Physics 202 MWF 10:20 Spring 2001 (Ford)

Exam II Chapts. 20--22 in Cutnell and Johnson 5e

Multiple choice questions. Circle the correct answer. No work need be shown and no partial credit will be given.

You have two identical resistors. Each has resistance R. When one resistor is connected to a voltage source the electrical power dissipated in the resistor is 80 W. Neglect the temperature dependence of the resistance.

(3 pts) 1. If the two resistors are connected to the voltage source in series, what is the total electrical power dissipated in the two resistors?

(a) 10 W
(b) 20 W
(c) 40 W <--------- answer
(d) 60 W
(e) 80 W
(f) 160 W

(3pts) 2. If the two resistors are connected to the voltage source in parallel, what is the total electrical power dissipated in the two resistors?

(a) 10 W
(b) 20 W
(c) 40 W
(d) 60 W
(e) 80 W
(f) 160 W <--------- answer


(3 pts) 3. Consider the circuit shown in the sketch. What is the charge on the capacitor when the capacitor is fully charged?



(a) zero
(b) 12 µC
(c) 48 µC
(d) 72 µC <--------- answer
(e) 108 µC
(f) 144 µC


(3 pts) 4. A conducting rod is free to slide along a pair of conducting rails in a region where there is a uniform magentic field B directed as shown. Just after the switch is closed, what direction does the rod move?



(a) to the left
(b) to the right <--------- answer


(3 pts) 5. A circular conducting loop rests on a table. A long, straight, current-carrying wire runs under the loop, along a diameter. The current in the wire is in the direction shown. If the current in the wire is increasing,



(a) the induced current in the loop is counterclockwise
(b) there is no induced current <--------- answer
(c) the induced current in the loop is clockwise


(3 pts) 6. A circular conducting loop rests on a table. A long, straight, current-carrying wire also lies on the table and passes close by the loop. The current in the wire is in the direction shown. If the current in the wire is increasing,



(a) the induced current in the loop is counterclockwise
(b) there is no induced current
(c) the induced current in the loop is clockwise <--------- answer


Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(16pts) 7. A conducting bar is free to slide along a pair of conducting rails in a region where a uniform magnetic field is directed into the plane of the paper. When a constant external force F_ext moves the bar to the right at a constant speed v the current in the 5 resistor is 4 A. The magnetic field has magnitude B = 6 T and the resistance of the rails and the bar can be neglected.



a) Which end of the resistor, a or b, is at higher potential?

Ans._____b____

b) What is the speed v of the bar?

Ans.____4.17 m/s___

c) What is the magnitude of the external force F_ext that is required to keep the bar moving at the speed calculated in part (b)?

Ans.___19.2 N_____


(17 pts) 8. A rectangular current loop is located near a long, straight wire that carries a current of 5 A. The current in the loop is 20 A and is counterclockwise. Determine the magnitude and direction of the net force that the long wire exerts on the loop.



Ans. magnitude ___2.4 × 10^-5 N____________

direction (show on sketch) ___toward top of page_____

(16 pts) 9. A particle with charge q = -3 × 10^-6 C enters a region where there is a magnetic field with magnitude B = 0.4 T and direction into the plane of the sketch, as shown. The speed of the particle is v = 50 m/s.



a) What are the magnitude and direction of the electric field in the region if the particle is to pass through undeflected and with no change in speed?

Ans. E = ____20 N/C_____

direction __toward bottom of page____

b) If the electric field is turned off before the particle enters and there is just the magnetic field in the region, the particle wil travel in a semicircle. Will the particle exit the magnetic field region at point P or at point Q?

Ans. ____Q____


(17 pts) 10. Consider the circuit sketched below. The two batteries have negligible internal resistance. The current I₁ through resistor R₁ is 3.0 A, in the direction shown.



a) Calculate the current through each of the other two resistors. (To receive any partial credit, when you use the loop rule you must clearly show the circuit loop to which you are applying Kirchhoff's loop rule and the direction you are going around the loop.)

Ans. current through R₂ ____7.4 A____

current through R₃ ____10.4 A___

b) Calculate the emf ₂.

Ans. ____74.2____



(16 pts) 11. Consider the circuit shown in the sketch. The current through the 6 resistor is 4 A, as shown. What is the current I₁ through the 25 resistor and the current I₂ through the 20 resistor?



Ans. current I₁ ____7 A____

current I₂ ____9.95 A____