Tag Archives: Electric Field

A2L Item 256

Goal: Hone the vector nature of the electric field

Source: CTQ283-6

Four charges are positioned as
shown. What is the direction of the electrical field at the origin?

  1. Along i or in the +x direction
  2. Along -i or in the -x direction
  3. Along j or in the +y direction
  4. Along -j or in the -y direction
  5. Along some other direction
  6. E=0 at the origin

Commentary:

Answer

(6) Difficulty with this question usually indicates that students are
not familiar with vectors. Good follow up question: ask whether it
matters if the charges are along a 45 degree line.

A2L Item 257

Goal: Hone the vector nature of the electric field

Source: CTQ283-7

Four
charges are positioned as shown. What is the direction of the
electrical field at the origin?

  1. Along i or in the +x direction
  2. Along -j or in the -y direction
  3. Along j or in the +y direction
  4. Along -i or in the -x direction
  5. Along some other direction
  6. E=0 at the origin

Commentary:

Answer

(3) Difficulty with this question indicates that students do not know
how the direction of the electric field relates to the sign of the
charge.

A2L Item 255

Goal: Hone the vector nature of the electric field

Source: CTQ283-5

Four
charges are positioned as shown. What is the direction of the
electrical field at the origin?

  1. Along i or in the +x direction
  2. Along j or in the +y direction
  3. Along -i or in the -x direction
  4. Along -j or in the -y direction
  5. Along some other direction
  6. E=0 at the origin

Commentary:

Answer

(6) Difficulty with this question usually indicates that students are
not familiar with vectors. Good follow up questions are to change the
sign of one or more charges.

A2L Item 237

Goal: Hone understanding of Faraday’s Law

Source: 283-715 bar with moving magnet

A
conducting bar is placed on a set of horizontal rails. A bar magnet is
positioned above the rails with its north pole facing the rails, and is
then released. While the magnet falls toward the rails, which of the
following statements are true.

  1. There is an electric field in the
    bar
  2. There is a current in the bar
  3. The bar remains
    stationary
  1. A only
  2. B only
  3. C only
  4. A and B
  5. A and C
  6. B and C
  7. A, B, and C

Commentary:

Answer

(4) As the bar magnet falls, the magnetic flux through the circuit will
change. This will cause an E field and current in the conducting bar.
The conducting bar will also experience a magnetic force due to the
current flowing in the bar.

A2L Item 235

Goal: Hone understanding of Faraday’s Law

Source: Hone understanding of Faraday’s Law

A long conducting bar moves with a constant velocity in a uniform magnetic
field. If the bar and the velocity of the bar are perpendicular to the
magnetic field as shown. Which of the following statements are true?

  1. At steady state there is an E field in
    the bar
  2. At steady state there is a current in the bar
  3. At steady state there is a mag. force on bar
  1. A only
  2. B only
  3. C only
  4. A and B only
  5. A and C only
  6. B and C only
  7. A, B, and C

Commentary:

Answer

(1) This question is often given as an example of Faraday’s law.
Relating Emf to flux change is difficult for some students to perceive
when there is no circuit. Creating an imaginary circuit helps, but many
students continue to get the direction of the field incorrect even
though the magnitude of the potential difference is now understood. It
is useful to view the process using the Lorentz force. This helps
students understand which charges migrate to which end and, therefore,
what the direction of the electric field in the bar is.

A2L Item 236

Goal: Hone understanding of Faraday’s Law

Source: 283-710 Conducting bar on rails

A
conducting bar is placed on a set of horizontal rails. After a uniform
magnetic field is set up perpendicular to the rails, the bar is given a
push. As the bar moves along the rails, which of the following
statements are true.

  1. There is an E field in the bar
  2. There is a current in the bar
  3. The bar moves with a constant
    speed
  1. A only
  2. B only
  3. C only
  4. A and B
  5. A and C
  6. B and C
  7. A, B, and C
  8. None are true

Commentary:

Answer

(4) In the absence of a force sustaining the motion, the bar slows down.
It is interesting to discuss what happens to the kinetic energy as the
bar slows down. Advanced students can work out that the energy is
dissipated in the resistance of the bar.

A2L Item 228

Goal: Reason regarding electrodynamics

Source: 283-635 Path of a charge in E&B fields.

A
charge has an initial velocity parallel to the y-axis in E and B fields.
Both fields point along the x axis. Which of the following statements
regarding the charge’s motion are correct?

  1. The charge will travel along a straight-line path.
  2. The charge’s speed will change as it travels.
  3. The charge will travel in a helical path.
  4. The charge will travel in a helical path of increasing pitch.
  5. The charge will travel in a circle in the x-y plane.
  6. 1 and 2 only
  7. 2 and 4 only
  8. None of the above

Commentary:

Answer

(7) A common response is #4 because they forget that increasing pitch
implies that the speed changes.

A2L Item 227

Goal: Reason regarding electrodynamics

Source: 283-630 Path of a charge in E&B fields.

A
charge is released from rest in E and B fields. Both fields point along
the x axis. Which of the following statements regarding the charge’s
motion are correct?

  1. The charge will travel along a straight-line path.
  2. The charge’s speed will change as it travels.
  3. The charge will travel in helical path.
  4. The charge will travel in helical path of increasing pitch.
  5. The charge will travel in a circle in the x-y plane.
  6. 1 and 2 only
  7. 2 and 4 only
  8. None of the above

Commentary:

Answer

(6) The different responses reveal the extent to which students
understand vector cross products and/or read the problem carefully. Some
students choose #8 because they do not like the way the motion is
expressed. They prefer descriptions such as, the charge first moves in a
straight line until it gets some speed then …

A2L Item 216

Goal: Hone the relationship between E and V

Source: 283-475 Must V=0 if E=0?

True or false: it is possible to have the electric field be 0 at some
point in space and the electric potential be non-zero at that same
point.

  1. True
  2. False

Commentary:

Answer

(1) Whichever answer students give, ask them to draw a charge
configuration which satisfies their answer. Often this is sufficient to
cause them to change their mind. If appropriate raise for discussion the
case of the interior of a uniformly charged spherical shell.

A2L Item 215

Goal: Relate flux and electric field

Source: 283-405 If phi = 0, is E=0?

True or False: If the electric flux = 0 over some closed Gaussian
surface, then this means that the electric field = 0 on that surface.

  1. True
  2. False

Commentary:

Answer

(2) A good followup question is; Even though the electric field is not
zero everywhere, can it be zero somewhere on the Gaussian surface? If
so, draw a charge configuration for which this is true?