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Unformatted text preview: PHYSICS 241  EXAM I September 25, 2007 This exam consists of 17 problems on 7 pages. Please check that you have them all.
The maximum score possible is 100. Useful Equations and constants: 1 91192 v + 1 dq
41mg 7‘2 MO dE 471‘60 7‘2
feltmi @57me 03A €0¢E=q
A W b _, _,
71—60]: go a
1 q dV
V = —— = = — _
411150 7" U Vq E (d! )
' A
qﬂCV 0:503“ C=HCO
L 1 2
R=9E V==2R 13:21; (1:501;
1 2
= V = __ e—t/RC)
8 —t/RC _¢/Rc
3 356 q : (Joe
50 =. 8.85 x10“? Clemz k = 8.99 x10“ NmZ/cZ = l/41teo
g = 9.8 111/32 G = 6.67x10'nNm2fkg2 weight = mg Avogadro‘s Number = 6.023x1023 e = 1.6x10'19 C and m = 9.11x10'3‘ kg for an electron. m = 1.67x10'2Jr kg for a proton PHYS 241 EXAM #1 September 25, 2007 l. (5 points) The electron volt is a unit of A. capacitance
B. charge C. energy D. momentum
E. potential 2. (5 points) An electric ﬁeld is E = (400 MC) E for x > 0 and E = (—400 N/C) ? for
x < 0. A cylinder of length 30 cm and radius 10 cm has its center at the origin and its axis along the x axis such that one end is at x = +15 cm and the other is at x = —15 cm.
What is the flux (in Nmz/C) through each end of the cylinder? A) zero B) 1.3x103
C) 13 D) 0.13 E) 0.25x103 3. (5 points) Two protons (pI and p2) and an electron (e) lie on a straight line, as shown.
The directions of the force of p2 on p1, the force of e on p1, and the total force on p1,
respectively, are: mow—M—
p1 p2 e A > I I I
B s I
C —'—P g 4—— ] 4—H"— D 4. ( 5 points) A uniform electric field of 300 MC makes an angle of 25° with the dipole
moment of an electric dipole. If the moment has a magnitude of 2.0 x 10‘9 C—m, the
torque exerted by the field has a magnitude Of: A. 6.7 x 10'12 N—m
B. 2.5 X 10‘7 N—m
C. 5.4 X 10'7 N—m
D. 6.0 X 10'7 Nm
E. 2.8 X 10'? Nm PHYS 241 EXAM #1 September 25, 2007 5. ( 5 points) If you increase the charge on a parallel~plate capacitor from 3 pic to 9 [JG and
increase the plate separation from 1 mm to 3 mm, the energy stored in the capacitor changes
by a factor of (1) ' (2)
an wiry
{3) (4) 6. ( 5 points) You want to use three capacitors in a circuit. If each capacitor has a
capacitance of 3 pF, the conﬁguration that gives you an equivalent capacitance of 2 pF between points x and y is
A. 1 B.2
C3
D4 E. None of these are correct PHYS 241 EXAM #1 September 25, 2007 7. ( 5 points) A bob of mass m (m = 0.500 g), and charge magnitude Q (Q = 50.0 pC) is
held by a massless string in a uniform electric ﬁeld E. If the bob makes an angle of 10.0
degrees with the vertical, what is the magnitude of the electric ﬁeld E (N/C) and the sign of
the bob charge Q. A. 17.3 and Q is negative
B. 17.3 and Q is positive C. 0.18 and Q is positive
D. 98.1 and Q is negative
E. 98.1 and Q is positive 8. ( 5 points) The voltage between the cathode and the screen of a television set is 22 kV. If
we assume a speed of zero for an electron as it leaves the cathode, what is its speed (In/s)
just before it hits the screen? A. 2.8x106
B. 5.3x107
C. 6.2x107
D. 8.8x107
E. 7.7x 1015 PHYS 241 EXAM #1 September 25, 2007 9. ( 5 points) Each of the four capacitors shown is
500 ,uF. The voltmeter reads 1000 V. The
magnitude of the charge, in coulombs, on each
capacitor plate is: A. 0.2
B. 0.5
C. 20
D. 50
E. none of these 10. ( 10 points) Positive charge Q is distributed uniformly throughout an insulating sphere of
radius R, centered at the origin. A positive point charge Q is placed at x = 2R on the x—axis. The
magnitude of the electric field at x=R/2 on the xaxis is: A. Q 2
475801?
B. Q 2
875801?
c. .Lz
727F801?
D. ———17g 2
727reaR E. none of these PHYS 241 EXAM #1 September 25, 2007 11. ( 5 points) A solid spherical conductor of radius 15 cm has a charge Q = 6.5 nC on
it. A second, initially uncharged, spherical conductor of radius 10 cm is moved toward
the ﬁrst until they touch and is then moved far away from it. How much charge is
there on the second sphere after the two spheres have been separated? A. 4.3 DC
B. 2.2 nC
C. 3.2 nC
D. 2.6 nC
E. 3.9 DC 12. ( 10 points) The capacitance of a spherical capacitor with inner radius a and outer
radius b is proportional to: PHYS 241 EXAM #1 September 25, 2007 13. ( 5 points) Find the magnitude of the electric field, E, in V/m at the point (1.5m, 
1.5m) in the x—y plane for a the potential function V(x,y,) = (x2y — yzx 10)V. A) zero B) 7.75 C) 9.55 D) 15.5 E) 77.5 14. (5 points) An active thundercloud has a potential exceeding 3 MV relative to the
Earth’s surface. A typical cloud system (whose bottom is 1.6 km above the earth) has
an area of 100 ka. Estimate the energy, U, stored in this cloud assuming you can treat
it and the Earth as a giant capacitor. A) 0.83 M] D) 8.30 M]
B) 2.49 M] E) 9.96 M]
C) 4.98 MJ 15. (5 points) Charge is distributed uniformly along a long straight wire. The electric
field 2 cm from the wire is 20 N/C. The electric field 4 cm from the wire is: A. S N/C B. 10 MC
C. 40 MC
D. 80 N/C
E. 120 MC PHYS 241 EXAM #1 September 25, 2007 16. ( 5 points) Three circuits, each consisting of a switch and two capacitors are initially charged as indicated. After the switches have been closed, in which circuit will the
charge on the left—hand capacitor decrease? (HE/S (2) L45
6;, 1 3:; 6q 1 3‘] MUG “UC A) l
B) 2
C) 3
D) None of these 17. ( 10 points) Four point charges q] = —1 ac, q2 = q1 5 £12
2 uC, q3 = 3 ac, and q4 = 4 tLC are located at the . .
corners of a square Whose side s = 1 m. The electrostatic potential energy of this system of
charges is A. 0.492 J
B. 0.337 J
C. 0.286]
D. 0.176 J s ‘13
E. 0.1405 ' ...
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