PHYS 272 Fall 2008
Monday, November 10, 2008
Exam 2 - A
There are two parts to Exam 2, this machine-graded test and a separate test that you turn
in to be graded by hand.
Machine Answer Sheet:
Using a pencil, fill in Last Name, First Name, & Middle Initial, plus your 10-digit Purdue
University ID number. Enter Instructor (Hirsch), Course (PHYS 272), Date (11/10/08),
and Test (2). You must include your Signature. Fill in circle “A” for Test Form.
Hand-Graded Sheet:
Enter your Name, Signature, PUID, and circle your Recitation Section.
When finished with both parts, bring them to the front of the classroom, show your
Purdue ID card to the Instructor, and turn in the machine-graded answer sheet and the
hand-graded answer sheet at the same time. These two parts together are worth 100
points total. Your grades will appear in the CHIP gradebook in a few days. You should
keep this copy of the exam in case you have questions for your TA.
1
Problem 1 [5 points]
In the diagram below, the electric field is uniform and has components <-200, -400, 0>
N/C. A is at <3, 2, 0> m. B is at <8, 2, 0> and C is at <8, 0, 0>. What is ΔV along a path
from A to C?
1.
2.
3.
4.
5.
6.
–200 V
+200 V
–1000 V
+1000 V
–800 V
+800 V
y
A
B
X
X
x
E
X
C
Problem 2 [5 points]
A bar magnet has a length L, cross sectional area A and magnetic dipole moment of µ1
A-m2. It is cut into two equal pieces, each with length L/2 and cross sectional area A.
Each of these bar magnets has a magnetic dipole moment µ2. The ratio µ2/µ1 is:
1.
2.
3.
4.
5.
6.
1.0
2.0
4.0
0.5
0.25
Not enough information
2
Problem 3 [5 points]
You place a long straight wire on top of your compass, and the wire is a height of 5
millimeters above the compass needle. If the conventional current in the wire is I = 0.2 A
and runs to the right as shown, the approximate angle the needle deflects away from
North is: [Assume that the magnitude of the Earth’s magnetic field is 2 x 10-5 T]
1.
2.
3.
4.
21.8 degrees to the west
21.8 degrees to the east
11.3 degrees to the west
11.3 degrees to the east
Problem 4 [5 points]
Originally, the potential difference across the capacitor was 1600 V. A dielectric with
K=2 was then inserted into the capacitor as shown. Now, |ΔV| = |VB-VA|=
1. 800 V
2. 400 V
3. 0 V
4. 1200 V
5. can not be determined from given information
A
B
+Q
1.5 mm
-Q
2 mm
0.5 mm
3
Problem 5 [5 points]
A positively charged particle carrying 2.0 x 10-8 C enters a region between charged
capacitor plates through a hole in one plate, as shown in the figure on the next page. The
potential difference between the plates is 1000 V, and the kinetic energy of the particle as
it enters the hole is 1.0 x 10-5 J. (Only electrical effects are to be considered. Gravitational
and air resistance are to be ignored.) Which statement below is correct?
1. The kinetic energy of the particle decreases as it moves towards the right-hand
plate. After reaching the plate, the particle reverses direction gaining kinetic
energy as it moves to the left.
2. The particle has insufficient kinetic energy to reach the right-hand plate, and it
reverses direction just before reaching the right-hand plate.
3. The particle has insufficient kinetic energy to reach the right-hand plate, and it
reverses direction just after passing through the hole.
4. The particle has insufficient kinetic energy to reach the right-hand plate, and it
reverses direction midway between the two plates.
Problem 6 [5 points]
Many commercial resistors are made of carbon, which has a very low conductivity, σcarbon
= 3x104 Am-2/(V/m). A 5 mm long carbon resistor with a cross-sectional area of 0.1 mm2
has a resistance of about 1.7 ohms. How long would a copper wire of the same diameter
have to be to obtain the same resistance?
(σcopper = 5.8x107 (Ωm)-1)
1.
2.
3.
4.
5.
1m
10 mm
10 m
0.1 m
0.01 m
4
Problem 7 [5 points]
In these circuits, identical bulbs, in different combinations, are lighted by connection to
identical, ideal batteries. Rank the five bulbs (A, B, C, D, E) in order of decreasing
brightness.
1.
2.
3.
4.
5.
6.
A, B=C, D=E
D=E, B=C, A
B=C, D=E, A
A, D=E, B=C
A=B=C, D=E
D=E, A=B=C
Problem 8 [5 points]
The electron mobility in copper is u = 4.5 x 10-­‐3 (m/s)/(N/C). If, after each collision with the lattice, a mobile electron is accelerated from rest under the influence of an applied electric field whose strength is E = 0.01 V/m, what is the magnitude of this acceleration and what is the average time between collisions? 1.
2.
3.
4.
5.
1.8 x 109 m/s2, 5.0 x 10-12 s
1.8 x 107 m/s2, 2.5 x 10-12 s
1.8 x 109 m/s2, 2.5 x 10-14 s
1.8 x 107 m/s2, 2.5 x 10-14 s
not enough information
5
Problem 9 [5 points]
MRI (magnetic resonance imaging) machines use a large coil of wire to generate a large
magnetic field within the coil. Suppose that the magnitude of the magnetic field at the
center of a coil of radius R used in an MRI machine is B. If the MRI coil can be treated as
a thin coil (i.e, equivalent to a single loop), what would be the magnitude of the magnetic
field at a distance 3R from the center of the loop, along the loop’s “z” axis?
1.
2.
3.
4.
5.
B/3
B/6
B/9
B/(10)3/2
B/27
Problem 10 [5 points]
The incomplete circuit below shows the surface charge distribution after static
equilibrium has been reached. The battery provides an emf = 1.5 V. What is the potential
difference, VB -­‐ VA? A
B
1. 0 V
2. 1.5 V
3. -1.5 V
4. VB -­‐ VA is not well defined because there is no conductor between points A and B.
6