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UW 115A
Winter 2015
Midterm Exam #2
Name (LAST, first): _________________________________ UW ID#: ___________________
Part I: (50 points) “Long answer” problems. Show your work. Show units. Mind significant figures.
#1 (25 points) The PV diagram shows two possible states of system containing 1.35 moles of ideal gas.
P1 = P2 = 455 N/m2, V1 = 2.00m3, V2 = 8.00m3.
a) (3 points) On this PV diagram, draw the process depicting
an isobaric expansion from state 1 to state 2, and label this process
A. Show the direction in which the process goes with an arrow.
b) (8 points) Find the work done by the gas and
the change in internal energy of the gas in this process.
c) (6 points) On the same PV diagram, draw the two-step process corresponding to an isothermal expansion from
state 1 to volume V2, followed by isovolumetric increase in temperature to state 2, and label this process B. Show the
direction in which the process goes with arrows.
d) (8 points) Find the change in internal energy of the gas for the two-step process B.
#2 (25 points) Three charges are positioned as shown. The values are q1 =+1.0C, q2 = -2.0C, q3 =+1.0C; the
distance between q1 and q2 is same as the distance between q2 and q3 and is equal to 0.3m.
a) (10 points) Draw a free-body diagram of forces acting on charge q1. Find the force on charge q1, magnitude and
direction.

P
b) (15 points) Draw a diagram of electric fields at point P due to all three charges. Find the magnitude and direction
of electric field due to the three charges at point P, 0.2m above the line of charges.
UW 115A
Winter 2015
Midterm Exam #2
Name (LAST, first): _________________________________ UW ID#: ___________________
Part II: Multiple choice problems (4 points each, except #2 and #13 are 3 points, 50 points total)
1. A turbine takes in 1000-K steam and exhausts the steam at a temperature of 500 K. What is the maximum
theoretical efficiency of this system?
a.
b.
c.
d.
24%
33%
50%
67%
2. Which of the following choices best corresponds to what is required by the second law of thermodynamics for
any real process taking place in an isolated system?
a.
b.
c.
d.
entropy decreases
entropy remains constant
entropy increases
entropy equals work done on the system
3. What is the change in entropy (∆S) when one mole of silver (108 g) is completely melted at 961°C? (The heat of
fusion of silver is 8.82 x104 J/kg.)
a.
b.
c.
d.
e.
5.53 J/K
7.72 J/K
9.91 J/K
12.10 J/K
Zero
4. Carnot engine is:
a. The hypothetical engine that is most energy-efficient
b. The first real engine that was built to experimentally prove the second law of theormodynamics
c. The engine that is capable of providing the fastest energy output
d. The engine with cold reservoir at the lowest attainable temperature
e. None of the above
5. Which of the following field line pictures best represents the electric field from two charges that have the same
sign but different magnitudes?
A
B
C
D
UW 115A
Winter 2015
Midterm Exam #2
Name (LAST, first): _________________________________ UW ID#: ___________________
6. A hollow metallic sphere with radius of 10 cm has a small object with a charge of +10 µC carefully placed at the
center of the sphere. The sphere itself carries a charge of -10 µC. What charge resides on the outer surface of the
sphere?
a.
b.
c.
d.
e.
zero
+ 20 µC
- 20 µC
- 10 µC
+10 µC
7. If a conductor is in electrostatic equilibrium near an electrical charge:
a.
b.
c.
d.
the total charge on the conductor must be zero.
the electric field inside the conductor must be zero.
any charges on the conductor must be uniformly distributed.
the sum of all forces between the conductor and the charge must be zero.
8. A charge, +Q, is placed inside a balloon and the balloon is inflated. As the radius of the balloon r increases the
flux of electric field through the surface of the balloon:
a.
b.
c.
d.
e.
increases proportional to r2.
increases proportional to r.
stays the same.
decreases as 1/r.
decreases as 1/r2
9. If an electron is accelerated from rest through a potential difference of 1,200V, find its approximate velocity at the
end of this process. (e = 1.6 10-19 C; me = 9.1 10-31 kg)
a.
b.
c.
d.
1.0  107 m/s
1.4  107 m/s
2.1 107 m/s
2.5  107 m/s
10. In which case does an electric field do positive work on a charged particle?
a.
b.
c.
d.
e.
a negative charge moves opposite to the direction of the electric field.
a positive charge is moved to a point of higher potential energy.
a positive charge moves opposite to the direction of the electric field.
a positive charge completes a closed path
a negative charge completes a closed path
11. When charge Q1 is placed at point P1, the resulting potential at point P is V1. When Q2 is placed at point P2 after
Q1 is in position at P1, the resulting potential at P becomes V 2. What is the potential at point P if charge Q1 is then
removed?
a.
b.
c.
d.
V1 + V2
V1 - V2
V2 - V1
(V1 + V2)/2
UW 115A
Winter 2015
Midterm Exam #2
Name (LAST, first): _________________________________ UW ID#: ___________________
12. Which of the points shown in the diagram are at the
same potential?
a.
b.
c.
d.
e.
2 and 5
2, 3 and 5
1 and 4
1 and 5
2 and 4
13. Doubling the potential difference across a capacitor
a. doubles its capacitance.
b. halves its capacitance.
c. quadruples the charge stored on the capacitor.
d. halves the charge stored on the capacitor.
e. does not change the capacitance of the capacitor.