Physics I - Waukesha South High School

fiftysixpowersElectronics - Devices

Oct 18, 2013 (4 years and 20 days ago)

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Physics I

Unit 2.6: Electromagnetism


Notes





Magnets




In Earlier times, Greeks and Etruscans found that a locally found rock, a
lodestone
, would always
point in the same direction.


This is the beginning of the study of magnetism.


Only a few natur
ally
occurring objects exhibit magnetic properties, but electrical currents are also affected

by magnetic fields,
which make

their study much more important.




The lodestone, now known as magnetite, is predominantly an iron alloy.


Other elements such
as
nickel and cobalt can also be magnetized.


Recently, many of the rare earth elements have been made
into magnets using a high temperature ceramic process.


The strongest of these are the ones made of
neodymium.




Some magnets are
permanent

magnets s
ince they do not lose their magnetic ability quickly, but
rather stay magnetized for a long time, often for years.


Other magnets are
temporary

magnets since
they lose their magnetic ability very quickly.


A refrigerator magnet would be a permanent, while
an
electromagnet would be a temporary one.


Materials which are easy to magnetize (such as iron) also lose
their ability soon, while those which are hard to magnetize (like nickel and cobalt) tend to hold on to it
much longer.





Magnetic polarity





Magnetic

materials possess a polarity; two distinct regions of opposite affect.


When suspended, a
magnetic object will rotate until one end will point toward the north pole of the earth..


If displaced, it
will return to this position.


This would be the
north pole

of the magnet and the opposite end would be
the
south pole
.




Poles of a magnet are similar to opposite electrical charges in their interactions: like poles repel and
unlike poles attract.


Unlike electrical charges, magnetic poles are never

found alone.


They always exist
in pairs.






Magnetic fields




Since magnetic forces are detected at a distance from the magnetic object their must be a magnetic
field.


Similar to electric fields
in shape and effect, magnetic fields can be drawn to show the intensity
and direction at any point around the source of the field.







.


Magnetic field lines

start and end on either magnetic poles

or infinity, they do not cross, and their density ind
icates

the strength of the field.


The direction of the field lines shows

the direction of the magnetic force on a north mono pole

(if one existed).











Electromagnetism





In the late 1700"s it was observed that magnetic compasses (small free

floating magnets) placed near
current carrying wires would be effected when the current was on, but returned to normal when the
current was off.


It wasn't until the mid 1800's that the connection between the two was formalized into
the science of electro
magnetism.


It is now well understood that moving charges create magnetic fields
and that changing magnetic fields cause forces on electrical charges





Electromagnetic fields




A moving charge creates a magnetic field which moves radially away from t
he path of the charge.


As
in electrical definitions, the direction of the magnetic field is defined by conventional current.


The
shape of the conductor has a great effect on the shape of the resulting magnetic field.




For a
single straight wire
, the

resulting field lines for a current in the wire look like concentric rings
around the wire.


There are no poles in this field, since the lines are circles and do not touch the wire.


Since the field gets weaker with distance from the wire, the lines shoul
d get farther apart.




The direction of the field can be determined by
applying a right hand rule; the thumb of the right
hand points in the direction of conventional current,
the fingers curl around the wire in the direction of
the magnetic field.





If the current conductor is wound into a coil, the individual magnetic fields of each of the loops of the
coil superimpose to create a stronger, more direct field.


Such a current carrying coil is called a
solenoid
.


The field for a solenoid has polar
ity, one end of the coil acts a
s

a north pole while the other
end becomes a south pole.


If a soft iron core is placed in the coil, the field becomes more concentrated
and stronger in the region of the iron.


Notice the similarity of the field for a soleno
id and for that of a
bar magnet.




The polarity of the coil can be determined
through the application of a second right hand
rule.


If the fingers of the right hand curl around
the coil in the same direction as conventional
current, the thumb points t
o the north pole of
the coil.








Magnetic effects on charged particles






If a charged particle sits in a magnetic field, nothing special happens.


If the particle is in motion,
however, it gets interesting.


When a charge moves through a magnetic

field, as a single particle or as a
current, there is a magnetic force on the particle perpendicular to its motion and also to the magnetic
field.




The magnitude of the force depends on three factors;
the size of the charge (q), the speed of the
particle (v), and the strength of the magnetic field (B).




This equation can be used to experimentally define the strength of a magnetic field.


If a one coulomb
charge moves through a magnetic field at one

m/s and experiences a force of one Newton, then the
strength of the field is one
Tesla
.


A Tesla is a large unit of field strength, with even the largest of
laboratory magnets reaching a few tens of Tesla.


Most everyday magnetic fields, like the earth's
or a
refrigerator magnet, are in the micro to nanoTesla range.






