# A2L Item 239

Goal: Reason with Lenz’s Law

Source: 238-730 Lenz’s Law 2

For which of the following situations will the current flow clockwise?
<table

A) A conducting rectangular loop falls from rest into a magnetic
field directed out of the page. B) A thin metal strip slides down parallel rails elevated at an
angle q. A constant B field is directed vertically upward. C) A conducting loop moves in a magnetic field produced by an
infinite current-carrying wire. 1. No situations
2. A
3. A and B
4. A and C
5. A, B, and C

### Commentary:

(4) There is usually a lot of confusion with Lenz’s Law. It is important
to determine what students are using to decide the direction of the
current. Some students despair of ever figuring it out and just guess.
Examining the microscopic motion of charges often helps.

# 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:

(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 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:

(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 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:

(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?

# A2L Item 214

Goal: Hone the concept of flux

Source: 283-400, Flux in and out of a balloon.

We construct a closed Gaussian surface in the shape of a sphericalWe construct a closed Gaussian surface in the shape of a spherical
balloon. Assume that a small glass bead with total charge Q is in the
vicinity of the balloon. Consider the following statements:

1. If the bead is inside the balloon, the electric flux over the
balloon’s surface can never be 0.

2. If the bead is outside the balloon,
the electric flux over the balloon surface must be 0.

Which of these statements is valid?

1. Only A is valid.
2. Only B is valid.
3. Both A and B are valid.
4. Neither one is valid.

### Commentary:

(3) Students may accept statement A but still think that the value of
the flux depends upon location of the bead in the sphere. Transition
from just inside to just outside poses particular difficulty to some
students. This usually derives from lack of experience with vectors and
dot products. Having the student draw field lines does help, but only
after they comprehend that the formal definition of flux is equivalent
to counting the net number of lines of E crossing the surface.

# A2L Item 127

Goal: Link flux to field lines.

Source: 283-415 nonzero flux?

The circles in the picture below are Gaussian surfaces. All other lines
are electric field lines. For which cases is the flux non-zero? 1. a
2. a, b, and f
3. a, b, e, and f
4. a, b, d, e, and h
5. a and b
6. All but g
7. All of them
8. None of the above
9. Cannot be determined

### Commentary:

between two or more of the situations. For example, which is flux is
larger, a or f? Such questions help determine what students are focused
upon; are they keying on number of lines, do they understand that the
flux can be negative.

# A2L Item 124

Goal: Link flux and enclosed charge.

Source: 283-410 Flux greatest?

Consider the flux through the following spherical Gaussian surfaces.
Take each charge distribution to be either a point charge or a spherical
shell of charge (uniformly distributed). For which situation is the flux
greatest? 1. a and c
2. a, b, and c
3. b
4. a
5. b, c, and e
6. a, f, and g
7. b, c, and g
8. Other