Transcript Magnetism

Magnetism
• Magnetic Poles: A magnet is a material that
creates a magnetic field. All magnets have two
ends, a north and south pole.
• Magnetic fields cause certain types of material to
become “magnetically polarized”
• A magnetically polarized material is a material
where the “domains” inside have a magnetic
“dipole moment,” that is held rigidly in place.
Groups of molecules inside the substance are like
little magnets, each with a north and south pole.
and when the material becomes polarized, these
line up together, producing a larger, stronger
magnet.
Magnetic Materials
• Ferromagnetic materials have a large and
positive susceptibility to an external
magnetic field. They exhibit a strong
attraction to magnetic fields and are able to
retain their magnetic properties after the
external field has been removed.
Ferromagnetic materials have some
unpaired electrons so their atoms have a net
magnetic moment. They get their strong
magnetic properties due to the presence of
magnetic domains.
• Permanent magnets are all ferromagnetic
materials
Paramagnetic Materials
• Paramagnetic metals have a small and
positive susceptibility to magnetic fields.
These materials are slightly attracted by a
magnetic field and the material does not
retain the magnetic properties when the
external field is removed. Paramagnetic
properties are due to the presence of some
unpaired electrons and from the realignment
of the electron orbits caused by the external
magnetic field. Paramagnetic materials
include magnesium, molybdenum, lithium,
and tantalum.
Diamagnetic Materials
• Diamagnetic metals have a very weak and
negative susceptibility to magnetic fields.
Diamagnetic materials are slightly repelled by a
magnetic field and the material does not retain the
magnetic properties when the external field is
removed. Diamagnetic materials are solids with all
paired electron and, therefore, no permanent net
magnetic moment per atom. Diamagnetic
properties arise from the realignment of the
electron orbits under the influence of an external
magnetic field. Most elements in the periodic
table, including copper, silver, and gold, are
diamagnetic.
Magnetic Fields of Magnets
• Like electric fields, magnetic fields are
indicative of lines of force. If you were to
sprinkle iron filing around a magnet, the
shape the filings would fall into naturally
show the lines of the magnetic field.
• Also like electric field, magnetic field lines
outside the magnet always have a defined
direction: from the north pole of the magnet
to the south pole.
• Inside the magnet the field lines run from
the South pole to the North Pole.
Electromagnetism
• Currents produce magnetic fields.
• A current carrying wire produces a
magnetic field around the entire length of
the wire. You can observe this phenomena
with a compass.
• Compasses employ a strip of ferromagnetic
material that aligns itself with the magnetic
field of the planet. If a compass is placed
near a wire carrying a current, the compass
needle will deflect and align with the
magnetic field.
Magnetic Dipoles
• All magnetic sources act as a dipole, that is a
North and South pole. The simple reason for this
is that B-field lines are continuous inside and
outside any magnetic structure.
– NS outside, SN inside
• There is no such thing as a magnetic monopole.
When a bar magnet is split in two, the result is two
smaller bar magnets.
• Even when magnetic materials are separated at the
microscopic level, a dipole is formed. It is
impossible to create a magnetic monopole.
B-Fields
• Magnetic Field: A force field created by the
presence of a magnetic material or any moving
charge such as a current. Magnetic fields create
forces on charged particles and objects and induce
currents in conductive materials.
• Symbol: B The magnetic field is a vector. It
always has a direction.
• Unit: Tesla (T)
Current Carrying Wires
and the Right Hand Rule (RHR)
REMEMBER: The B-Field inside the magnet flows from the South
Pole to the North Pole.
Magnetic Field of Coil
(Solenoid)
What is the direction of current in this solenoid?
Electromagnets
• An electromagnet is a solenoid wrapped
around a ferromagnetic (usually iron) core.
The core has the effect of amplifying the Bfield of the solenoid further, producing a
stronger magnet with same current.
Why does this “amplifying effect” occur?
The intensity of a B-Field due to
currents
• Geometry and matter can both affect the
strength of a B-field, but in general:
i
B
r
• For a very long, straight wire:
oi
B
2r
Magnetic Forces
Force on a single charge
• The force on a positively charged particle (test
charge) in a magnetic field (B) is given by the
equation:
F  qv  B
Where q is the quantity of charge, v is the velocity of
the charge and B is the magnitude of the magnetic
field. The x is the operation “cross product,”
basically a multiplication operation for vectors
where direction is taken into account.
