Falstad 3-D Electric Field and Potential Simulations
AP Physics, 2012
Instructions
 View each simulation
 Copy images into report file (screen dump)
 Summarize situation in paragraph form
(think of this as a caption in a physics book)
Make sure that you address all of the guide
questions in your paragraph.
 Use your textbook, chapter 23 as a
reference.
 Save document and rename it using your
first name as the filename
 Email final document to Dr. Schultz
http://www.falstad.com/mathphysics.html
Under “Electricity and Magnetism” find the applet called
“3D Electrostatic Fields.”

Change setting from Display: Particles (Vel) to Display:
Field Lines or Equipotentials

The mouse can be used to either adjust viewing angle or
adjust zoom

The “Slicing” option lets you take a 2-D slice of the field
pattern, making the pattern more similar to something
that you could draw.

The sliding bars can be used to control field strength and
field line density.
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GUIDE QUESTIONS: ANSWER THESE IN PARAGRAPH FORM IN
YOUR REPORT.
Point Charge Double
 Is there an axis of symmetry? Where is it?
 Where does the field equal zero? Does this imply zero potential?
 From the equipotential, how can you tell that the charges are opposite?
 Describe what would happen if you placed a proton at the midpoint between the
charges. What direction would it follow if you gave it a slight nudge?
 Can you see that the electric field is everywhere perpendicular to the equipotential
curves?
Dipole
 Is there an axis of symmetry? Where is it?
 Where does the potential equal zero? Does this imply equilibrium?
 From the equipotential, how can you tell from the shape of the lines that the charges
are opposite?
 How else does the program distinguish positive from negative potential contours?
 Can you see that the electric field is everywhere perpendicular to the equipotential
curves?
Quadrupole
 Which charges are opposite in sign? How can you tell from the E field? How can you
tell from the equipotentials?
 Could a proton be in equilibrium anywhere? Would it be stable in this location?
 Is it fair to say that the E field is always perpendicular to the equipotential curves?
Charged line
 What type of symmetry is evident here? Explain why in terms of the E-field.
 What is the distance (r) dependence of the electric field magnitude? How is this
evident from the line geometry?
 Describe the shape of the equipotential curves. How does their shape relate to the
direction of the E field?
Finite line
 How is the electric field different that from an infinite line?
 As you increase the length of the line, what happens to the spacing and direction of
the field lines?
Conducting sphere in field
 Describe how the electric field lines ‘connect’ to the surface of the sphere. Is this
the case for any field line density or sphere size?
 What does this imply about the parallel component of the net force on charges near
the surface?
 Describe the shape of the equipotential lines very close to the surface of the sphere.
Is the actual surface of the object is itself an equipotential? Justify this answer with
an explanation.
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Conducting sphere + pt
 Use the “sphere potential” control on the bottom of the screen to vary the charge on
the sphere from – to +. What angle do the E-field lines always make with respect to
the surface?
 Print an E-field slice showing a sphere with the same charge as the point, and another
with the opposite charge of the point. How do you know which is which?
 Print an equipotential slice of the sphere with same charge as point and with opposite
charge of the slice. How can you tell which is which?
 Is it fair to say that the E field is always perpendicular to the equipotential curves?
Infinite plane
 Describe the electric field in terms of magnitude and direction
 Describe the equipotential surfaces above and below the plane. If you travelled along a
line perpendicular to the plane, would anything happen to the magnitude of the
potential?
 Is it fair to say that the E field is always perpendicular to the equipotential curves?
Charged Plate
 How is the electric field of the charged plate similar to infinite plane? Different from?
 What happens to the field as the plate size is increased?
Charged plate pair
 Are the plates charged alike or opposite? How can you tell from the field lines?
 What does the spacing of the equipotentials between the plates tell you about the
voltage difference there compared to the region outside ofthe plates?
 In order for the field to do the greatest amount of work on the particle (for a given
distance) what path should it follow?
Cylinder in uniform field
 Is cylinder a conductor or insulator? How can you tell from the E field lines?
 Describe the shape of the equipotential lines very close to the surface of the cylinder.
Do you think it’s safe to assume that the actual surface of the object is itself an
equipotential?
Dielectric cylinder in uniform field
 Are the E-field lines perpendicular to the surface. To determine this, vary the
dielectric strength.
 Is there a field inside of the cylinder? How is this different than if the cylinder were
a conductor?
 Is the potential within the cylinder constant (as it is in a conductor?) If not, describe
how it varies.
Charged Ring
 If a particle was placed at the very center, would it be in equilibrium? If so, would it
be stable?
 Make the ring size very small. The potential now looks like the potential due to what
type of object?
 Is it fair to say that the E field is always perpendicular to the equipotential curves?
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