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Answer all of the following questions. You can earn up to 50 points back on your
midterm if you correctly answer all parts and return to me by Monday!
1. Two parallel plates separated by a distance d, are connected to a battery with an emf E.
Which of the following is correct if the plate separation is doubled while the battery
remains connected?
a. Electric charge on the plates is doubled.
b. Electric charge on the plates is halved.
c. The potential difference between the plates is doubled.
d. The potential difference between the plates is halved.
e. The capacitance is unchanged.
JUSTIFY your answer with text and equation:
2. A 4 F capacitor is charged to a potential difference of 100 V. The electrical energy stored in the
capacitor is
(A) 2 x 10-10 J
(B) 2 x 10-8 J
(C) 2 x 10-6 J
(D) 2 x 10-4 J
(E) 2 x 10-2 J
JUSTIFY your answer with text and equation:
3. In question #2, explain how the capacitor will operate in a simple series circuit that
connects the battery, capacitor, and a switch in the following situations. (be sure to justify
your explanations):
a. Just after the circuit is closed.
b. If after a significant amount of time, the switch is opened. Now, how does the
capacitor behave?
4. A metal spring has its ends attached so that it forms a circle. It is placed in a uniform magnetic
field, as shown above. Which of the following will NOT cause a current to be induced in the
spring?
(A)
(B)
(C)
(D)
(E)
Changing the magnitude of the magnetic field
Increasing the diameter of the circle by stretching the spring
Rotating the spring about a diameter
Moving the spring parallel to the magnetic field
Moving the spring in and out of the magnetic field
JUSTIFY your answer in terms of changes in magnetic flux with text and an equation:
5. The pressure exerted on the bottom of a dam by the water in the reservoir created by the
dam depends on the _____________________________.
JUSTIFY your answer with text and equation:
6. As a rock sinks deeper and deeper into water of constant density, what happens to the
buoyant force on it?
a. It increases.
b. It remains constant.
c. It decreases.
d. It may increase or decrease, depending on the shape of the rock.
JUSTIFY your answer with text and equation:
Questions 7-8: A magnetic field of 0.1 T forces a proton beam of 1.5 mA to move in a circle of radius
0.1 m. The plane of the circle is perpendicular to the magnetic field.
7. Of the following, which is the best estimate of the work done by the magnetic field on the
protons during one complete orbit of the circle?
(A) 0 J
(B) 10-22 J
(C) 10-5 J
(D) 102 J
(E) 1020 J
JUSTIFY your answer:
8. Of the following, which is the best estimate of the speed of a proton in the beam as it moves in
the circle?
(A)
(B)
(C)
(D)
(E)
10-2 m/s
103 m/s
106 m/s
108 m/s
1015 m/s
Write a dimensional analysis reasoning here that would allow you to ESTIMATE the correct
answer WITHOUT the benefit of a calculator. THEN, explain how this reasoning works using
math.
Questions 9 and 10.
9. The electrical resistance of the part of the circuit shown between point X and point Y is
(A) 1.3 
(B) 2 
(C) 2.8 
(D) 4 
(E) 6 

Simplify the circuit. Show every step in the simplification. Then complete a VIR chart for each
individual resistor and for the total circuit if X and Y were connected to a 12 V battery.
10. When there is a steady current in the circuit, the amount of charge passing a point per unit of
time is
(A) the same everywhere in the circuit
(B) greater at point X than at point Y
(C) greater in the 1 resistor than in the 2 resistor
(D) greater in the 1 resistor than in the 3 resistor
(E) greater in the 2 resistor than in the 3 resistor
11. One section of wire in the circuit above lies in a region containing a magnetic field B directed into
the page. The magnetic force on the section of wire is directed
a. Into the page
b. Toward the right
c. Toward the left
d. Toward the top of the page
e. Toward the bottom of the page
JUSTIFY your answer:
FOR #12 – Attach a separate sheet of unlined paper with your response. Follow my
steps that have been outlined for you carefully.
12. A particle of mass m and charge q is accelerated from rest in the plane of the page through a
potential difference V between two parallel plates as shown above. The particle is injected
through a hole in the right-hand plate into a region of space containing a uniform magnetic field
of magnitude B oriented perpendicular to the plane of the page. The particle curves in a
semicircular path and strikes a detector.
(a) Why must the sign of the particle’s charge be “Positive”?
Justify your answer:
(b) Draw a right hand. Label the pointer, fingers and thumb to explain the direction of the
Magnetic Field B.
(c) Explain why the particle’s path in the region of the magnetic field is semicircular.
Determine each of the following in terms of m, q, V, and B.
(d) The speed of the charged particle as it enters the region of the magnetic field B.
REMEMBER the speed of the particle as it enters is due to whatever happened BEFORE
it gets to the magnetic field!!
1. What is the F on the particle in the electric field in terms of q and V?
2. How can you find the final velocity if you know the F on the particle?
3. Write your speed in terms of m, q, and V.
(e) The distance along the plate from the hole to the detector.
1. What is the shape of the path? How can this be related to the hole and detector?
2. What is the F on the particle when it is travelling in the magnetic field?
3. Relate the shape of the path to “circular motion”. What is the relevant math?
4. Now find the distance in terms or m, q, V, and B
(f) By definition the work done on the particle on its trip is 0. Explain why must this be true?