Phy207    Exam II (Form1)          Professor Zuo           Fall Semester 2014  Signature:______________________   

Phy207 Exam II (Form1) Professor Zuo Fall Semester 2014 “On my honor, I have neither received nor given aid on this examination” Signature:______________________ Name:___________________ #1‐13 #14 Total ID number:___________________ Enter your name and Form 1 (FM1) in the scantron sheet. Attempt all problems. Multiple choice questions are worth 5pts each and the essay problem is 40 pts (5 extra pts if you do all). There is a formula sheet attached the end. This is a closed book exam, you must work independently! No collaboration is allowed. Prohibited items: any electronic devices including cell phones and calculators, pens, backpacks, notes, books. Anyone found cheating during the exam will automatically receive an “F” grade for the course and sent to the honor’s court. Put an X next to your discussion section:
[ ] Dr. Mezincescu 5P, 11:00 – 11:50 a.m.
[ ] Dr. Curtright 5Q, 12:30 – 1:20 p.m.
[ ] Dr. Zuo 5R, 2:00 – 2:50 p.m.
[ ] Dr. Zuo 5S, 3:30 – 4:20 p.m.
[ ] Dr. Liu 5T, 5:00 – 5:50 p.m.
(For the next two problems) A parallel plate capacitor is made of two square sheets of metals of side
length L with a separation d between each other. Half of the space is filled and the other half empty. The
filled half is filled with two dielectrics with dielectric
constant k1 and k2, as shown.
1) Find the capacitance of the capacitor
A)
B)
1
C)
D)
E)
2) Find the charge on the area just above the filled space if k1=2, k2=4.
A) Q , B) Q, C) Q, D) Q, E) none of above
3) In the circuit shown in the figure, the capacitors are initially uncharged. The switch is first thrown to position A
and kept there for a long time. It is then thrown to position B. Let the charges on the capacitors be Q1, Q2, and Q3
and the potential differences across them be V1, V2, and V3. Which of the
following conditions must be true with the switch in position B?
A) V1 = V2 = V3
B) V1 + V2 = V3
C) V3 = V0
D) Q1 = Q2 = Q3
E) Q1 + Q2 = Q3
4) When a potential difference of 10 V is placed across a certain solid cylindrical resistor, the current through it is 2
A. If the solid is compressed such that the diameter of this resistor is now twice as much as the original and the total
volume remains the same, the current will be
A) 8 A, B) 1/4 A, C) 1/8 A , D) 16 A, E) 32 A.
For the next three problems, consider a multiloop circuit shown in the figure. It is not necessary to solve the entire
circuit.
5) Compared to the polarity shown in the figure, the emf ε1 is closest to
A) -5 V,
B) 5 V, C) 45 V,
D) 51 V, E) -51 V.
6) What is current I1?
A) 5A, B) 2A, C) A, D)
A, E) none of above
7) What is current I2?
A) 5A, B) 7A, C) 3A, D) 0A,
E) -3A
(For the next three problems) Consider the circuit shown below, the capacitors are all initially uncharged, the
connecting leads have no resistance, the battery has no appreciable internal resistance, and the switch S is originally
open.
8) Just after closing the switch S, what is the current in the 15.0-Ω
resistor?
A) 0.00 A, B) 1.67 A, C) 2.50 A, D) 3.33 A, E) 5.00 A
9) What is the current going through the battery right after closing the
switch?
A) 0 A, B) A, C) A, D) A, E) A
10) After the switch S has been closed for a very long time, what is the potential difference across the 28.0-μF
capacitor?
A) 0.00 V,
B) 25.0 V, C) 3.33 V, D) 37.5 V, E) 50.0 V
11) Ions having equal charges but masses of M and 2M are accelerated through the same potential difference and
then enter a uniform magnetic field perpendicular to their path. If the heavier ions follow a circular arc of radius R,
what is the radius of the arc followed by the lighter?
A) 4R,
B) 3R,
C)
2 R,
D) R/ 2 ,
12) An electron moving with a velocity
= 5.0 × 107 m/s î enters a region of space where perpendicular electric and
a magnetic fields are present. The electric field is
through the region without being deflected?
A)
= +2.0 × 10-4 T ĵ
B)
= -2.0 × 10-4 T ĵ
C)
= +2.0 × 10-4 T
D)
= -2.0 × 10-4 T
E)
= +5.0 × 10-4 T
E) R/2
=10 kV/m ĵ. What magnetic field will allow the electron to go
13) A conducting bar of length 1.0 meter is being moved at a constant velocity of 1.0m/s from left to right on a track
by an external force F as shown. A uniform magnetic field B=2.0T is directed out of the plane. The bar makes a
good electrical contact and slides without friction. A constant force F=1.0N is applied. What is the current going
through the resistor?
A) ½ A
B) 1 A
C) 2 A
D) 0 A
E) I need to know the resistor R
15) (40pt ) Two concentric spherical conducting shells with inner radius a and outer radius b for the inner
shell and inner radius c and outer radius d for the outer shell are charged with q and –q, respectively.
a) Find the electric field everywhere.
b) Find the potential difference between the two conductors.
c) Find the electric energy contained within a thin shell of radius r (b<r<c) and thickness dr.
b) Find the total electric field energy.
e) Find the capacitance of the capacitor.
b a c d Solution: a) E is zero everywhere except for b<r<c, where E=kq/r2. Here you can derive it using Gauss’s law 4
∅
b) The potential difference is ∆V=Vb‐Vc=
). c) The energy contained within a thin shell can be calculated using ). d) The total energy is e) There are two ways to find the capacitance: one is using C=Q/V=q/(kq(1/b‐1/c))=4
The other way is to use U=(1/2)q2/C, C=(1/2)q2/U=4
4
bc/(c‐b). bc/(c‐b).