For circumstances where the B-Field is at a right
angle to the moving charge (current) this equation
can be re-written as:
F  qvB
Force on a current carrying wire
• In the case of a wire carrying a current, the
force on the wire by an external magnetic
field is given by the equation
F  iL  B
Where i is the current in the wire, L is the
length of the wire and B is the magnetic
field. As with the prior example, when the
length of wire in question is perpendicular
to the magnetic field. The equation
becomes:
F  iLB
Why do we care?
• Electric motors work as a result of this
principle.
• Electric generators employ the reverse of
this principle (electromagnetic induction),
which we will study later.
• The use of electric current to produce a
force and the reverse principle, the use of a
mechanical force to produce an electric
current, are foundational to modern
technology.
– Examples?
Representing a B-field
• Since current carrying wires create a
magnetic field that wraps around the wire,
perpendicular to every point on the surface,
we need a way to draw B-fields near wires
since we are restricted to a 2-D page.
Questions
• 1) A wire carries a current of 4.0 amps. The wire
is at right angles to a uniform magnetic field and
0.50 meters of the wire is in the field. The force
on the wire is 0.62 Newtons. What is the strength
of the magnetic field?
• 2) A high-speed electron travels at right angles to
a magnetic field that has an induction of 0.620
Tesla. The electron is traveling at 3.46 x 107 m/s.
What is the force acting on the electron?
Faraday’s Discovery
• We have worked with the idea that currents
always produce magnetic fields.
• Now, we have already used this principle in
our discussion of inductors, but we will
expand now upon the reverse of the above
principle.
• Faraday discovered that magnetic fields
create induced currents.
In a nutshell:
When a conductor is either moved through
a B-field, or a conductor is already inside
a B-field when that field begins changing,
there is an induced electric field inside
the conductor that produces a current.
This current will produce a magnetic field
to offset the change in the external field.
Electromagnetic induction
• Connect a wire to both terminals of an
ammeter. Now take a horseshoe magnet (or
two opposite facing bar magnets) and move
a wire vertically in between the poles. The
needle in the ammeter will jump when the
wire enters the strong B-field.
• As we discussed earlier, there will also be a
resulting force on the wire.
Direction of current
• Use the right hand rule to determine the
direction of the induced current in the wire.
– Put your fingers in the direction of the B-field
and your thumb in the direction of the motion
of the wire.
– The direction of your palm (imagine an arrow
straight out, perpendicular from the palm of
your hand) gives the direction of the force on
the wire from the B-field.
– Recall that F=iLB, current in the wire is in the
same direction as the force.
Electromotive force
• When you pass a wire of velocity v through a
magnetic field, the B-field produces a current in
the wire. But we know that in order for current to
flow we must have a potential difference or
voltage between the ends of the wire. The B-field
actually creates this voltage.
• In the case where a current is induced in a wire by
a B-field, the voltage create is referred to as an
electromotive force.
• You might recall that we referred to a voltage
source in a circuit as providing the “push” that
causes electrons to flow in the wires. The two
terms, voltage and Emf, are synonymous.
Faraday/Lenz Equation
The Physics I Version
• EMF = BLv
– B is the magnitude of the magnetic field, L is
the length of the wire in the field and v is the
velocity of the wire as it passes through the
field.
– The above equation is only true when the wire
moves through the field perpendicular to the
field. If it moves through at an angle, on the
component of the wires velocity perpendicular
to the field is involved in generating the EMF.
Applications of EMF
•
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Inductors
Speakers/Microphones
Electric Generators*
Electric Motors
Transformers
*Most EMF generators (like the kind that produce the electric
power in your home) produce an Alternating Current.
Faraday’s (Lenz’s) Law
• Any change in the magnetic environment of a
coil of wire will cause a voltage (emf) to be
"induced" in the coil. No matter how the
change is produced, the voltage will be
generated.
• As a result of this induced voltage (emf:
electro-motive force), there is an induced
current in the coil producing a B-field to offset
any change in the B-field already present.
• Faraday’s Law is written mathematically as:
emfinduced

 N
t
The negative sign actually comes from
Lenz’s Law, which postulates that the
current produced resists the change in
magnetic flux.
Galvanometers and induced
current.
