Tag Archives: Electric Field

A2L Item 201

Goal: Reason regarding electric fields

Source: 283-10, E at origin due to charged rods

All of the configurations shown below consist of charged rods of the
same length L. The magnitude of the total charge is also the same for
each rod. The total charge in each rod is distributed uniformly.

For which
configuration(s) is the electric field vector at the origin in the
positive x direction?

  1. 2 only
  2. 1 and 3 only
  3. 5 only
  4. 4 only
  5. 1 and 5 only
  6. None of the above

Commentary:

Answer

(4) Because they are given lots of examples involving point charges,
spheres and rings, students often miss the fact that there are many
situations for which the direction of the field can be deduced even
though determining the value or formal expression for the field is way
beyond them.

A2L Item 200

Goal: Distinguishing components of the Lorentz force

Source: 283 – effects of magnetic force

A charged particle moves into a region containing both an electric and
magnetic field. Which of the statements below are true?

  1. The particle cannot accelerate in the
    direction of B.
  2. The path of the particle must be a circle.
  3. Any change in the particle’s kinetic energy is caused by the E
    field.
  1. Only A
  2. Only B
  3. Only C
  4. Both A&B
  5. Both A&C
  6. Both B&C
  7. All are true.
  8. None are true.

Commentary:

Answer

(3) The only cases that most students see is the one having E and B
perpendicular. As a result they discount the case of E and B parallel
and think statement A is also true.

A2L Item 199

Goal: Hone the concept of electric field

Source: UMPERG-283-365

Two
uniformly charged rods are positioned horizontally as shown. The top
rod is positively charged and the bottom rod is negatively charged. The
total electric field at the origin

  1. is 0
  2. has both an x, and a y component
  3. points totally in the i direction
  4. points totally in the -i direction
  5. points totally in the j direction
  6. points totally in the -j direction

Commentary:

Answer

(6) By symmetry the field must point along the y-axis. Students
who do not understand that the field points away from positive charges
and towards negative charges would select #1 thinking that the fields
cancel.

A2L Item 174

Goal: Reasoning about electric fields

Source: 283 ring, E on axis

A ring
of radius R with charge +Q (uniformly distributed) is positioned as
shown. What is the electric field at a point on the axis, a distance x
from the origin?

  1. None of the above.


Commentary:

Answer

(4) Discuss how the form of the field can be reasoned from symmetry and units. Together with limiting value as x goes to zero, this uniquely singles out one answer.

A good follow-up activity is to have students sketch a graph of the field and potential along the x-axis.

A2L Item 123

Goal: Reasoning with electric fields.

Source: UMPERG-A2LEM7

The diagrams show two uniformly charged spheres. The charge on the
right sphere is three times as large as the charge on the left sphere.
Each vector represents the electric field at the center of one sphere
caused by the other sphere
. Which choice best represents the
magnitude and direction of the electric field vectors caused by one
sphere at the position of the other sphere?


Commentary:

Answer

(5) The electric field at the sphere with smaller charge is three times
larger than the field at the sphere with larger charge.

A2L Item 116

Goal: Hone vector nature of electric fields

Source: UMPERG-em97Q

Two uniformly charged rods are positioned horizontally as shown. The
top rod is positively charged and the bottom rod is negatively charged.
The total electric field at the origin

  1. Is 0.
  2. Has both an x and a y component.
  3. Points totally in the i direction.
  4. Points totally in the -i direction.
  5. Points totally in the j direction.
  6. Points totally in the -j direction.

Commentary:

Answer

(6) Along the y-axis, by symmetry, the electric field due to either rod
points along the y-axis. At the origin the contribution to each rod is
the same and points down.