Physics 202 MWF 10:20 Spring 1996 (Ford)
Final Exam
Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(9 pts) 1. Parallel rays from a very distant object are incident from the left onto two thin lenses as shown in the sketch. The diverging lens has f1 = -20 cm and the converging lens has f2 = +35 cm. The distance between the two lenses is 5 cm.

a) How far is the final image from the second lens?

Ans. __87.5 cm to left__

b) Is the final image real or virtual?

Ans. __virtual__


(12 pts) 2.
a) Charge Q is distributed uniformly throughout an insulating sphere of radius R. In terms of Q, what is the electric field

(i) for r < R

Ans. __Qr/40 R3__

(ii) for r > R

Ans. __Q/40 r2__

b) A solid conducting sphere of radius R has a net charge Q. In terms of Q, what is the electric field

(i) for r < R

Ans. __E = 0__

(ii) for r > R

Ans. __Q/40 r2__

NOTE: For each question, to receive credit you must give a clear derivation of your result.


(12 pts) 3. Point charges q1 = -5.0 × 10-6 C and q2 = +3.0 × 10-6 C are held fixed at the positions shown in the sketch, on the y-axis and 0.030 m above and below the origin. Point a is at the origin and point b is on the x-axis, 0.040 m from the origin.

a) What is the net electric field (magnitude and direction) at the origin (point a) produced by these two charges. (Show the direction of E on the sketch.)

Ans. E = __8.0 × 107 N/C__

direction __+y__

b) Calculate the work done by the resultant electric force of these two charges on a third charge q3 = -4.0 × 10-6 C when it moves from point a to point b. Is the work done positive or negative?

Ans. W = __0.96 J__

positive or negative? __positive__


(12 pts) 4. Three capacitors are connected to an emf of 80 V, as shown in the sketch. The capacitors have capacitances C1 = 6.0 × 10-6 F, C2 = 3.0 × 10-6 F, and C3 = 5.0 × 10-6 F. After the charges have reached their final values, calculate the magnitude of the charge Q on each capacitor and the potential V across each capacitor.

Q1 = __2.74 × 10-4 C__

V1 = __45.7 V__

Q2 = ____1.03 × 10-4 C__

V2 = __34.3 V__

Q3 = ____1.72 × 10-4 C__

V3 = __34.3__


(12 pts) 5. Consider the circuit shown in the sketch. The resistances are R1 = 30 and R2 = 15 . The capacitor is initially uncharged. The switch is closed at t = 0.




a) At some later time t, the current i1 through R1 is 9.0 A and the current i2 through R2 is 2.0 A.

(i) What is the emf of the battery?

Ans. __270 V__

(ii) What is the charge q on the capacitor at this instant?

Ans. ____9.6 × 10-4 C__

b) What are i1, i2, and q a long time after the switch is closed?

Ans. i1 = __9 A__

i2 = __0__

q = ____10.8 × 10-4 C__________________


(9 pts) 6. An L-C circuit consists of an inductor with L = 0.800 H and a capacitor with C = 6.00 × 10-4 F. The initial charge on the capacitor is Q and the initial current in the inductor is zero.

a) During the current oscillation, the maximum current in the inductor is 8.0 A. What was the initial charge Q on the capacitor?

Ans. __0.175 C__

b) At the instant when the current in the inductor is 4.0 A, what is the charge on the capacitor?

Ans. __0.152 C__


(9 pts) 7. A circular loop of wire with resistance R and radius r = 0.30 m is in a region of uniform magnetic field. The magnetic field is directed out of the plane of the paper and is increasing at a uniform rate of B/t = +0.030 T/s. The current induced in the wire loop has magnitude 0.70 A.

a) Is the induced current clockwise or counterclockwise?

Ans. __clockwise__

b) What is the resistance R of the loop?

Ans. __0.0121 __


(10 pts) 8. A long, straight wire carries a current I = 6.0 A in the direction shown in the sketch. An electron (charge q = -1.6 × 10-19 C) is at point P and has velocity v = 3.0 × 104 m/s in the direction away from the wire, as shown in the sketch.

a) What is the magnetic field (magnitude and direction) at point P due to the current in the wire? (Show the direction of B on the sketch.)

Ans. B = __4.0 × 10-5 T__

direction __into page__

b) What is the force (magnitude and direction) that the magnetic field of the wire exerts on the electron when the electron is at point P? (Show the direction of F on the sketch.)

Ans. F = ____1.92 × 10-19 N__

direction __to right__


(7 pts) 9.
a) Calculate the binding energy (in MeV) of the lithium isotope 73Li. The mass of 73Li is 7.016003 u.

Ans. __39.2 MeV__

b) The nucleus 23090Th undergoes alpha decay. What are the nucleon number A and atomic number Z of the nucleus that is produced by this decay?

Ans. A = __226__

Z = __88__


(8 pts) 10. When light of wavelength 400 nm is incident on a metal surface, the maximum kinetic energy of the photoelectrons that are produced is 1.6 eV.

a) What is the maximum kinetic energy of the photoelectrons that are produced when light of wavelength 300 nm is incident on the same surface?

Ans. __2.63 eV = 4.22 × 10-19 J______________

b) What is the longest wavelength of light that will cause photoelectrons to be emitted from this surface?

Ans. __825 nm__