10/12/2010
Electric Charges in Magnetic Fields
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Right Hand Rule #1
Moving charges feel magnetic force
perpendicular to path of motion
This force has a maximum value when
the charge moves perpendicularly to the
magnetic field lines
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This force is zero when the charge moves
along the field lines
This force is zero if the charge is stationary
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F  Bqv sin 
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Superconducting magnets
 300000 Gauss or 30 Tesla
Earth’s magnetic field
-5 T
 0.5 G or 5 x 10
Quick Quiz
 How many G in a T?
D. 105
A.
10-5
E. 0.5 x 10-5
B.
10-4
C.
104

Direction of B Field produced by a
Long Straight Current carrying Wire

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Current in wire is moving charge
 Produces Magnetic Field
Right Hand Rule #2
 Grasp the wire in your
right hand
 Point your thumb in the
direction of the current
 Your fingers will curl in the
direction of the field

The strength of a
magnetic field
produced by a wire
can be enhanced by
forming the wire into
a loop
All the segments,
∆x, contribute to the
field, increasing its
strength
If the charge is negative,
the force is opposite that
determined by the right
hand rule
André-Marie Ampère


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1775 – 1836
Credited with the
discovery of
electromagnetism
 Relationship between
electric currents and
magnetic fields
Mathematical genius
evident by age 12
Magnetic Field of a Solenoid
Magnetic Field of a Current Loop

Place your fingers
in the

direction of v
Curl the fingers in the
direction of the 
magnetic field, B
Your thumb points in
the direction
of the

force, F , on a positive
charge


If a long straight
wire is bent into a
coil of several
closely spaced
loops, the resulting
device is called a
solenoid
It is also known as
an electromagnet
since it acts like a
magnet only when
it carries a current
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10/12/2010
Electromagnetic Induction
Your Experiment
Michael Faraday

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1791 – 1867
Great
experimental
scientist
Invented electric
motor, generator
and transformers
Discovered
electromagnetic
induction
Discovered laws
of electrolysis

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
Electromagnetic Induction
Results of the Experiment

A current is set up in the circuit as long
as there is relative motion between the
magnet and the loop


The same experimental results are found
whether the loop moves or the magnet
moves
The current is called an induced current
because is it produced by an induced emf
(electromotive force)
When
When aa magnet
magnet moves
moves
toward
toward aa loop
loop of
of wire,
wire, the
the
ammeter
ammeter shows
shows the
the
presence
presence of
of aa current
current (a)
(a)
When the magnet is held
stationary, there is no
current (b)
When the magnet moves
away from the loop, the
ammeter shows a current in
the opposite direction (c)
If the loop is moved instead
of the magnet, a current is
also detected
Magnetic Flux


The emf is actually induced by a change in the
quantity called the magnetic flux rather than
simply by a change in the magnetic field
Magnetic flux is proportional to both the
strength of the magnetic field passing through
the plane of a loop of wire and the area of the
loop
Lenz’ Law

Applications of Induction Law
Ground Fault Interrupters
Lenz’ Law—Case #1
The magnetic field B becomes smaller with time
 This reduces the flux

 The induced
current will produce an induced
field, B ind, in the same direction as the
original field

B
The ground fault interrupter (GFI) is a safety device
that protects against electrical shock


Quick Quiz
Is there an induced field if there is no
Conductor present?
A. Yes
B. No
C. Question makes no sense
The current caused by the induced emf
travels in the direction that creates a
magnetic field with flux opposing the change
in the original flux through the circuit


Wire 1 leads from the wall
outlet to the appliance
Wire 2 leads from the
appliance back to the wall
outlet
The iron ring confines the
magnetic field, which is
generally 0
If a leakage occurs, the field
is no longer 0 and the
induced voltage triggers a
circuit breaker shutting off
the current
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10/12/2010
Applications of Induction Law
Apnea Monitor
Applications of Induction Law
Electric Guitar

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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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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
Lenz’ Law – Case #2
Moving Magnet Example
Induced Field

A bar magnet is moved to the right toward a stationary
loop of wire

As the magnet moves, the magnetic flux increases with time
The induced current produces a flux to the left, so the current is
in the direction shown
Quick Quiz: In what direction would the induced flux point if
the bar magnet were moved AWAY from the ring?
A. LEFT B. RIGHT
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