Electromagnetism Notes
Oersted’s Discovery:
Whenever current moves through a conductor, a magnetic
field is created in the region around the conductor. The
direction of current flow is taken to be the direction of
positive charge flow (the opposite direction of electron
flow) and is called conventional current flow.
The Right Hand Rule #1
(Current in a Conductor):
Grasp the conductor in such
a way that your thumb points
in the direction of
conventional current flow.
The curvature of your fingers point in the direction of the
concentric circles of the magnetic field around the
conductor.
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What if you coil the wire?
The field lines look very similar to that of the field around
a simple bar magnet (below). But how can we predict
which end of the electromagnet is north or south?
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To predict electromagnet polarity we use another right
hand rule.
Right Hand Rule #2
(A Coil/Solenoid):
Grasp the coil in such a
way that your four fingers
points in the direction of
conventional current flow.
Your thumb points in the direction of the magnetic field
inside the coil that happens to be the direction of the
north end of the electromagnet.
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Applications of Electromagnets
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Drawing Field Maps
The Motor Principle and Its Applications
What happens when two magnetic fields interact?
A force is produced on both of the magnets!
Showing current direction in three dimensional (3D)
diagrams is done with the following convention.
When the cross-section of a
conductor is shown, a dot (•)
represents current flowing out of
the page, toward you.
An X represents current flowing into the page. The • and
X notation for current flow relates to the two ends of an
arrow. The dot (•) represents the point of the arrow
coming towards you. The X represents the feathered
fletching of the arrow moving away from you.
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This is what happens when a current carrying conductor
(circular field) is placed inside the uniform field of a
horseshoe magnet.
The result is a force on the conductor to the right.
The Motor Principle:
Two magnetic fields interact with one another to produce
a force. If the conductor that carries a current cuts
through an existing magnetic field then it experiences a
force that is directed at 90 degrees to both the conductors
charge flow and the external magnetic field. The strength
of this force depends on the strength of the external
magnetic field and the current through the conductor.
Is there a way to predict which way this force will act?
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Right Hand Rule #3 (Motor Principle):
Point your fingers in the
direction of the external
magnetic field (from north to
south), the thumb points in
the direction of conventional
current flow.
The palm will point in the
predicted the direction of the force on the current carrying
conductor (which is at right angles to the other two).
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Try these examples:
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The Action in a Direct Current (D.C.) Electric Motor
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A Commutator allows a motor to run on D.C. current
because it alternates the current in the armature.
An A.C. Motor is created when the split ring commutator
is replaced with slip rings and the A.C. current is supplied
to the armature.
A Sample Electric Motor (A St. Louis Motor)
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Example Questions
1.
For each current-carrying conductor, sketch a view of the magnetic field,
based on the direction of current flow shown.
(a)
(b)
(d)
2.
(c)
(e)
For each current-carrying conductor, show the direction of current flow,
based on the structure of the magnetic field shown.
(a)
(d)
(b)
(e)
(c)
(f)
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Electromagnet Questions
1.
For each current-carrying coil, sketch a view of the magnetic field around the
coil, based on the direction of current flow shown. On each, label the polarity
(north and south) of the electromagnet.
(a)
(c)
(b)
(d)
2. For each coil, show the direction of current flow that would cause the labeled magnetic polarity.
(a)
(b)
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Motor Principle Questions
1. For all cases, indicate the field magnet poles (N or S), the current direction, field
lines, and the direction of force on the conductors.
Add the missing piece of information; the magnetic poles, field lines, direction of
conventional current flow or force in each of the figures below.
(a)
(b)
(d)
(c)
(e)
3. Which way will these two coils turn?
(a)
(f)
4. Identify the direction of current flow
and motor rotation and label the
armature polarity.
(b)
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Electromagnetic Induction
Faraday’s Principle: A magnetic field that is moving or
changing in intensity in the region
of a conductor causes or induces
electrons to flow in the conductor.
In the two cases above and the one
below, induced current will flow
only if there is motion between the conductor and the
magnetic field that is inducing the current. The back and
forth motion that is required to do this creates an
alternating and not a direct current in the conductor.
The turning on and shutting of the primary current
produces an alternating current in the secondary loop.
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Which way will the current flow and why?
Lenz’s Law: The direction of the induced current acts in
such a way as to create an induced magnetic
field that opposes the motion of the inducing
magnetic field. Note the operative word is to
“oppose” and not “repel.”
Which way would current flow in the two coils below if
the magnets were moved as shown?
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Electric Generators
A Simplified AC Generator
A generator output when a commutator is used. Direct
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current is generated.
Transformers
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Power Distribution Using Transformers
Alternating Versus Direct Current
In North America, AC is supplied at a standard rate of 60
Hz. Not only is it the simplest form of current to generate
because there is no commutator required, but it is
transformable which minimizes power loss during
transmission over long distances. In your home, electrical
energy can be transformed to suit almost any voltage
requirement. The standard rate of 60 cycles per second
can then be used by many electrical devices in your home
to keep time, such as clocks and your desktop computer.
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Electromagnetic Induction questions
1. Show the direction of
induced current flow and
the induced magnetic
polarity.
2. Show the polarity of the
inducing magnet
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3. Show the direction of magnet motion.
4. Predict which way the current will flow in this moving
conductor.
5. A primary voltage of 12 V AC was applied to the
transformer illustrated in Fig. 18.32.
a) What is the ratio of primary to secondary turns
Np
Ns ?
b) What is secondary voltage?
c) Is this a step-up or a step-down transformer? (circle 1)
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