• Changing the magnetic field around a wire,
especially a coiled wire produces a counter current
in the wire or coil. This current is in the reverse
direction of the forced current flow in the wire and
occurs naturally as nature’s offsetting response to
a change in magnetic flux (the amount of magnetic
field flowing around the wire).
• A galvanometer is a special type of ammeter
designed to measure this “induced” or changing
current.
• Induced currents can be created even in a coiled
wire that is not part of a larger circuit.
Loudspeakers
• An application of the magnetic force on a
current carrying wire is a loudspeaker. In
order to understand how a loudspeaker
works, one must first examine a device
known as an inductor.
• A microphone is the reverse of a speaker.
Inductors and Inductance
• An inductor is a solenoid inserted into an
electric circuit designed to slow the growth
of current flowing in the circuit by
producing a reverse current. This occurs
according to Faraday’s (Lenz’s) Law.
• http://www.ngsir.netfirms.com/englishhtm/
RL.htm
• Notice the reverse current that slows the
build of forward current in the circuit when
the switch is closed.
Switch is open, circuit is off
Switch is closed
(current flows from battery)
Switch closed a long time…
So when an inductor is in place, the amount of
current flowing in the circuit depends on time!
In this case, after the circuit has been operating for
a long time, the current has stabilized at a maximum
and the inductor simply functions as a wire, or short circuit.
Speakers
• A speaker is a special type of inductor.
• A sound wave can be stored as an electrical signal.
Basically the pattern of increasing and decreasing
sound waves is saved as a corresponding
increasing and decreasing voltage/current. This
electric signal is amplified many times and sent to
the speaker. As the current to the speaker changes,
the current in the speaker/inductor changes. There
is a corresponding change in the B-field. This
change in B-field produces a force which causes
the inductor to vibrate against the cone of the
speaker. This vibration is picked up by the air
around the speaker and eventually by your ears.
What it looks like…
http://electronics.howstuffworks.com/speaker6.htm
Speakers and Microphones
• A speaker produces mechanical vibrations (sound)
when the changing current in an inductor produces
a variable force on the wire. Thus, a speaker
converts an electrical signal (waveform) to a
mechanical wave.
• A microphone converts a mechanical wave, like
sound, to an electrical signal.
– That signal can be stored, or saved, in many forms as an
electronic media file, either in computer memory (on a
re-writable “hard disk” or RAM), or on some sort of
physical data storage device (magnetic tape like floppy
disks, cassettes, VCR tapes, or other device like vinyl
records, CD’s, DVD’s etc…)
– All these devices are a medium for saving a sample of,
or for reproducing an electric signal that can be
converted into sound.
Electric Motors
• We know that currents in B-field experience
a force due to the magnetic field’s effect on
the moving charges in the current.
What will be the
direction of current in
the loop?
Recall:
EMF = BLv
So, when v=0, EMF=0
As the motor turns, a switching
circuit can be used to keep the
direction of current flow
constant relative to the space.
Start the motor. v  0,
EMF = #, current flows
in the loop.
If we have current, we
have a force on the wire.
Fon a wire = iLB
Generators
• An electric generator is a device that
provides a voltage source by means of an
induced EMF.
• In a simple generator, a loop of wire turns in
a magnetic field. The turning is usually
provided by some mechanical process
(steam rising, or water falling). The B-field
induces a current in the wire that can be
used in a circuit to do work.
EMF = BLv
In a generator, v is maximized perpendicular to the field when
the sides of the loop are in line with the source of the magnetic
field. Since B and L would both be constant, this condition
would maximize the output EMF and the output current.
Current and voltage output both follow a sine wave.
Improvements
• Electric motors and generators are both
applications of the same physical principles.
• We use generators to produce a strong EMF
or voltage that we can harness for
electricity. We use motors to do mechanical
work. The induced EMF in a motor is an
added bonus. The alternator in your car is
basically a generator driven by you’re the
turning of your car’s motor.*
*The motor in your car is actually a thermodynamic
“heat engine,” using the heat from combustion to do
mechanical work. This is not the same as an electric motor!
Although, the end result (mechanical work) is the same in both cases.
Questions:
• Are there ways that we can increase the
output of motors and generators?
• What variables can we change to increase
the output?
– Hint: Think about physical changes we can
make to the systems.
