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Teacher’s Guide
How Electrons Move
Overview
Students explore how electrons create fields and are influenced by fields. Students
begin by learning how to use vectors as a concise way of visualizing force fields. Then
students learn how to interpret electric field representation and how to use charged
particles to deflect the motion of an electron. Students also learn how to design a
uniform field that will accelerate an electron along a straight path. Finally, students
explore the movement of electrons in magnetic fields.
Learning Objectives
Students will be able to:
 Identify the relationship between the magnitude and direction of a force vector
and the distance between charged objects
 Interpret electric field representations of forces present around a charged
particle.
 Determine how the motion of an electron is affected by the forces created by
electric field surround charge particles.
 Explain how movement of electrons in magnetic fields is related to charge, mass,
velocity, and magnetic intensity.
Prerequisite Knowledge
Students should already have a basic understanding of:
 Charge
 Coulomb’s Law
 Vector
 Field
 Magnet
Background and resources
The article “General Students’ Misconceptions Related to Electricity and Magnetism” by
Christian Raduta from the physics department at The Ohio State University discusses on page
10 how students typically see the electric and magnetic fields as having a static nature. It is
important to notice whether or not your students think whether or not a field exists in a space
and applies forces on charges, and/or whether they think it does not change even when a new
charged particle enters a region. This conception should change after working with the models
in the activity (but it may not).
URL: http://www.google.com/search?q=Misconceptions+electric+field&ie=utf-8&oe=utf8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
A simple explanation of magnetic fields can be found in the following video
http://www.youtube.com/watch?v=uj0DFDfQajw
In the following video natural magnetic fields are revealed as chaotic, ever-changing geometries
as scientists from NASA's Space Sciences Laboratory excitedly describe their discoveries.
http://www.animateprojects.org/films/by_date/2007/mag_mov/1/
Approximate time for lesson completion: 60 minutes
Activity Answer Guide
Page 1:
No questions.
Page 2:
1. Place a snapshot here with an annotation
showing where you think the hidden charge
is in the model above.
Note that the invisible charge is used to pair with
the visible negative charge at the lower-right
corner.
Page 3:
1. Did you guess incorrectly about what
particles were present in any of the four
examples above? If so, explain what
mistakes you made and why you think you
made them.
The invisible charge is somewhere in the area of
the red circle.
2. If you have two charged particles side by
side and no other particles present, which of
the following is possible concerning their
force vectors?
(a)
In some cases I guessed correctly and in others
incorrectly. When I guessed incorrectly, it was
because I was not able to detect when a neutral
particle was present. Otherwise, the direction of
the force field vectors made it possible to guess
where and what type of charge was present.
2. Electric field vectors all pointing towards a
single point indicates what?
3. What is the relationship between the
length of a force vector (the magnitude of the
force) and the distance between charged
objects?
(b)
(b)
(c)
4. Place a snapshot here with annotations
proving your answer to the question to the
left. If necessary, return to the model and
create the snapshot you need
Page 4:
3. Which of these subatomic particles would
NOT have an electric field?
1. Prediction: Do you think the electron in the
Electron Cannon Game would travel on the
same path if you changed all of the charges
by the same amount?
(c)
2. Was your prediction correct? Explain what
happened when you increased or decreased
the amount of charge on the stationary
particles.
Yes, my prediction was correct. When I changed
all the charges by the same amount, the
interaction of the electron with the charges
closest to it significantly changed the path of the
electron.
3. Place the snapshot of your winning game
here
Snapshot solutions will vary.
4. Describe what you needed to do in order
to win the game.
I had to carefully arrange the attraction and
repulsion forces that would guide the electron’s
path to the target. I had to try to avoid the
obstacles.
Page 5:
1. Place an image of a uniform field that
accelerates an electron in a straight line
here.
2. Which of the following would NOT
increase the average strength of the electric
field within the box in the above model? Note
that some of these options can be tested
with the model.
(c)
Page 6:
1. The picture to the right was formed by
placing a bar magnet under a piece of paper
and then sprinkling iron filings on top.
Describe the relationship between the
pattern formed and the magnetic field
created by the magnet.
The iron filings line up along the magnetic field
created by the magnet.
Page 7:
1. Place a snapshot of your attempt to match
Challenge A here.
5. Which of the following images was created
by gradually decreasing the field intensity
from a large positive value to a small positive
value?
(b)
Page 8:
1. If you have two charged particles
positioned one above the other, which of the
following is NOT possible concerning their
force vectors?
(a) (b) (d)
2. Place a snapshot of your attempt to match
Challenge B here.
2. In which of the following setup of electric
field or magnetic field can a single charged
particle possibly stay motionless?
(a) (d)
3. Which of the following can move an
electron in a straight line?
(d)
4. Which factors affect the strength of an
electric field?
(d)
3. If you have a magnetic field pointing out of
the screen, moving electrons, which are
negative, will
(d)
4. Which of the following would make a
charged particle traveling clockwise in a
magnetic field change direction and start
moving counter-clockwise?
(c)
5. Mass spectrometry is an analytical
technique for determining the elemental
composition of a sample. Some mass
spectrometers use a magnetic field to deflect
the trajectories of ions (see the above image
on the left), something you have learned in
this activity. Explain why this method can be
used to sort ions of different masses and
charges using the above model.
Ions of different masses and same charge have
different deflections after moving across the
same magnetic field. They end up in difference
places in the detector pane. Looking at how
many ions arrive in different spots of the
detector pane, we can deduce how many
different kinds of atoms the sample has.