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MOTORS AND GENERATORS
MAGNETIC FIELDS
Powerful magnets are essential components in motors and generators. Some electric
motors and generators rely upon a combination of a permanent and an electromagnet.
An electric motor converts electrical energy into mechanical energy by the torque
acting on a conductor carrying an electric current in a magnetic field. A generator
converts mechanical into electrical energy by moving a conductor through a magnetic
field.
But, what is a magnet and what is meant by a magnetic field?
Magnetic phenomena have been known for thousands of years. Certain
elements are magnetic such as iron, cobalt and nickel. Strong magnets can now
be made from certain alloys including (neodymium iron boron) and (alnico aluminium, iron, cobalt and nickel). Flexible magnets can be made from ceramic
material.
Magnets can attract ferromagnetic materials such as iron. Magnets can attract
or repel other magnets as shown in figure (1).
N
S
N
S
opposite poles attract each other
N
S
S
N
S
N
N
S
similar poles repel each other
Fig. 1.
Forces between a pair of bar magnets.
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To explain these magnetic forces, we introduce the concept of magnetic field.
A field is a region of influence. For example, an object because of its mass
experiences a force in a gravitational field. A charged object experiences a force
in an electric field. A field for a vector quantity is shown by a pattern of lines.
The lines indicate the field pattern and the density of the lines indicates the
strength of the field. The closer the field lines are together, then the stronger
the field and hence the stronger the force.
A magnet has a north pole and south pole. The magnetic field of a magnet can
be shown by a set of continuous loops that exit from the north pole of the
magnet and enter at the south pole as shown in figure (2), (3) and (4). The Bfield lines indicate how a small magnet (or compass) will align itself in the field.
strong field:
high density of B-lines
B
S
weak field:
low density of B-lines
N
S
N
compass needles / magnets
align along B-field
Fig. 2. Magnetic field lines for a bar magnet. The magnetic field
lines are continuous loops. The field lines exit away from the north
pole and enter the magnet at the south pole. The magnetic field is
strongest where the field lines are closest together at the two poles.
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Fig. 3. Magnetic field lines for a bar magnet showing the external
magnetic field only and the magnetic field pattern due to iron
filings. The iron filings align themselves as if they were small
magnets and more of them accumulate where the B-field is
strongest.
N
S
the magnetic field in the region
between the poles is nearly uniform
Fig. 4. Magnetic field lines for a horse-shoe magnet. The
magnetic field lines are continuous loops. The field lines exit away
from the north pole and enter the magnet at the south pole.
The Earth has a magnetic field surrounding it. The field lines are similar to a
huge bar magnet. The south pole of the magnet is located near the geographical
north pole and the north magnetic pole near the south geographical pole as
shown in figure (5).
south magnetic hole
north magnetic hole
Fig. 5. The Earth is surrounded by a magnetic field.
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Figure 6 show that if you break a magnet into pieces you create smaller magnets.
Fig. 6. Breaking a magnet produces new smaller magnets.
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ELECTRIC CURRENTS AND MAGNETISM
When electric charges are in motion they exert forces on each other that can’t
be explained by Coulomb’s law. If two parallel current carrying conductors are
near each other they attract each other when the currents are in the same
direction and repel each other when the currents are in opposite directions.
Such forces are called magnetic forces.
Hans Oersted (1777 – 1815) placed a wire near a compass needle and switch on
the current. When the wire was parallel to the compass needle, the compass
needle was deflected by the current. When the wire was perpendicular to the
compass needle, there was no deflection when the current was switched on. He
made two conclusions: (1) the electric current somehow exerts a twisting force
on the magnet near it and (2) the magnitude of the force depends upon the
relative orientation of the current and the magnet.
A current (moving charges) through a wire alters the properties of space near it
such that a piece of iron will experience a force. Hence, surrounding a wire
carrying a current is a magnetic field. It is the interaction of the magnetic field
and the iron that leads to the force, rather than the current and iron acting
upon each other.
The magnetic field surrounding a straight conductor carrying a current I can be
visualized as a series of circles. The closer the lines are together, the stronger
the magnetic field. A compass placed near the wire will align itself with the field
lines. The direction of the magnetic field is determined by the right hand screw
rule. Using the right hand: the direction of the thumb represents the current
(direction in which positive charges would move) and the curl of the fingers
represents the direction of the magnetic field as shown in figure (7). The
magnetic field strength is given by the vector quantity B where B stands for the
B-field or magnetic induction or magnetic flux density. The S.I. unit for the Bfield is the teslas [T].
I
B
Fig. 7. The right hand screw rule is used to determine the direction of
the magnetic field produced by moving charges.
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The B-field surrounding a conductor carrying a current I at a distance R from the
conductor is given by equation (1) as shown in figure (8)
(1)
B=
0 I
2 R
 0 = 4 × 10-7 T.m.A-1
where  0 is a constant called the permeability of free space.
direction of B-field given by right hand rule
B-field increases with increasing I
B
B
R
I
B-field decreases with increasing R
B
current I into page
R
Fig. 8. B-field surrounding straight conductor carrying a current.
A conductor carrying a current produces a magnetic field similar to a bar
magnet as shown in figure (9).
current I
B-field
Fig. 9.
Magnet field due to a
current loop. The
direction of the
B-field is given
by the right hand
screw rule.
B-field
stro
ngB-field
strong
Bfield
A solenoid is a conductor wound into a long set of coils
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Solenoids are often used as electromagnets where a ferromagnetic substance
placed inside the coils greatly increases the strength of the magnetic field.
Figure (10) shows the magnetic field patters for an air filled solenoid and when a
rod of ferromagnetic material is placed inside the coils.
Fig. 10. Magnetic field patterns for a solenoid (air and ferromagnetic cores) .
