Session 20: Induction and
Lenz’s Law
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Garrett Schieber
Phys 222
Dr. Al-Saqer
10/20/14
Part 1: Conceptual Quesitons
1) A circular loop of wire is in a region of spatially uniform
magnetic field, as shown in the figure. The magnetic field is
directed into the plane of the figure.
a) Determine the direction of the induced current in the loop
when B is increasing.
The induced current always wants to oppose the change in
flux, so when B is increasing, the circular loop will want
to induce a current so that the current creates a B field
against the increasing B-field. Thus, the induced current
will be Counter Clockwise.
b) Determine the direction of the induced current in the loop when B is constant with value
B0.
2) Using Lenz's law, determine the direction of the current in resistor ab of the figure when:
(Book 29.17)
a) Switch S is opened after having been closed for
several minutes.
When the switch is closed, there is a current in
coil A. When the switch opens, the current
decreases (to zero), thus the induced current in
coil B wants to produce a B-field in the same
direction as originally. In order to produce this
new B-field, the current must flow through the resistor in the direction from a to b.
b) Coil B is brought closer to coil A with the switch closed.
c) The resistance of R is decreased while the switch remains closed.
See Lenz’s Law Practice worksheet for more problems.
Supplemental Instruction
1060 Hixson-Lied Student Success Center  294-6624  www.si.iastate.edu