The direction of the magnetic force on a charged particle can be
determined using another right hand rule;


the fingers of the right hand point
in the direction of the magnetic field,
the thumb points in the direction of the
moving charge (current), and the force on the particle is perpendicularly
away from the palm.











Since the force is always perpendicular to the motion of the charge, a single
charge will experience a centr
ipetal force and travel in a circular path, or at least
part of one.


A device known as a mass spectrometer uses this principle to identify
the size and charge of unknown particles.




















Magnetic effects on current carrying co
nductor






If the moving charge is part of a current in a conductor then the con
d
uctor will
experience a force perpendicular to itself and the field.


The rules for the strength of
the force and its direction on the conductor are the same as for a single charge.









When two parallel current carrying conductors are close to one another, their respective magnetic
fields will superimpose and create a force between the conductors.


If the currents are in the
same

direction, the field between the conductors is wea
ker than the field outside which causes an unbalanced
force pulling the two together (attraction).


If the currents are in the
opposite

direction, then the inner
field is stronger and the conductors will be pushed apart (repulsion).









For many applications this causes little concern, but if you are working with power lines, or electrical
cords to appliances which use large currents the effect can be noticeable and must be considered.





Applic
ations of electromagnetic concepts





The use of electromagnetic factors in mass spectrometers and power lines has already been
mentioned.


A use closer to home would be their use in audio speakers.





The guts of a speaker includes a large permanen
t magnet with a
voice coil wrapped around its core.


The cone of the speaker (the part
we see) is attached to the coil.


As the current in the coil changes in
response to the sound originally recorded, the force on the coil changes
size and direction.


Thi
s makes the coil move forward and backward in
response to the force.


As the coil moves, so does the cone, which
pushes the air in accordance with the sound and electrical signals
involved.





Induced current




You have already seen that a moving char
ge in a magnetic field can experience a force.


The key
elements are; a moving charge (current), a magnetic field, and a force.


A current created the force.




Now, just turn that around: a wire forced through a magnetic field may have a current create
d in it.


No source required, just move a wire through the magnetic field.


Of course, as before, certain factors
determine the amount and direction of the current.




The speed, direction, and orientation of the movement of the conductor through the fi
eld determines
the aspects of the
induced current
. The maximum amount of current will be induced if the conductor
moves perpendicular to the field, is moved at a faster speed, and if there is more conductor present
(more wire).





The direction of the induced current is a little more complicated because once the charges are forced
to move (current) by the magnetic field they possess a magnetic field of their own.


These two fields (the
one creating th
e force and the one of the current) will superimpose to create a separate, new force on the
conductor.




If you think about it, the direction of this additional force can not be in the same direction as the


force
on the conductor in the first place.


If it was then a small nudge of the wire would create the current,
which would create a force on the wire in the same direction, which would make the wire move faster,
which would create more current, more force, etc.


It would be a run away situation.


So
mebody along
time ago would have started a wire, and it would continue to move faster ever since.




This situation compels the new force to be in the
opposite

direction of the force which creates the
motion.


Think of it as induced electrical friction.


The larger the induced current, the larger this
opposing force.


Many times it is referred to as back emf, or back current.




The formal statement of this phenomenon is
Lenz's law
;


The magnetic field of the induced current
opposes the change in the
applied magnetic field.


In other words, the direction of the induced current is
opposite to the current which would create the motion in the first place.





We usually think of wires moving in a stat
ionary magnetic field, but the
wire could be stationary and the field move past it.


The results would be
the same, but sometimes the direction of the currents and fields can be a
little challenging to
determine.















Self inductance

is a term
used to describe the creation of a back current in a conductor when the
current in a wire changes.


If the current is increasing then the induced current tend to keep the current
value from increasing as fast.


If the current is decreasing, then the induce
d current would tend to keep it
from decreasing as fast.


Since solenoids have a larger magnetic effect due to the interaction of one
section's magnetic field on another section, the self inductance of a solenoid is more pronounced than for
a single, strai
ght wire.





Applications of induction




There are many applications of induced currents and their magnetic fields.


One of the more common
ones is the recording of sound on magnetic tape (cassettes) or recording computer data on disks.


Both of
these

syste
ms are essentially the same.




A wire carrying a current which changes its magnitude and direction
according to the information it carries is wrapped around a piece of iron.


The
changing current induces a changing magnetic field in the iron.


T
he recording
medium is a thin film of iron particles embedded in plastic.


As they pass by the
"head" of the iron magnet the regions of the iron particles align themselves with
the magnetic field at that point in time.


As the pattern of current changes, t
he
pattern of the iron particles in the plastic changes accordingly.