The magnetic field of a solenoid is very similar to that of a bar magnet as shown
in figure (11).
Fig. 11. Magnetic field patterns for a bar magnet and solenoid.
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MAGNETIC FLUX
B
[T.m-2]
A useful quantity is to consider the number of magnetic field lines crossing an
area. This concept is called the magnetic flux and is illustrated and defined in
figure (12).
Magnetic flux for constant B-field
 B  B A cos
Each point on a surface is associated
with a direction, called the surface
normal; the magnetic flux through a
point is then the component of the
magnetic field along this direction
B
A

  90o
cos  1
 B  B A cos
B  B A
  0o
cos  0
B  0
Fig. 12. Magnetic flux when the magnetic field B is uniform over an area A.
ORIGIN OF THE MAGNETIC FORCE
Permanent Magnets
A moving charge gives rise to a magnetic field. An electron is not a spinning or
orbiting particle, but to account for the magnetism of materials it is useful to
view the electron as a charged particle spinning as it orbits the nucleus. Every
electron, on account of its spin, is a small magnet. In most materials, the
countless electrons have randomly oriented spins, leaving no magnetic effect on
average. However, in a bar magnet many of the electron spins are aligned in the
same direction, so they act cooperatively, creating a net magnetic field. In
addition to the electron's intrinsic magnetic field, there is sometimes an
additional magnetic field that results from the electron's orbital motion around
the nucleus. This effect is analogous to how a current-carrying loop of wire
generates a magnetic field. Ordinarily, the motion of the electrons is such that
there is no average field from the material, but in certain conditions, the motion
can line up so as to produce a measurable total field as in iron.
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Origin of magnetic effects
Consider a charged particle A with its electric field surrounding it. Another
charged B placed in this electric field will experience an electric force as
described by Coulomb’s law. But what happens when the charge particle A
moves. About 1880 the famous English scientist James Clerke Maxell stated that
the charge B was not instantaneous aware that charge A had moved but the
change in the electric field due to A moving, spreads out from A at the speed of
light. The time required for the influence of the electric field to travel causes a
time lag. This time lag for the change to be travel or due to the fact that one
electric charge does not influence another instantaneously gives rise to the
magnetic force. Thus, the force between two moving becomes dependent upon
their speeds. At any instant of time one charge is feeling the electric field of
influence of the other, not where it is now, but where it was a short time before.
Hence, all magnetic effects are simply magnetic effects.
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Common uses of magnets
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Magnetic recording media: VHS tapes and audio cassettes contain a reel of
magnetic tape. The information that makes up the video and sound is encoded on
the magnetic coating on the tape. Computers, floppy and hard disks record data on
a thin magnetic coating.
Credit, debit, and ATM cards: have a magnetic strip on one side.
Common televisions and computer monitors: TV and computer screens containing
a cathode ray tube employ an electromagnet to guide electrons to the screen.
Speakers and microphones: Most speakers employ a permanent magnet and a
current-carrying coil to convert electric energy (the signal) into mechanical energy
(movement which creates the sound). The coil is wrapped around a bobbin
attached to the speaker cone, and carries the signal as changing current which
interacts with the field of the permanent magnet. The voice coil feels a magnetic
force and in response moves the cone and changes the pressure the neighbouring
air, thus generating sound. Dynamic microphones employ the same concept, but in
reverse. A microphone has a diaphragm or membrane attached to a coil of wire.
The coil rests inside a specially shaped magnet. When sound vibrates the
membrane, the coil is vibrated as well. As the coil moves through the magnetic
field, a voltage is induced across the coil. This voltage drives a current in the wire
that is characteristic of the original sound.
Medicine: Hospitals use Magnetic Resonance Imaging (MRI) to spot problems in a
patient's organs without invasive surgery.
Transformers: are devices that transfer electric energy between two windings of
wire that are electrically isolated but are coupled magnetically.
Compasses: is a magnetized pointer free to align itself with a magnetic field, most
commonly Earth's magnetic field.
Art: Vinyl magnet sheets may be attached to paintings, photographs, and other
ornamental articles, allowing them to be attached to refrigerators and other metal
surfaces.
Toys
Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are
either too small, too hard to reach, or too thin for fingers to hold. Some
screwdrivers are magnetized for this purpose. Magnets can be used in scrap and
salvage operations to separate magnetic metals (iron, steel, and nickel) from nonmagnetic metals (aluminium, non-ferrous alloys, etc.). The same idea can be used
in the so-called magnet test, in which an auto body is inspected with a magnet to
detect areas repaired using fiberglass or plastic putty.
Magnetic levitation transport, or maglev, is a form of transportation that suspends,
guides and propels trains through electromagnetic force. The maximum recorded
speed of a maglev train is 581 km.h-1.
Health: human tissues have a very low level of susceptibility to static magnetic
fields, there is little mainstream scientific evidence showing a health hazard
associated with exposure to static fields. Dynamic magnetic fields may be a
different issue however; correlations between electromagnetic radiation and
cancer rates have been postulated due to demographic correlations. If a
ferromagnetic foreign body is present in human tissue, an external magnetic field
interacting with it can pose a serious safety risk. A different type of indirect
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magnetic health risk exists involving pacemakers. If a pacemaker has been
embedded in a patient's chest (usually for the purpose of monitoring and
regulating the heart for steady electrically induced beats), care should be taken to
keep it away from magnetic fields or metal detectors at airports. It is for this
reason that a patient with the device installed cannot be tested with the use of an
MRI, which is a magnetic imaging device. Children sometimes swallow small
magnets from toys; and this can be hazardous if two or more magnets are
swallowed, as the magnets can pinch or puncture internal tissues; one death has
been reported.
Motors and generators
P6354 P6428 P6998
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