Direct Current
• Batteries produce a steady potential difference
(high vs. ground) and with the exception of time
dependent components in a circuit (like capacitors
and inductors), the current flow in a circuit
powered by a battery is constant. We call this
Direct Current or DC.
• Generators can be designed to produce a direct
current if a switch is employed (like a motor)
Alternating Current
• We have already seen how an “alternating
current” or AC generator works. An electric
voltage source that is oscillating produces
current that varies with time.
• The type of current produced by an AC
generator is analogous to creating a simple
DC circuit with a battery, then flipping the
battery over again and again. Sort of…
• Basically, this type of system produces current that
flows in one direction for a short amount of time
and then reverses to flow in the opposite direction.
Since most appliances either don’t “care” or are
designed to operate in this type of environment,
everybody wins!
• In the U.S. and many other countries, electric
generators are standardized to produce an average
frequency at 60 Hz, that is to say they reverse 60
times each second.
• Likewise, the voltage pumped into your home is
standardized between 110V and 120V. Most of
your appliances (except your dryer) are designed
to operate on an AC power supply of 120V at
60Hz.
AC vs. DC
• In the late 1800’s (c. 1880) the race was on that would
determine the course of electric power generation to the
present day.
• The vision of Edison’s General Electric company was to
provide DC electric power to consumers in cities. His idea
was that each city block would have its own power station.
• In 1893 at the Chicago World’s Fair, N.Tesla powered (lit)
the entire fair with electricity transmitted all the way from
his hydroelectric plant on Niagara Falls.
• Edison fought back with a public campaign of
misinformation, going so far as to blow up cattle in a
display of the “danger of alternating current.” Of course,
you could achieve the same result with DC current too, but
Edison didn’t tell anybody that.
AC Advantages
• Since you are only pushing current in any one direction
for a very short amount of time (1/60th of a second) AC
power generation is highly advantageous as it allows you
to provide a very high voltage, low current signal that can
be transmitted, hundreds, or potentially, thousands of
miles.
• The single disadvantage to AC power is that circuits must
be designed to operate in an environment where the
voltage was constantly changing. In the modern era, this
disadvantage is no longer a large problem since, while
AC circuits are slightly more complex than a DC circuit
designed for the same function, we now have 100+ years
of experience behind us in designing circuits.
• In addition, we now have portable
transformers (those heavy little black boxes
that some of your appliances use as plugs)
that will convert a 120V, 60 Hz AC signal to
a DC signal for use in some appliances.
Transformers
• A transformer is a device that uses B-fields
and induced currents to create and exchange
between current and voltage.
• For example, let’s say you have a 120V
input line but only require 6V to operate a
certain appliance. If you were to hook up
the appliance directly to the 120V source,
what do you suppose would happen?
• Therefore, we need something to “stepdown” the voltage of our input line before it
gets to the device.
• Transformers are analogous to a system of locks
on a river or canal. They allow you to step down
the input voltage, but with a trade off. As the
voltage is decreased, the current output of the
transformer increases.
• A transformer is basically a core made of soft iron
formed into a hollow square. On one side of the
core the input wire comes in and wraps around the
core forming a solenoid. When current flows, a
magnetic field is created in the iron core. This Bfield induces a current in the output wire, which is
formed into a solenoid around the other side.
“Step up” vs. “Step down”
• Some transformers increase voltage at the
expense of current. This is a step up
transformer. Step-up transformers have
fewer coils in the input solenoid and more
coils on the output side of the core.
• A step down transformer (like those at
power switching stations, the grey cylinders
on phone poles, the green boxes in yards
and the little black “plug boxes” you use at
home sometimes) increases current at the
expense of a decreased voltage. These
transformers have many more coils on the
input side of the core.
Also see This Link
http://www.osha.gov/SLTC/etools/electric_power/
illustrated_glossary/substation_equipment/
power_transformers.html
Questions:
•
A wire 20.0 m long moves at 4.0 m/s perpendicularly
through a magnetic field. An EMF of 40 V is induced
in the wire. What is the strength of the magnetic
field?
•
A straight wire, 0.75 m long moves upward through a
horizontal 0.30T magnetic field at a speed of 16 m/s.
What is the EMF induced in the wire?
•
A hair dryer uses 10 A at 120 V. It is used with a
transformer in England where the line voltage is 240
V.
A) What should be the ratio of the number of turns in
the transformer?
B) What current will the hair dryer now draw?