Electromagnetic Induction and Faradays Law

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Chapter 21 Electromagnetic Induction and
Faradays Law
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Units of Chapter 21

• Induced EMF
• Faradays Law of Induction Lenzs Law
• EMF Induced in a Moving Conductor
• Changing Magnetic Flux Produces an Electric
  Field
• Electric Generators
• Back EMF and Counter Torque Eddy Currents
• Transformers and Transmission of Power

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Units of Chapter 21

• Applications of Induction Sound Systems,
  Computer Memory, Seismograph, GFCI
• Inductance
• Energy Stored in a Magnetic Field
• LR Circuit
• AC Circuits and Reactance
• LRC Series AC Circuit
• Resonance in AC Circuits

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21.1 Induced EMF
Almost 200 years ago, Faraday looked for evidence
that a magnetic field would induce an electric
current with this apparatus
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21.1 Induced EMF
He found no evidence when the current was steady,
but did see a current induced when the switch was
turned on or off.
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21.1 Induced EMF
Therefore, a changing magnetic field induces an
emf. Faradays experiment used a magnetic field
that was changing because the current producing
it was changing the previous graphic shows a
magnetic field that is changing because the
magnet is moving.
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21.2 Faradays Law of Induction Lenzs Law
The induced emf in a wire loop is proportional to
the rate of change of magnetic flux through the
loop. Magnetic flux
(21-1)
Unit of magnetic flux weber, Wb. 1 Wb 1 Tm2
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21.2 Faradays Law of Induction Lenzs Law
This drawing shows the variables in the flux
equation
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21.2 Faradays Law of Induction Lenzs Law
The magnetic flux is analogous to the electric
flux it is proportional to the total number of
lines passing through the loop.
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Example 21-2
A square loop of wire 10.0 cm on a side is in a
1.25 T magnetic field B. What are the maximum
and minimum values of flux that can pass through
the loop?
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21.2 Faradays Law of Induction Lenzs Law
Faradays law of induction
1 loop (21-2a)
N loops (21-2b)
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21.2 Faradays Law of Induction Lenzs Law
The minus sign gives the direction of the induced
emf A current produced by an induced emf moves
in a direction so that the magnetic field it
produces tends to restore the changed field.
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21.2 Faradays Law of Induction Lenzs Law
Magnetic flux will change if the area of the loop
changes
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21.2 Faradays Law of Induction Lenzs Law
Magnetic flux will change if the angle between
the loop and the field changes
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21.2 Faradays Law of Induction Lenzs Law

• Problem Solving Lenzs Law
• Determine whether the magnetic flux is
  increasing, decreasing, or unchanged.
• The magnetic field due to the induced current
  points in the opposite direction to the original
  field if the flux is increasing in the same
  direction if it is decreasing and is zero if the
  flux is not changing.
• Use the right-hand rule to determine the
  direction of the current.
• Remember that the external field and the field
  due to the induced current are different.

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Example 21-5
A square coil of wire with a side of length 5.00
cm contains 100 loops and is positioned
perpendicular to a uniform 0.600 T magnetic
field. It is quickly pulled from the field at
constant speed to a region where B drops abruptly
to zero. At t0, the right edge of the coil is
at the edge of the field. It takes 0.100 s for
the whole coil to reach the field-free region.
The coils total resistance is 100 ?. Find (a)
the rate of change in flux through the coil, and
(b) the emf and current induced. (c) How much
energy is dissipated in the coil? (d) What was
the average force required?
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Exercise B
What is the direction of the induced current in
the circular loop due to the current shown in
each part?
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21.3 EMF Induced in a Moving Conductor
This image shows another way the magnetic flux
can change
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21.3 EMF Induced in a Moving Conductor
The induced current is in a direction that tends
to slow the moving bar it will take an external
force to keep it moving.
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21.3 EMF Induced in a Moving Conductor
The induced emf has magnitude
(21-3)
Measurement of blood velocity from induced emf
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Example 21-6
An airplane travels 1000 km/h in a region where
the earths magnetic field is 5.0x10-5 T and is
nearly vertical. What is the potential
difference induced between the wing tips that are
70 m apart?
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Example 21-7
The rate of blood flow in our bodys vessels can
be measured using the apparatus shown below since
blood contains charged ions. Suppose that the
blood vessel is 2.0 mm in diameter, the magnetic
field is 0.080 T, and the measured emf is 0.10
mV. What is the flow of velocity of the blood?
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21.4 Changing Magnetic Flux Produces an Electric
Field
A changing magnetic flux induces an electric
field this is a generalization of Faradays law.
The electric field will exist regardless of
whether there are any conductors around.
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21.5 Electric Generators
A generator is the opposite of a motor it
transforms mechanical energy into electrical
energy. This is an ac generator
The axle is rotated by an external force such as
falling water or steam, inducing an emf in the
rotating coil. The brushes are in constant
electrical contact with the slip rings.
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21.5 Electric Generators
A dc generator is similar, except that it has a
split-ring commutator instead of slip rings.