Cabrillo College
Physics 4B
PLC Activity #9
Magnetic Fields & Forces
Due: see website for due dates
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
Part 1: Magnetic Fields & Forces Physlets
Go to Chapter 27: Magnetic Fields & Forces. Do the following Physlet Physics
exercises and answer the questions listed.
Physlet Problem 27.6
An electron is shot through four regions of constant magnetic field (position is given in
centimeters and time is given in seconds).
a. In which direction is the magnetic field in each region?
b. Rank the magnitude of the magnetic fields of the four regions, from smallest to
greatest.
c. How would the path change if we inverted the direction of the magnetic field in every
region? Draw the new path of the electron.
d. If you wanted to ensure that the particle did not enter Region II, would you increase
or decrease the speed of the electron entering Region I? Give a mathematical proof
of your answer.
1
Physlet Problem 27.8
The animation shows a particle passing through a mass spectrometer (position is given
in meters and time is given in seconds). There is a constant magnetic field throughout
the region directed into the screen. There is a constant electric field in the first region
only.
a. Is the particle charged? How do you know? If it is charged, does it have a positive
or negative charge? Justify your answer.
b. If the electric field produced by the charged plates in the first region is 80 N/C, what
is the value of the magnetic field in that region?
c. What is the charge to mass ratio (q/m) of the particle?
Physlet Problem 27.7
Six objects (all with a mass of 5 grams) are shot into a region of constant magnetic
field. The blue object has a charge of 6 mC. The position and velocity of the blue object
are given in the table (position is given in centimeters and time is given in seconds). All
objects are located at x = 50 cm at time t = 0 s.
a. Rank the objects based on their velocity when they first encounter the magnetic field,
from lowest to highest. Calculate the speeds to confirm your rankings.
b. How does the magnitude of the velocity of each object change as it interacts with the
magnetic field?
c. Rank the objects based on their charge, from lowest to highest. Calculate the
charges to confirm your rankings
d. What are the direction and magnitude of the magnetic field?
2
Physlet Problem 27.10
The blue rod has a current flowing through it and sits in a uniform external magnetic field
that points out of the page (as represented by the gray circles with white dots). The
probe at the top records the force required to support the rod (position is given in
centimeters, magnetic field is given in Tesla, current is given in amperes, and force is
given in newtons).
a. In which direction does the current flow through the rod?
b. What is the mass of the rod?
Part 2: Ranking Problems
Question 1
The figure shows crossed and uniform electric and magnetic fields E and B, at a certain
instant, the velocity vectors of the 10 charged particles
listed in Table. (The vectors are not drawn to scale.) The
table gives the signs of the charges and the speeds of
the particles; the speeds are given as either less than or
greater than E/B. Which particles will move out of the
page toward you after the instant?
Particle Charge
Speed
1
2
3
4
5
Less
Greater
Less
Greater
Less
+
+
+
+
-
Particle Charge
6
7
8
9
10
+
+
-
Speed
Greater
Less
Greater
Less
Greater
Question 2
The figure shows the path of a particle through six regions of uniform magnetic field,
where the path is either a half-circle or a quarter-circle. Upon leaving the last region, the
particle travels between two charged, parallel plates and is deflected toward the plate of
higher potential. What are the directions of the magnetic fields in the six regions?
3
Question 3
The figure shows 11 paths through a region of uniform magnetic field. One path is a
straight line; the rest are half-circles. The Table gives the masses, charges, and speeds
of 11 particles that take these paths through the field in the directions shown. Which path
in the figure corresponds to which particle in the table?
Particle Mass Charge Speed
1
2m
q
v
2
m
2q
v
3
m/2
q
2v
4
3m
3q
3v
5
2m
q
2v
6
m
-q
2v
7
m
-4q
v
8
m
-q
v
9
2m
-2q
3v
10
m
-2q
8v
11
3m
0
3v
Question 4
The figure shows eight wires that carry identical currents through the same uniform
magnetic field (directed into the page) in eight separate experiments. Each wire consists
of two straight sections (each of length L and either parallel or perpendicular to the x and
y axes shown) and one curved section (with radius of curvature R). The directions of the
currents through the wires are indicated by the arrows next to the wires. (a) Give the
direction of the net magnetic force on each wire in terms of an angle measured
counterclockwise from the positive direction of the x axis. (b) Rank wires 1 through 4
according to the magnitude of the net magnetic force on them, greatest first. (c) Do the
same type of ranking for wires 5 through 8.
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