You can see why
bringing a strong magnet close to these materials could ruin the data stored on them.




A device which utilizes the concept of
induction is the transformer.


A
trans
former consists of two separate coils
of wire wrapped around the
same piece of
iron.


Any change

in current in one of the
coils creates a changing magnetic field in
the iron.


Since the other coil is also
wrapped around the iron, any change in
the iron's m
agnetic field causes a change
in the current in that coil.




Notice that a transformer only transfers electrical energy between the two coils when the current
changes. If the current doesn't change in the first coil, there is no current in the second c
oil.


Transformers are usually only used in alternating current circuits, where the current is continually
oscillating back and forth.


Transformers provide a way of isolating one electrical circuit from another
since there is no electrical connection betw
een the two coils, but a magnetic link which connects the
two.


The amount of energy (voltage and current) which is transferred depends on the sizes of each of
the coils as well as their orientation on the iron core.




Generators





A
generator

is a
device which converts mechanical energy into electrical energy.


By moving a wire
through a magnetic field, you have seen that a current can be produced.


If this process can be done
repeatedly the current can be maintained.


In simplest terms a generator
consists of a coil of wire which
rotates in a magnetic field.


Permanent magnets are usually used to create the magnetic field, and the coil
is wrapped around an iron core to concentrate its field.


In the diagram below only a single loop of the
coil is sh
own for simplicity





As you can see from the diagram the magnitude and direction of the current changes as the loop turns
through the magnetic field.


The only parts of the loop which are involved in the
generation of current
are the sides which are perpendicular to the field (colored in yellow and blue).


The other sides are
parallel to the field and do not create current as they rotate.




In positions A and C the loop is creating the maximum current
since the wires are moving
perpendicular to the field.


In positions B and D there is no current generated since the wires are moving
parallel to the field.


Notice that the current, or EMF, follow
s

a sinusoidal curve.


Since the current
changes its magnit
ude and direction in a cyclic pattern it is referred to as an alternating current.





Characteristics of alternating current




An
alternating current

follows the same concepts as the waves you have studied in the past; they
have an amplitude and freq
uency.


The amplitude of an alternating current is referred to as its
maximum

value.


This is not that useful of a number since the maximum value only exists for a fraction of each
rotation.




A more useful value for EMF or current is the
effective val
ue
.


Consider a light bulb connected to
both a battery and a generator.


The bulb receives energy when the charges move through its filament,
no matter which direction they are traveling.


The power of the bulb at any point in time is the product of
the cu
rrent and voltage at that point in time (Watt's law).




Since the battery has a constant voltage and current, this calculation is simple to make and the result
remains constant.


For the generator, these values are changing so the calculation is not co
nstant.


This is
where the effective value of AC comes in.



If you want both bulbs to have the same brightness (power) then the effective value of the current, or
EMF, is 0.707 times the maximum value.


If the maximum voltage of a generator is 100 V, t
hen its
effective voltage would be 70.0 V.


This means that it would create the same brightness in a light bulb as
one connected to a 70.7 V DC source.




It is the effective voltage that is usually expres
sed for AC.


American household voltage is around 110
Vac.


It is the effective voltage which is 110 V, the maximum voltage would be around 164 V, and the
voltage swing during one cycle of the AC would be close to 328 V.


That's one reason being shocked by

household current is so dangerous.





Motors





The easiest way to
think of

motors
is

that they are generators run in reverse.


Since a generator
converts mechanical energy into electrical energy, a motor changes electrical energy into mechanical
ene
rgy.


The parts of a motor can be identical to the parts of a generator, but instead of turning the loop
to induce a current, current is sent through the loop to make it turn.




A motor involves a turni
ng coil in a magnetic field.


This was the condition that created current in the
generator.


That means that a motor also generates current while it is running.


Remember, according to
Lenz's law, this generated current is opposite to the direction which i
s making the motor turn.


Because
of friction and lack of involvement of the entire magnetic field, the back current is only a fraction of the
motor's current.




When a motor first starts to turn there is almost no back current since the coil is barely

turning.


This
means that the motor is drawing the maximum current from the source. As the motor increases the speed
of the coil, the back current increases until it reaches a steady value.


At this point the motor draws the
least current from the source.





You may have noticed this when a large motor is started in your home.


For a brief time as the motor
is starti
ng, the lights in the house dim
, but come back to normal when the motor is running at full speed.





This situation is also why it is mo
re economically responsible to leave a motor running rather than
turn it on and off.


Also, when a load is attached to the shaft of the motor, the speed of the coil is
reduced, this reduction in speed reduces the back current, which increases the current i
n the coil, and
gives the motor more power to maintain its speed as the load is applied.


Pretty neat, huh?