Exam #2, March 15th, 2007
Name: _______________________________________________________________
EXAM II
Total of 103 points on this exam (7 extra credit points offered)
Physics 216
Thursday March 15th 23, 2007
Dr. Urquidi
Please show your work. Answers without work will not be given credit and partial
credit is impossible if your work is not shown.
Please do your work in the space provided. If you need more room then proceed to
the back of the paper but be sure to note that you have done so.
Please make your answers conspicuous by boxing or underlining them.
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Exam #2, March 15th, 2007
1.(3pts) _______(T/F) The unit of Capacitance is the Farad.
2.(3pts) _______(T/F) Capacitance ONLY depends on geometry
3.(3pts) _______(T/F) The Capacitance of case 2 (see Fig) is twice that of case 1
4.(3pts) _______(T/F) In a parallel connection, capacitors have the same charge
stored in them.
5.(3pts) _______(T/F) A dielectric is a conducting material.
6.(3pts) If the area of a parallel plate capacitor is doubled, what happens to the
capacitance?
7. (8pts) A parallel plate capacitor is to be constructed by submersing the plates in
transformer oil. If the capacitance of the device is to be 4 F and is to be operated at
80,000 V and the dielectric constant of transformer oil is 2.24 and the dielectric
strength is 12X106 V/m, then…
a) Calculate the minimum plate area required
b) What is the energy stored in the capacitor at the
operating voltage?
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Exam #2, March 15th, 2007
8.(4pts) Work is done on a dielectric as it is inserted into a capacitor. Where does
the force that does this work originate?
9.(8pts) Water has a dipole moment of 6.3X10-30 C m. If a sample contains 1023
molecules whose dipole moments are all oriented in the direction of an electric field
of 7.8X107 N/C how much work is required to rotate the dipoles from their
orientation of 30° with respect to the electric field to –30° with respect to the
electric field.
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Exam #2, March 15th, 2007
10.(8pts) A coaxial cable has an outer conductor of inside diameter 1.2 cm and is
filled with polyethylene, which has a dielectric constant of 2.3. If the capacitance per
unit length of this cable is 40 microfarad for every meter of cable then what is the
diameter of the inner conductor?
11.(8pts) Consider the following parallel plate capacitor with plates 11mm to a side
and having a separation of 7mm. The dielectric constant of the insulating material is
2 and does not fill the entire gap. What is the capacitance?
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Exam #2, March 15th, 2007
12. (5pts) Rank in order, from largest to smallest, the potential energies Ua to Ud of
these four pairs of charges. Each + symbol represents the same amount of charge.
1.
2.
3.
4.
5.
Ua = Ub > Uc = Ud
Ua = Uc > Ub = Ud
Ub = Ud > Ua = Uc
Ud > Ub = Uc > Ua
Ud > Uc > Ub > Ua
13.(5pts) A proton is released from rest at point B, where the potential is 0 V.
Afterward, the proton
1. moves toward A with an increasing speed.
2. moves toward A with a steady speed.
3. remains at rest at B.
4. moves toward C with a steady speed.
5. moves toward C with an increasing speed.
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Exam #2, March 15th, 2007
14.(5pts) Rank in order, from largest to smallest, the potentials Va to Ve at the points
a to e.
1.
2.
3.
4.
5.
Va = Vb = Vc = Vd = Ve
Va = Vb > Vc > Vd = Ve
Vd = Ve > Vc > Va = Vb
Vb = Vc = Ve > Va = Vd
Va = Vb = Vd = Ve > Vc
15.(5pts) Rank in order, from largest to smallest, the potential differences ∆V12,
∆V 13, and ∆V 23 between points 1 and 2, points 1 and 3, and points 2 and 3.
1.
2.
3.
4.
5.
6
∆V12 > ∆V13 = ∆V23
∆V13 > ∆V12 > ∆V23
∆V13 > ∆V23 > ∆V12
∆V13 = ∆V23 > ∆V12
∆V23 > ∆V12 > ∆V13
Exam #2, March 15th, 2007
16.(3pts) What are the units of potential difference?
17.(3pts) The units of the electric field used in this last section of the course are
1. V/C.
2. N/C.
3. V/m.
4. J/m2.
5. W/m.
18.(5pts) The electric potential inside a capacitor
1. is constant.
2. increases linearly from the negative to the positive plate.
3. decreases linearly from the negative to the positive plate.
4. decreases inversely with distance from the negative plate.
5. decreases inversely with the square of the distance from the negative
plate.
19.(5pts) The electric field
1. is always perpendicular to an equipotential surface.
2. is always tangent to an equipotential surface.
3. always bisects an equipotential surface.
4. makes an angle to an equipotential surface that depends on the amount of
charge.
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Exam #2, March 15th, 2007
20.(8pts) Rank in order, from largest to smallest, the equivalent capacitance (Ceq)a to
(Ceq)d of circuits a to d.
21(8pts) Three charged, metal spheres of different radii are connected by a thin
metal wire. The potential and electric field at the surface of each sphere are V and
E. Which of the following is true?
1.
2.
3.
4.
5.
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V1 = V2 = V3 and E1 = E2 = E3
V1 = V2 = V3 and E1 > E2 > E3
V1 > V2 > V3 and E1 = E2 = E3
V1 > V2 > V3 and E1 > E2 > E3
V3 > V2 > V1 and E1 = E2 = E3