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PHYS 212 First Hour Exam Name ______________
February 17, 2004 Problem Session:
Hr _____ Instr _____
Show all work for full credit! Answers must have correct units and
appropriate number of significant digits. For all the problems
(except for multiple choice questions), start with either (a) a
fundamental equation (b) a sentence explaining your approach; or
(c) a sketch.
k =9.0x10^9 Nm^2 /C^2 me =9.1x10-31^ kg = 511 keV/c^2 mp =1.67x10-27^ kg = 938 MeV/c^2
e = 1.6x10-19^ C 1 eV = 1.6x10-19^ J μo = 4πx10-7^ Tm/A m earth = 5.98x10^24 kg
1. (12 pts) Consider charges q 1 =− 5 μC
6 μC
, and positioned as shown. Find the total electric force on q
q 2 (^) =− 10 μC q 3 =+
3
μ C
μ C μC
y
x
(m)
(m)
q = −5 q = − 1 2
3 q = +
F E
D
A
B C
P
2. (8 pts) Consider five point charges arranged
along the horizontal axis as illustrated. All the charges are negative, and they all have same magnitude. Which arrow best represents the electric field vector at the point P? (Circle one.)
A B C D E F
Briefly explain your reasoning:
3. (12 pts) Three infinite parallel plates are
arranged as shown. The electric potential for each plate is shown in the figure. Determine the magnitude and direction of the electric field in the region between plates 2 and 3.
V 1 =200 V V 2 = 0 V V 3 = −60 V
1.0 cm 2.4 cm
1 2 3
6. (12 pts) A 5 cm by 5 cm square loop of wire lies in the plane
of the page, and a uniform magnetic field of T p out of the page. A current I = 3 A flows clockwise around the loop. Determine the torque on the loop about its center.
2 × 10 −^5
I
5 cm
5 cm
B
oints
. (12 pts) An electron moves with velocity v = ( 2 i ˆ− 3 ˆ j ) × 106 m/s
r in a uniform magnetic field B = ( 4 i ˆ+ 5 ˆ j )× 10 −^2 T
r
. Calculate the ma
gnetic force on the electron.
8. (6 pts) An 8 inch length of scotch tape is pressed sticky side down on a table. A
second identical piece of tape is pressed down on the table beside the first piece of tape. The two pieces of tape are then ripped quickly from the table. The strips of tape are allowed to hang vertically, and brought next to each other. Do the strips of tape attract, repel, or hang unaffected by each other? Explain your reasoning.
x
y
L
d
P
9. (15 pts) A line of charge with positive linear
charge density λ and length L is located on the y axis with one end at the origin as shown Set up the integral that you would evaluate to determine the y component of the electric field Ey at the point P on the y axis a distance d below the origin, as shown. You do not need to evaluate the integral. Your expression should contain only given quantities, physical constants, and your integration variable.
