PHY3054 - Exam 2 - Fall 1994 - Selman Hershfield

1. A copper wire has resistance 5 Ohms. Given that the resistivity of silver is 85 percent of the resistivity of copper, what is the resistance of a silver wire three times as long with twice the diameter?
   
   
   1. 3.2 Ohms
   2. 4.4 Ohms
   3. 5.6 Ohms
   4. 6.4 Ohms
   5. 8.8 Ohms
   
   
2. A 12 V battery with an internal resistance of 5 Ohms is connected in a circuit with an ideal voltmeter as shown in Fig. 1. What is the difference between the voltmeter reading when the switch is open and when it is closed?
   
   
   1. 1.7 V
   2. 5 V
   3. 7 V
   4. 10.3 V
   5. 12 V
   
   
3. What is the magnitude of the current flowing in the circuit shown in Fig. 2?
   
   
   1. 0 A
   2. 0.5 A
   3. 1.0 A
   4. 1.5 A
   5. 2 A
   
   
4. In Figure 3 which resistor dissipates the most power and which dissipates the least power?
   
   
   1. The 1 Ohm resistor dissipates the most power, and the 4 Ohm the least.
   2. The 1 Ohm resistor dissipates the most power, and the 6 Ohm the least.
   3. The 4 Ohm resistor dissipates the most power, and the 1 Ohm the least.
   4. The 4 Ohm resistor dissipates the most power, and the 6 Ohm the least.
   5. The 6 Ohm resistor dissipates the most power, and the 1 Ohm the least.
   
   
5. A resistor draws a current of 1A when connected across an ideal 3V battery. Another resistor draws a current of 2A when connected across an ideal 3V battery. What current do the two resistors draw when they are connected in series across an ideal 3V battery?
   
   
   1. 0.33 A
   2. 0.5 A
   3. 0.67 A
   4. 1.5 A
   5. 3 A
   
   
6. Put your name, social security number, and test number correctly on the bubble sheet and the exam printout.
   
   
   1. Done correctly.
   2. Done incorrectly.
   3. Done incorrectly.
   4. Done incorrectly.
   5. Done incorrectly.
   
   
7. What is the effective resistance of the resistor network shown in Fig. 4?
   
   
   1. 5 Ohms
   2. 7 Ohms
   3. 13 Ohms
   4. 20 Ohms
   5. 54 Ohms
   
   
8. In the circuit shown in Fig. 5 what is the current labelled I?
   
   
   1. 0 A
   2. 0.2 A
   3. 0.4 A
   4. 0.6 A
   5. 1 A
   
   
9. In the circuit shown in Fig. 5 what is the magnitude of the voltage difference between points A and B?
   
   
   1. 1 V
   2. 1.4 V
   3. 1.6 V
   4. 2.0 V
   5. 2.6 V
   
   
10. The RC circuit shown in Fig. 6 is closed at time t=0. Which of the following is incorrect?
    
    
    1. The characteristic time scale for this circuit is RC.
    2. A long time after the switch is closed the charge on the capacitor is CV.
    3. A long time after the switch is closed the current is V/R.
    4. The current through the resistor is the rate of change of the charge on C.
    5. The voltage between points A and B is constant in time.
    
    
11. For a uniform magnetic field going into the plane of the page what is (i) the direction of circular motion of a positively charged particle and (ii) the direction of the force on a wire with current moving from left to right? Assume that both the particle and the wire are in the plane of the page.
    
    
    1. (i) Clockwise, (ii) towards the top of the page.
    2. (i) Clockwise, (ii) towards the bottom of the page.
    3. (i) Counter-clockwise, (ii) towards the top of the page.
    4. (i) Counter-clockwise, (ii) towards the bottom of the page.
    5. None of the above.
    
    
12. Current flows in a loop as shown in Fig. 7. What direction must the magnetic field be in order for (i) the magnitude of the torque on the loop to be a maximum and (ii) the potential energy of the loop to be a maximum?
    
