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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:
 Know the dependence of the magnitude and direction of an electrostatic force on
the distance between charged objects and their charges ( Understand Coulomb's
Law).
 Understand the concept of electric fields created by charged particles.
 Determine how the motion of an electron is affected by electrostatic forces.
 Explain how the movement of electrons in magnetic fields is related to charge,
mass, and velocity and magnetic field intensity.
Prerequisite Knowledge
Students should already have a basic understanding of:
 Charge
 Coulomb’s Law
 Vector
 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://arxiv.org/pdf/physics/0503132
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. What is the relationship between the
magnitude of the force (the length of the
arrow) and the distance between two
charged particles?
(b)
2. What is the relationship between the
direction of the electric force (the direction of
the arrow) and the charges of the two
particles?
(d)
3. Place a snapshot here with an annotation
showing where you think the hidden charge
is in the model above.
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.
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?
(b)
3. Which of these subatomic particles would
NOT have an electric field?
(c)
Page 4:
1. Place the snapshot of your winning game
here
Snapshot solutions will vary.
The invisible charge is somewhere in the area of
the red circle. (Note: There are many possible
good images that could be used here. This is
just one example.)
2. Describe what you needed to do in order
to win the game.
4. Explain why you think the hidden particle
is at the position you marked.
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.
Using all negative particles shows us that the
hidden charge must also be negative because
the visible particles are repelled by the similarly
charged hidden particle. The red circle shows
the area where all the force vectors are being
repelled from.
3. 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)
4. Was your prediction correct? Explain what
happened when you increased or decreased
the amount of charge on the stationary
particles.
field and it would travel straight (along the field
lines).
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.
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.
Page 5:
Page 6:
The iron filings line up along the magnetic field
created by the magnet.
Page 7:
1. Place an image of an electric field that
moves an electron in an approximately
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)
3. Describe what you needed to do in order
to get an electron to move in a straight line
and explain how it works.
I needed to create a wall of opposite charges on
each side. This created a uniform field in the
middle so I could put a charge anywhere in that
1. Place a snapshot of your attempt to match
Challenge A here.
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
negatively charged, 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. 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)
5. Mass spectrometry is used to determine
which elements are in a substance. Some
mass spectrometers use a magnet like the
one modeled above. Using what you
observed in the model above, explain how a
magnet can be used to sort ions of different
mass.
Ions of different masses and same charge have
different deflections after moving across the
same magnetic field. Heavier ones are not
deflected as much as lighter ones, so you can
sort the ions by mass. Because each element
has atoms with a certain mass you can tell what
elements are present if each ion is a single
atom.
6. On Page 1, you learned about the
discovery of the electron. Thomson
measured the mass/charge ratio of the
electron and excluded the possibility that the
particle was an ion. Based on the model
above about mass spectrometry, can you
explain how the mass/charge ratio can be
measured?
The assumption is that all the ions have the
same charge, so you can tell the relative
masses of the ions by seeing how much they
are deflected by the magnetic field. Lighter ions
are deflected more than heavier ones. An
electron is so much lighter than any other single
atom that it was apparent to Thomson that the
particles he saw in the cathode ray tube could
not be atoms, but something smaller.
7. Run the model to the left. Use the
"Reverse charge" button to reverse the
charge on the green particle. Can you
explain why the green particle moves in a
straight line, regardless of its charge?
The green particle is exactly between the two
other charges, and is given some initial velocity
in the vertical direction. Because it is exactly
between the two other charges it feels exactly
the same attraction or repulsion from those
charges, so it continues to move in a straight
vertical path. It doesn’t matter what the charge is
in the middle if both charges outside of it are the
same charge and distance from it. The middle
particle will feel the same force from the other
particles.