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PHYSICS DEPARTMENT
PHY 2054
1st Exam [CJ Ch. 25{27]
Name (print):
08 June 1999
Signature:
On my honor, I have neither given nor received unauthorized aid on this examination.
YOUR TEST NUMBER IS THE 5-DIGIT NUMBER AT THE TOP OF EACH PAGE.
DIRECTIONS
(1) Code your test number on your green answer sheet (use 76{80 for the 5-digit number). Code your name
on your answer sheet. Darken circles completely (errors can occur if too light). Code your student number
on your answer sheet.
(2) Print your name on this sheet and sign it also.
(3) Do all scratch work anywhere on this exam that you like. At the end of the test, this exam printout is to be turned
in. No credit will be given without both answer sheet and printout with scratch work most questions demand.
(4) If there is no answer close enough to be correct, the correct answer is to leave the question blank. You should not
expect many of these.
(5) Blacken the circle of your intended answer completely, using a number 2 pencil. Do not make any stray
marks or the answer sheet may not read properly.
(6) Good luck!!!!
Have fun!!!
>>>>>>>>BEFORE YOU FINISH <<<<<<<<
Make sure your test number is coded in correctly. Your test score will be posted under this number, so write it down and
take it with you Hand in the exam and the green answer sheet separately.
Useful (??) Constants:
k = 9 £ 109 Nm2 =C 2
²0 = 8:85 £ 10¡12 C2 /(Nm2 )
electron charge = ¡1:6 £ 10¡19 C
electron mass = 9:11 £ 10¡31 kg
V=volt
N=newton
J=joule
C=coulomb ¹ = \micro-" = 10¡6
m=meter
A = ampere \pico" = 10¡12 n = \nano" = 10¡9 proton charge = +e
proton mass = 1:67 £ 10¡27 kg
acceleration due to gravity = g = 9:8 m/s2
1. Charge A of +5 microC and charge B of ¡25 microC are placed 10 cm apart. The ratio of the electrostatic force on
charge A to the force on charge B is:
(1) 1:1
(2) 5:25
(3) 25:5
(4) zero
(5) none of these
2. In the previous question, what is the electric ¯eld (in megavolts/m) at a point halfway between charge A and charge
B?
(1) 108, toward B
(2) 108, toward A
(3) 72, toward A
(4) 72, toward B
(5) 92, toward B
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3. Three identical charges of 40 microC are placed at the corners of an
equilateral triangle of side length 20 cm. What is the magnitude of the
electric ¯eld (in megavolts/m) at the center of the triangle?
(1) zero
(2) 144
(3) 25
(4) 1.26
(5) 0.42
4. What is the electrostatic potential energy (in J) of the charge con¯guration described in the previous question?
(1) 216
(2) 144
(3) 540,000
(4) 72
(5) zero
5. Select all of the following statements that correctly describe a conservative ¯eld such as the electric ¯eld:
A. The force is independent of the ¯eld direction.
B. The work done by the ¯eld on a charge is independent of the path.
C. The ¯eld does no work on a moving charge.
D. The work done against the ¯eld by an external force on a charge in the ¯eld is path-independent.
(1) B, D
(2) A
(3) B, C, D
(4) A, B
(5) A, C
6. A large parallel-plate capacitor has a capacitance of 100 picofarads. If the spacing between the plates is doubled, what
is the new capacitance, in pF?
(1) 50
(2) 200
(3) 25
(4) 400
(5) none of these
7. In the capacitor described above, if the plate spacing is doubled without altering the charge Q on the plates, what
change (if any) occurs to the potential di®erence V between the plates?
(1) V is doubled
(2) V is halved
(3) V is unchanged
(4) V increases by
p
2
(5) V decreases by
p
2
8. Last week I examined a heart de-¯brillator on a hospital \crash cart". The device delivers an electric shock to the
heart muscle(s) to (hopefully) regularize an erratic heartbeat. The model I saw can deliver up to 360 J of electric
energy through its two conducting plates or \paddles" that measure about 100 square cm each. Suppose that the
paddles are placed on a patient's chest and back, opposite each other, separated by the thorax of thickness 25 cm.
