Download Charged Particles

CHARGED PARTICLES USER'S GUIDE (rev. 2/13/02)
The CHARGED PARTICLES software is intended to provide you with the opportunity to "experiment" with charged
particles moving through magnetic and electric fields. You control the field strengths and the speed of the particle.
Its path will be displayed graphically. You will be presented with a series of "challenges". A challenge is a task
which you are to accomplish using this simulation. For example, the first challenge is to cause an electron to move
in a circular path of radius 5 cm by using a magnetic field.
USING THE PROGRAM:

To start the program on the network, select Academic Programs (under Start)
and then select Charged Particles
The program will first display a "menu" of options. Beginning users should proceed through the options identified as
"CHALLENGES", starting with the lower numbered challenges first. Brief instructions for running each challenge
are provided on the screen, but you should refer to the information below for more complete instructions. You may
also want to choose the automatic demonstration option to get an idea of the kinds of things the program does.
THE CHALLENGES
In each challenge, an ion gun, which is the source of charged particles, will eject the particle toward the top of the
screen into a region with magnetic and electric fields which you control. The directions of these fields are indicated
by at the upper right corner of the screen. The magnetic field (B) direction is either into or out of the screen. This is
indicated by either a circle with a "X" in it (the tail of an arrow pointing away from you) or a circle with a dot in it (the
tip of the arrow pointing toward you). Do NOT use the software’s sign convention (+ or -). Use “into” (circle with an
X) or “out of” (circle with a dot) instead for magnetic field. The electric field is directed toward either the left or right
as shown by the arrow. The magnitudes of the fields are indicated at the bottom of the screen. SI units are used.
The units of B are newtons/ampere-meter, and the units of E are newtons/coulomb. The type of particle and the
speed of the particle as it leaves the ion gun are displayed at the bottom of the screen.
Use the following commands to control the fields:
B = increase magnetic field B
E = increase electric field E
Ctrl B = decrease magnetic field B
Ctrl E = decrease electric field E
Alt B = change the direction of B
Alt E = change the direction of E
Each time you press one of these keys (or pair of keys) a field will step upward and downward through a series preprogrammed values. You may hold down a key to change the field by several steps. Decreasing a field may
eventually cause it to become negative, which will reverse the field direction. A positive B field is directed into the
screen. A negative B field is directed out of the screen (toward you). A positive E field is directed toward the right, a
negative E field is pointing left.
One or both of the fields may be already "on" when the challenge begins. You should set the fields as needed
before each experiment to accomplish the goal of the challenge. Since you can control the field only in steps, in
some cases you may have to settle for a field that is slightly stronger or weaker than you want.
When you have the field set as you want it, press the ENTER key. If you have chosen fields which could cause
accelerations which are too large for the simulation to properly reproduce graphically, the message FIELDS TOO
INTENSE ! will be displayed. If this happens, reduce the magnitude of the fields to decrease the acceleration of the
particle. When you have set the fields as you want them, press ENTER.
The charged particle is now ejected from the ion gun. It will leave a trail on the screen as it moves. The particle will
always move in the two-dimensional plane of the screen, because the forces always act in that plane. The motion
of the particle will continue until it leaves the field area or until you interrupt the motion. You do this by simply
pressing any key on the keyboard. When the motion of the particle is interrupted, you will be offered the following
choices to allow you to try again or move on.
press SPACE BAR to have the simulation pause; to continue the simulation,
press SPACE BAR again
ENTER to repeat the experiment without clearing the screen
C to clear the screen and repeat the experiment
V to change the speed of the particle
X to quit the challenge and return to the main menu
Q to exit the program once you are back to the main menu
No score is kept in the challenges. You decide when you have accomplished your task. If at first you don't succeed,
try, try again.
THE CHARGED PARTICLES ASSIGNMENT FORMAT
Unless stated otherwise, turn in the following for each challenge assigned you:
1. Standard net force problem solution, including
i. force diagram,
ii. net force equation(s),
iii. solution,
with this exception: If you've done an identical net force problem earlier on your paper, you may simply
refer to that without repeating it. Then make the appropriate substitutions for the problem of interest.
2. If asked for an E- or B-field, the magnitude(s) and direction(s) of the field(s) that you selected.
3. A listing of the following values given on the screen after you run the challenge. (These are considered
your data.)
i.
ii.
Time
(x,y) coordinates
4. Answers to any additional questions. Always explain your answers.
CHALLENGE 1 (ORBITING ELECTRON)
In this challenge you will be working with electrons traveling at 2.6 x 10 5 m/s.
