Physics 108
Homework #5
Electromagnetic Induction
Name:
TA:
Day/Time:
Section:
Spring 2014
1) Induction with a bar magnet
a) (1 pt) Lenz’s law states (choose one):
i) The current induced in the coil will create a magnetic field that resists this change in flux.
ii) The magnetic field in space around an electric current is proportional to the electric current.
iii) A steady current can create a steady induced magnetic field.
iv) Induced emf is always positive.
A bar magnet is moving in the direction as shown below. Find the direction of induced current
and status on magnitude of the flux of the coil. The current direction is observed from the right.
(Circle one for each)
b) (1 pt)
Direction of Current
Magnitude of flux
Clockwise
Increasing
Counterclockwise
Decreasing
c) (1 pt)
Direction of Current
Magnitude of flux
Clockwise
Increasing
Counterclockwise
Decreasing
d) (1 pt)
Direction of Current
Magnitude of flux
Clockwise
Increasing
Counterclockwise
Decreasing
2) Rotating a coil in a uniform magnetic field
A square loop of side length 2 m is placed in a magnetic field (out of the page) as show.
a) (2 pts) Define the magnetic flux with magnetic
field (B), enclosed an area (A) where the magnetic
field passes, and the angle (θ) between the normal
axis of the area and the magnetic field direction.
Φ = ABcosθ
b) (2 pts) What is the magnetic flux through the loop when the strength of magnetic field is
0.3T?
Φ = ABcosθ = 2*2 m2 * 0.3T * 1 = 1.2 Wb
c) (2 pts) If this square loop rotates 45° and 90° through the axis (counterclockwise from the
right viewpoint), respectively, what is the magnetic flux for each?
For 45°, Φ = ABcosθ = 2*2 m2 * 0.3T * 1/√2 = 0.849 Wb
For 90°, Φ = ABcosθ = 2*2 m2 * 0.3T * 0 = 0 Wb
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