If the average resistance of the patient (between the paddles) is 100 ohms and 200 J of energy is delivered in 50
milliseconds, what was the approximate initial potential di®erence between the paddles (in volts)?
(1) 630
(2) 12,000
(3) 1500
(4) 12
(5) 2350
9. Identical 10 microfarad capacitors A and B are charged to 100V and 200V, respectively. They are then connected
together, + to { and { to +. After charge equilibrium is reached, what is the potential di®erence across capacitor B
(in volts)?
(1) 50
(2) 100
(3) 150
(4) 200
(5) 300
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10. For the above situation, the total stored energy in both capacitors after charge equilibrium is, with respect to the
initial stored energy U,
(1) U/10
(2) U/5
(3) 1.8 U
(4) 3 U
(5) 5 U
11. An unknown resistance r is connected in series with a 10-ohm resistor. When the combination is connected across a
12V battery, a total of 12 watts of power is dissipated. When a second 12V battery is added in series, the total power
dissipated increases to 48 W. What is the value of r, in ohms?
(1) 2
(2) 4
(3) 6
(4) 8
(5) 12
12. Three copper wires a, b, and c are of identical length. Wire B has twice the radius of wire A and C is twice the radius
of B. The three wires are connected together in parallel and the combination is then connected across a 25-V battery.
The potential di®erence across each wire (Va , Vb , Vc ) has the relationship Va :Vb :Vc equal to
(1) 1:1:1
(2) 1:4:16
(3) 8:4:1
(4) 16:4:1
(5) 4:2:1
13. Two parallel conducting plates are mounted one above the other, 5 cm apart. A voltage V is placed across the plates
creating a vertically oriented electric ¯eld between them. A 1-gram particle carrying a charge of ¡1 microCoulomb
placed halfway between the plates is held stationary by the ¯eld. What is V (in volts) and which plate (upper U or
lower L) is positive?
(1) 490, U
(2) 490, L
(3) 9800, U
(4) 9800, L
(5) insu±cient data for solution
14. A 1 microfarad capacitor is connected in series with a 500-ohm resistor. This RC combination is connected across a
50-volt battery at time t = 0. What is the potential di®erence (in Volts) across the resistor at time t = 1 millisecond?
Assume the capacitor is initially uncharged.
(1) 6.8
(2) 4
(3) 320
(4) 30
(5) 12
15. Find the equivalent resistance (in ohms) in the following circuit. All
the resistances are in ohms.
(1) 10
(2) 100
(3) 16
(4) 36
(5) 6
16. In the previous circuit, what is the magnitude (ignore the sign) of the potential di®erence VAB (in Volts) between
points A and B?
(1) 1.5
(2) 3.0
(3) 26.5
(4) 13.5
(5) 9.5
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17. In the following capacitor circuit, ¯nd the emf (in V) of the battery if
the charge on the 7 microFarad capacitor is 140 microCoulombs.
(1) 40
(2) 20
(3) 10
(4) 25
(5) 30
18. Three charges of magnitude ¡1 microCoulombs, 2 microCoulombs, and
3 microCoulombs are arranged as shown. Find the electric °ux through
the surface S (in 105 N/(Cm2 ).
(1) 1.13
(2) 2.26
(3) 5.6
(4) 4.5
(5) 0
19. An electron is shot towards a ¡1 nanoCoulomb point charge. How close does the electron come to the ¡1 microCoulomb charge (in m) if its velocity is 3:7 £ 106 m/s when it is far away? Assume that the ¡1 microCoulomb charge
remains at rest.
(1) 0.23
(2) 0.46
(3) 0.12
(4) 0.34
(5) 0.60
20. A charge Q is a distance d from another charge ¡9Q. Somewhere along the line joining these two charges is a point
where the electric ¯eld is zero. How far away from the Q charge is this point?
(1) d/2
(2) 2d
(3) 3d
(4) 9d
(5) d