A. Detemine the magnetic field (direction and magnitude) that will cause the electron to move clockwise in a
circle of radius approximately 5 cm. Do not use an electric field. The electron will not actually be able to
move in a complete circle, but you should be able to get it to move in a semicircle with a radius of 5 cm as
shown by the linear scale. You should be able to calculate the proper B field and succeed the first time.
B. What happens to the speed of the particle as it moves? Explain.
C. Now try 3 other speeds for the electron using the same value of the magnetic field used above. Record
time, x, and y for each in a table. What do you notice about the times to travel a half period? Use your
physics equations to prove your answer to the previous question.
D. Optional: Find one additional relationship from your table in part c. Again, prove the relationship.
CHALLENGE 2 (ORBITING ALPHA PARTICLE)
This challenge is the same as the first one, except that this time the particle is an alpha particle.
A. Find the magnetic field needed to cause the particle to move in a clockwise circle with a radius of
approximately 5 cm. The alpha particle's speed is 2.6 x 105 m/s. Do not use any electric field.
B. Do you need a larger or smaller B field than in Challenge 1? Explain.
CHALLENGE 3 (UNKNOWN X)
UNKNOWN X is a real particle which is sometimes studied using magnetic fields. Find out as much as you can
about it by varying the magnetic field.
A. Is the particle positive or negative? How do you know?
B. Determine the ratio of the particle's charge to its mass.
C. What do you think the particle might be? Find a particle from the table of isotopes that would have the
same charge-to-mass ratio. Calculate the ratio to verify your claim.
D. Optional: Find a second particle with the same charge-to-mass ratio.
CHALLENGE 4 (EXPLORE ELECTRIC FIELD)
In this challenge, you investigate the effect of an electric field on the motion of an electron.
Experiment first with an electric field alone.
A. Can you cause the electron to move in a circle? How?
B. How does the electron move in relation to the direction of the E field?
C. The shape of the electron's path should remind you of something you studied in 1 st semester physics. What
is that something and why should the paths be similar?
Now try using both a magnetic and an electric field that are set up such that the forces act in opposite directions on
the electron. For example, you might set E = 8 N/C and B = 5 x 10-5 N/amp-m.
D. What is the shape of the path produced in this way? Using vector diagrams to show such things as forces,
velocities, and fields, explain what causes the paths to have the shapes that they do.
CHALLENGE 5 (e/m RATIO)
In this challenge you are to determine the ratio of the charge to the mass of a positron. A positron is an antielectron; it has the same mass as but opposite charge to an electron. Use only a magnetic field. The speed is the
same as it has been in all of the preceding challenges (2.6 x 105 m/s).
A. Find the e/m ratio of the positron.
B. How does the e/m ratio compare to that calculated using textbook values?
C. How does the positron's reaction to the magnetic field compare with the reaction of the electron which you
used in an earlier challenge?
CHALLENGE 7 (VELOCITY FILTER)
One interesting use of perpendicular electric and magnetic fields is as a velocity filter. A velocity filter permits
particles (in this case electrons) which are traveling a certain speed to go straight through but deflects particles
traveling at any different speed. Your job here is to set up a velocity filter for electrons traveling at 5 x 10 5 m/s. The
magnetic field is established at 2 x 10-5 N/amp-m. Do not change it. The first electron will be ejected automatically
and it will be deflected due to the magnetic field.
A. Begin by finding the magnitude and direction of the electric field such that it balances the magnetic field
and the electron travels straight up. (This is a net force problem!)
B. Now try altering the speed of the electrons (by entering "V" at the end of a run) to see if the velocity filter
works. Electrons traveling at any other speed should be deflected. What did you find?
C. Suppose an electron is traveling too slow. Which way would it be deflected? Why?
D. Would the velocity filter sort out the particles with different charges traveling at the same speed? Explain.
CHALLENGE 8 (MASS SPECTROMETER)
In this challenge you'll be working with UNKNOWN Z. This interesting substance consists of singly charged ions of
a common element. The first ion will be ejected automatically. Try sending the ions through the magnetic field given
at the beginning of the challenge (0.7 N/A-m out of the screen). Repeat this several times without changing the
fields or the speed. Notice that the path of the ion is not always the same even though you did not change anything.
The different paths are caused by ions of different mass (but with the same charge). These are different isotopes of
the element.
A. What charge do the ions have? Give sign and magnitude. Tell how you know.
B. How many different isotopes does this sample have?
C. Determine the mass (in kg) of each isotope, and then convert to the mass in atomic mass units (u).
D. Which isotope is most common in this sample? (Run the simulation enough times to be confident of your
answer. Note the value of X at the end of each trial.) Give the number of trials you ran and how many of
them were of each isotope.
E. What is the relative abundance of each isotope in a sample of the element? (Note: the sum of the relative
abundances must be 100%.)
F. Use your answer to part C along with Appendix F to identify the element.