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Unformatted text preview: PH 112 MIDTERM EXAM SPRING 2006
Instructor: Dr. Andrew Pakhomov Your name: , 6&1, In this test you can make up to 31/25 pts Good [email protected] \ Problem 1. (5 points): An electron is placed in a uniform electric ﬁeld of
1.5x104 N/C. Determine its acceleration. Mass of electron: 9.1x10’31 kg. _ _.'(/c»</0 c)(/ sic/09W)
F: med: 6E >0»; me (?.I x/O‘g’ n33 61:2,4 x to" “/sz Problem 2. (5 points) A glass rod of length 27 cm lies along the positive xaxis with one end at the / origin (x = 0). Given that it is charged positively in such a way that its linear
charge density is a function of coordinate x: Mx) = (30.0 tie/maxi, ﬁnd the total charge on the rod. L‘ GEM
L ,Z?~m X 6?: j>t(x)olx = ](37</o“%3)><204>< = Canto“ 3—3 )x m 3 mm 0
an} a (/o~?c/ma>(mm>3= 0.2 nc
0 .— Problem 3. (5 points) Consider a uniform spherical shell of charge +Q and radius r surrounded by
a concentric spherical shell of uniform charge Q and radius R (R>r).
Determine the Eﬁelds inside, between and beyond the shells. Assume vacuum.
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E20 Problem 4. (5 points)
An electric ﬁeld in a region is given by a function: oni A 1'0 E: where E; and x0 are constants. Find the potential as a ﬁmction of position,
taking the zero of potential at x= O. VgVA‘: JE Jew? V3: (If—EXXON  EOXZ [I Problem 5. (5 points)
A little ﬂat ceramic capacitor consists of two circular plates 0.50 cm in diameter separated by a dielectric 1/3 mm thick with a relative permittivity
of 4.8. Compute its capacitance. Bonus Problem (6 points) The earth carries a net charge of about 4x105 C a) If no others charges were present, what would be the potential at the
earth's surface, with respect to inﬁnity?; b) Actually, this net negative charge is balanced by a positive charge layer
in the earth's ionosphere, an electrically conducting layer about 60 km
above the earth. Estimate the potential difference between the earth and
ionosphere. Earth radius R9 = 6.37 x106 vm
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 Spring '08
 Dr.JunLin

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