1. Week # 15
Electromagnetic induction: Induction experiments, Faraday’s law,
Eddy Current & its applications
Book: “University Physics” by Young & Freedman 13th edition, page: P-957”
Applied Physics (Ph-1003), 4 (3, 3)
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2. Michael Faraday
• 1791 – 1867
• Great experimental
scientist
• Invented electric
motor, generator and
transformers
• Discovered
electromagnetic
induction
• Discovered laws of
electrolysis
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3. 3
Faraday’s law of induction states:
The induced emf in a closed loop equals the negative of the time rate
of change of magnetic flux through the loop.
In symbols, Faraday’s law is
If we have a coil with N identical turns, and if the flux varies at the
same rate through each turn, the total rate of change through all the
turns is N times as large as for a single turn. If øBis the flux through
each turn, the total emf in a coil with N turns is
The negative sign in Faraday’s Law is included to indicate the
polarity of the induced emf, which is found by Lenz’ Law
10. 10
Application: Exploring the Brain with Induced emfs
Transcranial magnetic stimulation (TMS) is a technique for studying
the function of various parts of the brain. A coil held to the
subject’s head carries a varying electric current, and so produces a
varying magnetic field. This field causes an induced emf, and that
triggers electric activity in the region of the brain underneath the
coil. By observing how the TMS subject responds (for instance,
which muscles move as a result of stimulating a
certain part of the brain),
a physician can test for various
neurological conditions
11. Applications of Faraday’s Law – Electric Guitar
• A vibrating string induces an emf in a
coil
• A permanent magnet inside the coil
magnetizes a portion of the string
nearest the coil
• As the string vibrates at some
frequency, its magnetized segment
produces a changing flux through the
pickup coil
• The changing flux produces an
induced emf that is fed to an amplifier
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12. Applications of Faraday’s Law – Apnea Monitor
• The coil of wire attached to
the chest carries an
alternating current
• An induced emf produced
by the varying field passes
through a pick up coil
• When breathing stops, the
pattern of induced voltages
stabilizes and external
monitors sound an alert
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13. 13
Induction stove
Figure below, shows two pots of water that were placed on an
induction stove at the same time. There are two interesting features in
this drawing. First, the stove itself is cool to the touch. Second, the
water in the ferromagnetic metal pot is boiling while that in the glass
pot is not. How can such a “cool” stove boil water, and why isn’t the
water in the glass pot boiling?
The water in the ferromagnetic
metal pot is boiling. Yet the water in
the glass pot is not boiling, and the
stove top is cool to the touch. The
stove operates in this way by using
electromagnetic induction
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Reasoning for Induction stove: The key to this puzzle is related to the fact that
one pot is made from a ferromagnetic metal and one from glass. We know that
metals are good conductors, while glass is an insulator. Perhaps the stove causes
electricity to flow directly in the metal pot. This is exactly what happens. The stove is
called an induction stove because it operates by using electromagnetic induction. Just
beneath the cooking surface is a metal coil that carries an ac current (frequency
about 25 kHz). This current produces an alternating magnetic field that extends
outward to the location of the metal pot. As the changing field crosses the pot’s
bottom surface, an emf is induced in it. Because the pot is metallic, an induced
current is generated by the induced emf. The metal has a finite resistance to the
induced current, however, and heats up as energy is dissipated in this resistance. The
fact that the metal is ferromagnetic is important. Ferromagnetic materials contain
magnetic domains (see Section 21.9), and the boundaries between them move
extremely rapidly in response to the external magnetic field, thus enhancing the
induction effect. A normal aluminum cooking pot, in contrast, is not ferromagnetic,
so this enhancement is absent and such cookware is not used with induction stoves.
An emf is also induced in the glass pot and the cooking surface of the stove.
However, these materials are insulators, so very little induced current exists within
them. Thus, they do not heat up very much and remain cool to the touch.
18. Applications of Induction: Sound Systems,
This microphone works by induction; the vibrating membrane
induces an emf in the coil
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19. Applications of Induction: Sound Systems,
Differently magnetized
areas on an audio tape
or disk induce signals
in the read/write
heads.
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20. Applications of Induction: Seismograph
A seismograph has a fixed coil and a magnet hung on a
spring (or vice versa), and records the current induced
when the earth shakes.
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