    
    1. The torque is a maximum with B in the x direction, and the energy is a maximum with B in the y direction.
    2. The torque is a maximum with B in the -x direction, and the energy is a maximum with B in the -y direction.
    3. The torque is a maximum with B in the y direction, and the energy is a maximum with B in the x direction.
    4. The torque is a maximum with B in the -y direction, and the energy is a maximum with B in the -x direction.
    5. None of the above.
    
    
13. A wire carries a current of 3A in a uniform magnetic field. If the wire experiences a force per unit length of 2 N/m, what is the magnitude of the magnetic field?
    
    
    1. 0.67 Tesla
    2. 0.75 Tesla
    3. 1.5 Tesla
    4. 3 Tesla
    5. Not enough information given.
    
    
14. An electron moves is a circular orbit with diameter 10 cm in a 5 Tesla magnetic field. What is the time it takes the electron to complete one orbit?
    
    
    1. 1x10^-12 s
    2. 3x10^-12 s
    3. 5x10^-12 s
    4. 7x10-12 s
    5. 1x10-11 s
    
    
15. An electron moves with velocity 1x10⁵ m/s perpendicular to a uniform magnetic field. If after traveling 1 m, the electron has deviated 1 cm from its original path, what the magnitude of the magnetic field?
    
    
    1. 10^-5 Tesla
    2. 10^-6 Tesla
    3. 10^-7 Tesla
    4. 10^-8 Tesla
    5. 10^-9 Tesla
    
    
16. Which of the three sketches of field lines in Figure 10 is/are wrong?
    
    
    1. Figs. 8.1 and 8.2
    2. Figs. 8.1 and 8.3
    3. Figs. 8.2 and 8.3
    4. Fig. 8.2
    5. Fig. 8.3
    
    
17. As shown in Fig. 9, four wires have current flowing perpendicular to the page. Assuming that each wire carries the same current, what is the direction of the magnetic field at the point P as the center of the square?
    
    
    1. 0 degrees
    2. 45 degrees
    3. 90 degrees
    4. 180 degrees
    5. There is no magnetic field at P.
    
    
18. What is the magnetic field at point Q in Fig. 10?
    
    
    1. 0.6x10^-3 Tesla into the page
    2. 0.6x10^-3 Tesla out of the page
    3. 1.5x10^-3 Tesla into the page
    4. 1.5x10^-3 Tesla out of the page
    5. 2.3x10^-3 Tesla out of the page
    
    
19. What is the magnitude and direction of the force per unit length on the top wire shown in Fig. 11?
    
    
    1. mu I² / (4 Pi d) down
    2. mu I² / (2 Pi d) up
    3. mu I² / (2 Pi d) down
    4. mu I² / (Pi d) up
    5. The force is zero.
    
    
20. Which of the following is true about the rectangular current carrying loop shown in Fig. 12?
    
    
    1. The loop feels a force towards the long wire.
    2. The loop feels a force away from the long wire.
    3. The loop feels no net force, but there is a torque rotating it in the plane of the page.
    4. The loop feels no net force, but there is a torque rotating it out of the plane of the page.
    5. The loop feels no net force, and no torque.
    
    

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Answers

1. Answer = (1) 3.2 Ohms
2. Answer = (3) 7 V
3. Answer = (1) 0 A
4. Answer = (4) The 4 Ohm resistor dissipates the most power, and the 6 Ohm the least.
5. Answer = (3) 0.67 A
6. Answer = (1) Done correctly.
7. Answer = (1) 5 Ohms
8. Answer = (4) 0.6 A
9. Answer = (4) 2.0 V
10. Answer = (3) A long time after the switch is closed the current is V/R.
11. Answer = (3) (i) Counter-clockwise, (ii) towards the top of the page.
12. Answer = (1) The torque is a maximum with B in the x direction, and the energy is a maximum with B in the y direction.
13. Answer = (5) Not enough information given.
14. Answer = (4) 7x10-12 s
15. Answer = (4) 10^-8 Tesla
16. Answer = (3) Figs. 8.2 and 8.3
17. Answer = (4) 180 degrees
18. Answer = (3) 1.5x10^-3 Tesla into the page
19. Answer = (1) mu I² / (4 Pi d) down
20. Answer = (2) The loop feels a force away from the long wire.