1. Cutnell/Johnson Physics 8th edition
Classroom Response System Questions
Chapter 20 Electric Circuits
Interactive Lecture Questions

2. In which one of the following situations does a conventional electric current flow due north?
a) Protons in a beam are moving due south.
b) A water molecule is moving due north.
c) Electrons in a beam are moving due south.
d) Electrons in a wire connected to a battery are moving from south to north.

3. The battery capacity of a lithium ion battery in a digital music player is 750 mA-h. The manufacturer claims that the player can operate for eight hours if the battery is initially fully charged. Given this information, determine the number of electrons that flow through the player as you listen to your favorite songs for three hours.
a) 6.2  1018 electrons
b) 1.0  103 electrons
c) 2.4  109 electrons
d) 6.3  1021 electrons
e) 8.1  1028 electrons

4. In a certain circuit containing a battery and a resistor, Ohm’s law is obeyed. An instrument to measure the current in the circuit, an ammeter, is connected in between one of the terminals of the battery and one end of the resistor. The ammeter indicates that the current in the circuit is I. The battery is then removed and replaced with another battery. This time, the ammeter indicates the current is 2I. Which one of the following statements concerning the resistor is true?
a) When the second battery was placed in the circuit, the resistance increased to twice its initial value.
b) When the second battery was placed in the circuit, the resistance decreased to one half its initial value.
c) When the second battery was placed in the circuit, the resistance increased to four times its initial value.
d) When the second battery was placed in the circuit, the resistance increased to one fourth its initial value.
e) When the second battery was placed in the circuit, the resistance did not change.

5. Consider the circuit containing a battery and a resistor shown. For which one of the following combinations of current and voltage does R have the smallest value?
a) V = 9 V and I = A
b) V = 12 V and I = 0.5 A
c) V = 1.5 V and I = A
d) V = 6 V and I = 0.1 A
e) V = 4.5 V and I = A

6. 20. 2. 3. A certain circuit contains a battery and a resistor
A certain circuit contains a battery and a resistor. An instrument to measure the current in the circuit, an ammeter, is connected in between one of the terminals of the battery and one end of the resistor. The graph shows the current in the circuit as the voltage is increased. Which one of the following statements best describes the resistor in this circuit?
a) The resistor does not
obey Ohm’s law.
b) The resistor obeys Ohm’s
law for voltages between zero
and twenty-five volts.
c) The resistor obeys Ohm’s law
for voltages between zero and
thirty-five volts.
d) The resistor obeys Ohm’s law for
voltages between zero and forty volts.
e) The resistor obeys Ohm’s law for voltages between thirty and forty volts.

7. For which combination for the length L and radius R of a wire will the resistance have the smallest value?
a) L = 0.50 m and R = 0.03 m
b) L = 0.25 m and R = 0.08 m
c) L = 0.40 m and R = 0.2 m
d) L = 0.80 m and R = 0.1 m
e) L = 0.10 m and R = 0.05 m

8. The ends of a wire are connected to the terminals of a battery. For which of the following changes will the resulting current in the circuit have the largest value?
a) Replace the wire with one that has a larger resistivity.
b) Replace the wire with one that has a larger radius.
c) Replace the wire with one that has a longer length.

9. An automatic coffee maker uses a resistive heating element to boil the 2.4 kg of water that was poured into it at 21 C. The current delivered to the coffee pot is 8.5 A when it is plugged into a 120 V electrical outlet. If the specific heat capacity of water is 4186 J/kgC, approximately how long does it take to boil all of the water?
a) 5 minutes
b) 8 minutes
c) 10 minutes
d) 13 minutes
e) 15 minutes

10. The insulated wiring in a house can safely carry a maximum current of 18 A. The electrical outlets in the house provide an alternating voltage of 120 V. A space heater when plugged into the outlet operates at an average power of 1500 W. How many space heaters can safely be plugged into a single electrical outlet and turned on for an extended period of time?
a) zero
b) one
c) two
d) three
e) four

11. A portable CD player was recently introduced that has a “special power saving technology.” The manufacturer claims that with only two standard AA batteries (together: 3.0 V, 20 kJ energy storage) that the player can be played for about 25 hours. What is the approximate resistance in the CD player’s electrical circuitry?
a) 40 
b) 
c) 300 
d) 1.5 
e) 15 

12. A wire is used as a heating element that has a resistance that is fairly independent of its temperature within its operating range. When a current I is applied to the wire, the energy delivered by the heater each minute is E. For what amount of current will the energy delivered by the heater each minute be 4E?
a) 2I
b) 4I
c) 0.5I
d) 0.25I
e) 8I

13. The graph shows the current as a function of time for an electrical device plugged into a outlet with an rms voltage of 120 V. What is the resistance of the device?
a) 24 
b) 21 
c) 17 
d) 14
e) 12 

14. 20. 5. 2. Consider the circuits shown in parts A and B in the picture
Consider the circuits shown in parts A and B in the picture. In part A, a light bulb is plugged into a wall outlet that has an rms voltage of 120 volts. A current I passes through the circuit and the bulb turns on. In part B, a second, identical light bulb is connected in series in the circuit. How does the current in circuit B compare with that in circuit A?
a) The current is the same, I, as in part A.
b) The current is twice as much, 2I, as in part A.
c) The current in part B is zero amperes.
d) The current is one fourth as much, 0.25I, as in part A.
e) The current is one half as much, 0.5I, as in part A.

