Download Lecture 17: Magnetic induction: Faraday`s law

Phy2005
Applied Physics II
Spring 2016
Announcements:
Syllabus error: read rest of Ch. 22 (DC motors, etc.)
Science news page
“The flight of the sea snail”
NYT link to article
Last Time
Michael Faraday
invented DC
motor
DC motor activity
Last time cont’d
Solenoid: how to make a nearly
constant magnetic field
B
For infinitely long solenoid
with tight coils, inside
B = monI (const.!)
n: number of turns/m
Note: for such a solenoid
B=0 outside
Today: magnetic induction
INDUCTION
Michael Faraday (1791 – 1867)
…it appeared very extraordinary, that as every
electric current was accompanied by a corresponding
intensity of magnetic action at right angles to the current,
good conductors of electricity, when placed within the
sphere of this action, should not have any current induced
through them, or some sensible effect produced equivalent
in force to such a current.”
Primary Coil
Secondary coil
G
?
Summary of Experimental Findings
EMF (voltage) is induced in the secondary coil
Only when the magnetic field through it changes.
EMF induced is bigger if the area of coil is bigger.
EMF is induced always in the opposite direction of
change in magnetic field.
Vind = - A (dB/dt) rate of ch of B with time
= - d/dt (rate of ch of  with time)
 = B A: magnetic flux
(A is cross-sectional area  to field)
Flux: suppose a loop of area 0.2m2 is
placed in a magnetic field. The normal
to the plane of the loop makes an angle
of 40o with the field of B=0.3T.
B
 = B┴ A
A
= (0.3 x 0.2) cos(40)
= 0.06 x 0.643
= 0.046 T.m2
B┴
B
A
B//
1 loop
+
+
+
+
-
Vind = - d/dt
Vtot = -3 d/dt
Net number of loops
Example 23.2
150 turn loop with a 0.75 cm2 cross-section
Magnetic field: 0 T  0.25 T in 3.6 s
5 Ohm
What is the induced current in the coil?
Persistence: The action or fact of persisting in a particular state,
opinion, purpose, or course of action, esp. despite opposition,
setback, or failure; the quality or virtue of being persistent.
Issac Newton: INERTIA
Newton’s First Law
An object will remain its state (at rest or in motion)
provided that
no external net force acts on the subject.
A conductor wants to maintain its current
state despite changes in flux.
loop = 0
loop = m + s
This self-generated compensating flux is
produced by flowing current inside
the conductor (Lenz’s law).
However, the induced current dies out
due to a finite resistance in the conductor.
I = Vind/R
The smaller R, the larger I
 stronger B
In a perfect conductor (R = 0),
the induced current
continues to flow unless no further
flux change is produced in the loop.
AC Generator
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motorac.html#c2
ACADEMIC HONESTY
Each student is expected to hold himself/herself to a high standard
of academic honesty. Under the UF academic honesty policy.
Violations of this policy will be dealt with severely. There will be
no warnings or exceptions.
Q1. ( 23.3) The earth’s B-field in
Tennessee has an average value of
0.58 x 10-4 T, and is directed at an
angle 67o below the horizontal. Find
the flux through a circular coil (r=20cm)
lying on a tabletop.
A. 6.7 x 10-2 T-m2
B. 6.7×10-6 T-m2
C. 1.6 x 10-2 T-m2
D. 1.6 x 10-4 T
E. 8.3 x 10-14 m2
Q=67o
B