reduces to one half.
dimensions.
B. increases by two.
E. becomes negative.
C. drops to zero.
-7-
E___ 17. Consider some material that has been cooled until it has become a superconductor. If it is cooled even further
its resistance will:
A. increase.
D. oscillate around zero
B. decrease.
E. none of the above.
C. stay constant and non-zero.
A___ 18. When current is flowing in a superconductor, which statement is not true?
A. A battery is needed to keep the current going.
B. Electrical charges are moving.
C. The resistance is zero.
D. No power is given off in the form of heat.
E. The conductivity is infinite.
A___ 19. A 60-W light bulb is in a socket supplied with 120 V. What is the current in the bulb?
A. 0.50 A
D. 7 200 A
B. 2.0 A
E. 10 000 A
C. 60 A
D___ 20. The quantity volt is equivalent to which of the following?
A. J×m
D. J/C
B. J×C
E. C/J
C. C/W
C___ 21. The unit for rate of energy transformation, the watt, in an electric circuit is equivalent to which of the
following?
A. V/s
D. V/W
B. A×W
E. A/V
C. V×A
B___ 22. If a 500-W heater carries a current of 4.00 A, what is the voltage across the ends of the heating element?
A. 2 000 V
D. 0.008 V
B. 125 V
E. 0.125 V
C. 250 V
C___ 23. A light bulb has resistance of 240 W when operating at 120 V. Find the current in the light bulb.
A. 2.0 A
D. 0.20 A
B. 1.0 A
E. 0.30 A
C. 0.50 A
D___ 24. Which is a unit of power?
A. kWh
B. W/s
C. A×W
D. J/s
E. J/kg
C___ 25. Which is not a force?
A. gravity
B. electrical force
C. voltage
D. friction
E. sliding resistance force
-8-
B___ 26. A light bulb with a tungsten filament is attached to a source of variable voltage. As the voltage is increased
on the bulb,
A. the bulb's resistance decreases.
D. the power dissipated remains constant.
B. the bulb's resistance increases.
E. the bulb's resistance remains constant.
C. the current in the bulb decreases.
-9-
PHYSICS 212
1.
2.
CHAPTER 17 ADDITIONAL QUESTIONS
CURRENT AND RESISTANCE
A 12 gauge aluminum wire has a diameter of 2.0 mm. If the wire carries a 15 A current, find the
drift velocity of the electrons in aluminum. The electron density of aluminum is 6.0 × 1028 electrons
per m3.
A certain copper wire is 1.0 m long, 0.5mm in radius. What is the resistance of the wire?
-10-
3.
A gold wire 2mm in diameter and 100 m long is heated to 300 oC. What is its resistance at this
temperature?
4.
If the gold wire in problem 3 is connected to a 24 V power supply, what current will flow through
the wire and what power will be supplied?
-11-
PHYSICS 212 CHAPTER 17 PROBLEMS
CURRENT AND RESISTANCE
PRACTICE PROBLEMS:
HOMEWORK PROBLEMS:
9, 17, 19, 21, 25, 31, 45, 47
1, 4, 7, 11, 18, 27, 28, 35, 39
Include diagrams wherever possible.
1.
If a current of 80.0 mA exists in a metal wire, how many electrons flow past a given cross section of
the wire in 10.0 min? Sketch the direction of the current and the direction of the electrons’ motion.
The charge that moves past the cross section is
, and the number of electrons is
The negatively charged electrons move in the direction opposite to the conventional current flow.
4.
In a particular television picture tube, the measured beam current is 60.0 :A. How many electrons
strike the screen every second?
and the number of electrons is
7.
A 200-km-long high-voltage transmission line 2.0 cm in diameter carries a steady current of 1 000
A. If the conductor is copper with a free charge density of 8.5 × 1028 electrons per cubic meter, how
many years does it take one electron to travel the full length of the cable?
The drift speed of electrons in the line is
, or
-12-
The time to travel the length of the 200-km line is then
11.
A person notices a mild shock if the current along a path through the thumb and index finger exceeds
80 :A. Compare the maximum possible voltage without shock across the thumb and index finger
with a dry-skin resistance of 4.0 × 105 S and a wet-skin resistance of 2 000 S.
Thus, if
and if
18.
,
,
A rectangular block of copper has sides of length 10 cm, 20 cm, and 40 cm. If the block is connected
to a 6.0-V source across two of its opposite faces, what are (a) the maximum current and (b) the
minimum current that the block can carry?
With different orientations of the block, three different values of the ratio
These are:
,
,
and
(a)
(b)
-13-
are possible.
27.
(a) A 34.5-m length of copper wire at 20.0 OC has a radius of 0.25 mm. If a potential difference of
9.0 V is applied across the length of the wire, determine the current in the wire. (b) If the wire is
heated to 30.0 OC while the 9.0-V potential difference is maintained, what is the resulting current in
the wire?
(a)
The resistance at 20.0/C is
and the current will be
(b)
At 30.0/C,
Thus, the current is
28.
A toaster rated at 1 050 W operates on a 120-V household circuit and a 4.00-m length of nichrome
wire as its heating element. The operating temperature of this element is 320 OC. What is the crosssectional area of the wire?
The resistance of the heating element when at its operating temperature is
From
, the cross-sectional area is
-14-
35.
The heating element of a coffee maker operates at 120 V and carries a current of 2.00 A. Assuming
that the water absorbs all of the energy converted by the resistor, calculate how long it takes to heat
0.500 kg of water from room temperature (23.0 OC) to the boiling point.
The energy required to bring the water to the boiling point is
The power input by the heating element is
Therefore, the time required is
39.
A copper cable is designed to carry a current of 300 A with a power loss of 2.00 W/m. What is the
required radius of this cable?
The resistance per unit length of the cable is
From
sectional area is
, the resistance per unit length is also given by
, and the required radius is
-15-
. Hence, the cross-
-16-