15. 20. 6. 1. Consider the circuit shown in the drawing
Consider the circuit shown in the drawing. Two identical light bulbs, labeled A and B, are connected in series with a battery and are illuminated equally. There is a switch in the circuit that is initially open. Which one of the following statements concerning the two bulbs is true after the switch is closed?
a) Bulbs A and B will be off.
b) Bulbs A and B will be equally
illuminated.
c) Bulb A will be brighter and bulb B
will be off.
d) Bulb A will be off and bulb B
will be brighter.
e) Both bulbs will be dimmer than before the switch was closed.

16. Consider the three resistors and the battery in the circuit shown. Which resistors, if any, are connected in series?
a) R1 and R2
b) R1 and R3
c) R2 and R3
d) R1 and R2 and R3
e) No resistors are connected in series.

17. Consider the three resistors and the battery in the circuit shown. Which resistors, if any, are connected in parallel?
a) R1 and R2
b) R1 and R3
c) R2 and R3
d) R1 and R2 and R3
e) No resistors are connected in parallel.

18. 20. 7. 2. Consider the circuits shown in parts A and B in the picture
Consider the circuits shown in parts A and B in the picture. In part A, a light bulb is plugged into a wall outlet that has an rms voltage of 120 volts. A current I passes through the circuit and the bulb turns on. In part B, a second, identical light bulb is connected in parallel in the circuit. How does the total current in circuit B compare with that in circuit A?
a) The current is the same, I, as in part A.
b) The current is twice as much, 2I, as
in part A.
c) The current in part B is zero amperes.
d) The current is one fourth as much, 0.25I, as in part A.
e) The current is one half as much, 0.5I, as in part A.

19. Two light bulbs, one “50 W” bulb and one “100 W” bulb, are connected in parallel with a standard 120 volt ac electrical outlet. The brightness of a light bulb is directly related to the power it dissipates. Therefore, the 100 W bulb appears brighter. How does the brightness of the two bulbs compare when these same bulbs are connected in series with the same outlet?
a) Both bulbs will be equally bright.
b) The “100 W” bulb will be brighter.
c) The “50 W” bulb will be brighter.

20. Consider the three identical light bulbs shown in the circuit. Bulbs B and C are wired in series with each other and are wired in parallel with bulb A. When the bulbs are connected to the battery as shown, how does the brightness of each bulb compare to the others?
a) Bulbs B and C are equally bright,
but bulb A is less bright.
b) Bulbs B and C are equally bright,
but less bright than bulb A.
c) All three bulbs are equally bright.
d) Bulbs A and B are equally bright, but bulb C is less bright.
e) Only bulb A is illuminated.

21. A circuit is formed using a battery, three identical resistors, and connecting wires as shown. How does the current passing through R3 compare with that passing through R1?
a) I3 < I1
b) I3 = I1
c) I3 > I1
d) This cannot be determined without knowing the amount of current passing through R2.

22. 20.8.3. What is the approximate equivalent resistance of the five resistors shown in the circuit?
b) 7 
c) 11 
d) 14 
e) 19 

23. In physics lab, two students measured the potential difference between the terminals of a battery and the current in a circuit connected to the battery. The students then made a graph of the two parameters as shown. They then drew a best fit line through the data. From their results, determine the approximate internal resistance of the battery.
a) 
b) 
c) 0.1 
d) 0.3 
e) 0.6 

24. 20.10.1. What is the current through the 4- resistor in this circuit?
a) A
b) A
c) 1.0 A
d) 1.3 A
e) 1.5 A

25. 20.10.2. What is the current through the 1- resistor in this circuit?
a) 2.8 A
b) 3.0 A
c) 3.4 A
d) 4.3 A
e) 4.8 A

26. Which one of the following equations is not correct relative to the other four equations determined by applying Kirchoff’s Rules to the circuit shown?
a) I2 = I1 + I4
b) I2 = I3 + I5
c) 6 V  (8 ) I1  (5 ) I2  (4 ) I3 = 0
d) 6 V  (6 ) I4  (5 ) I2  (2 ) I5 = 0
e) 6 V  (8 ) I1  (6 ) I4  6 V  (2 ) I5  (4 ) I3 = 0

27. A parallel plate capacitor is connected to a battery and becomes fully charged. A voltmeter is used to measure the potential difference across the plates of the capacitor. Then, an uncharged thin metal plate is inserted into the gap between the parallel plates without touching either plate. What affect, if any, does the insertion of the plate have on the potential difference across the plates?
a) The potential difference will not change.
b) The potential difference will increase to twice its initial value.
c) The potential difference will decrease to one half its initial value.
d) The potential difference will increase to a value that cannot be determined without having more information.
e) The potential difference will decrease to a value that cannot be determined without having more information.

28. Three parallel plate capacitors, each having a capacitance of 1.0 µF are connected in series. The potential difference across the combination is 100 V. What is the charge on any one of the capacitors?
a) 30 C
b) C
c) C
d) C
e) C

29. In physics lab, Rebecca measured the voltage across an unknown capacitor in an RC circuit, every ten seconds after a switch in the circuit that allows the capacitor to discharge is closed. The capacitor was initially fully charged. Using the graph, estimate the time constant.
a) 7.5 s
b) 15 s
c) 30 s
d) 45 s
e) 60 s

30. An RC circuit contains a battery, a switch, a resistor, and a capacitor – all connected in series. Initially, the switch is open and the capacitor is uncharged. Which one of the following statements correctly describes the current in the circuit during the time the capacitor is charging?
a) The current is increasing with increasing time.
b) The current is constant with increasing time.
c) The current is decreasing with increasing time.
d) The current increases for the first half of the time until the capacitor is fully discharged, and then decreases during the second half of the time.
e) The current can either increase or decrease with increasing time depending on the value of the time constant.