Tag Archives: Lenz’s Law

A2L Item 244

Goal: Hone skill at using Lenz’s Law

Source: 283-CTQsas36

Consider the four situations below in which a wire loop lies in the
plane of a long wire. In which case(s) is the induced current in the
loop in the counterclockwise direction? [Note: if no velocity is
indicated, the loop is stationary.]

  1. A only
  2. B only
  3. C only
  4. D only
  5. A and B
  6. C and D
  7. All cases.
  8. None of the above.

Commentary:

Answer

  1. Discussing each of the cases is valuable. Students have difficulty
    recognizing that the induced current is in a direction that causes the
    field of the induced current to compensate for the change in flux. Even
    when they nominally understand that, they have trouble reliably applying
    the right hand rule.

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:

Answer

(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.