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Discussion Notes for Monday, June 30, 2008
Prepared by: Aaron Wessell
Conceptual Problems
(Kim Rosenthal borrowed it from pg. 495 in Essentials of College Physics)
1) A charged particle moves in a straight line through a region of space. Which of the
following answers must be true? (assume any other fields are negligible). The magnetic
field
(a) has a magnitude of zero
(b) has a zero component perpendicular to the particle's velocity
(c) has a zero component parallel to the particle's velocity in that region.
-C can be either True or False, it does not necessarily have to be true.
* A magnetic field parallel to the direction of the charge’s velocity would not affect
the motion of the charge.
(Kim Rosenthal borrowed it from pg. 515 # 10 in Essentials of College Physics)
2) Will a nail be attracted to either pole of a magnet? Explain what is happening inside
the nail when it is placed near the magnet.
-Atoms can possess dipole moments. When brought close to the magnet, the dipole
moment aligns with the magnetic field and the atom becomes a magnet of its own.
Therefore, the nail can be attracted to either pole of the magnet.
(Monique Pogreba borrowed it from pg. 514, #6 in Essentials of College Physics)
3) Which way would a compass point if you were at the earth's North magnetic pole?
Why?
-The needles of a compass align with, and point in the direction of, the magnetic
field. In order to know what direction the needle will point, one must know what
the magnetic field of Earth looks like. The Earth’s geographic North pole is actually
a magnetic South pole.
-If a vertical compass is placed at the North pole it will point directly down towards
the North pole.
(Monique Pogreba borrowed it from pg. 514 #2 in Essentials of College Physics)
4) Can a constant magnetic field set a proton at rest into motion? Explain your answer.
-According to the following equation: FB=qvBsinθ, a constant magnetic field cannot
set a proton at rest into motion. If the charge is not moving, its velocity is zero, and
if the velocity of zero, the force applied by the magnetic field is zero.
-Shushaku also reminded us that in dealing with electrostatics, the force on a charge
in an electric field is in the direction of the electric field. However, with magnetism,
the force applied by the magnetic field is in a direction perpendicular to the
magnetic field.
Quantitative Problems
(Tim Ford borrowed it from Essentials of College Physics, pg. 516 - # 8)
1) An electron is accelerated through 2,400 V from rest and then enters a region where
there is a uniform 1.70-T magnetic field. What are (a) the maximum and (b) the minimum
magnitudes of the magnetic force acting on this electron.
-As the electron is accelerated through 2,400 V from rest its potential energy will be
converted into kinetic energy, which allows for one to solve for the velocity:
PE = Vq
PE = (2400 V)(-1.6e-19C)
PE = 3.84e-16J= (1/2)mv2
v = 2.903e7 m/s
Fmax = qvB
(no need for sinθ when dealing with the max)
= (-1.6e-19C)(2.903e7 m/s)(1.7)
= 7.88e-12 N
Fmin = 0, if the magnetic field is parallel to the motion of the electron.
If sinθ = 90 or 180 degrees, then F is max
= 0 or 360 degrees, then F is min
(Katie Daniel borrowed it from Essentials of College Physics, pg. 516 # 14)
2) A wire having a mass per unit length of 0.500 g/cm carries a 2.00-A current
horizontally to the south. What are the direction and magnitude of the minimum magnetic
field needed to lift this wire vertically upward?
-We will focus on this problem later on because it is more involved.
CLOSING REMARKS:
-Tomorrow will be used as a Q + A session in preparation for Wednesday’s exam.
-Shushaku would like for questions to be sent to him via email, along with a sentence
or two describing what you are having trouble with. He will try to address common
questions in discussion.
-Tomorrow’s discussion attendance is optional. Those who wish to use the 50 min
for independent study time may do so.
-Shushaku has to leave discussion 10 min early but may be seen during his office
hours before lecture in Room 4257, Chamberlin.
-Reminder: Lab has been changed to 12:45 pm on Tuesday, except for those
attending the lab session on Wednesday.