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Electric Induction
In 1820 Hans Christian Oersted found
that an electric current flowing through a wire
created a magnetic field. This was the first
discovery linking electricity and magnetism.
British scientist Michael Faraday believed that if
a current could induce a magnetic field, then a
magnetic field should, inversely, induce a
current.
The main idea behind electric
induction, as investigated by Faraday, is that an
electric current can be “induced” (generated) by
moving a magnet through a closed coil of wire.
Also, reversing the direction in which the magnet
is moved (thus reversing the magnetic field)
reverses the electric flow. The results were the
same whether the magnetic field moved past a
stationary conductor (coil), or whether the
conductor moved past a stationary magnetic
field.
Oersted
Michael Faraday
1791-1867 British
‘natural philosopher’
http://www.bookrags.com/sciences/sciencehistory/electric-induction-wsd.html
Electromagnetism
The main ideas
A) A current generates a
magnetic field
induced magnetic field
B) Changing magnetic
fields generate current
current
http://micro.magnet.fsu.edu/electromag/java/compass/index.html
http://micro.magnet.fsu.edu/electromag/java/faraday2/index.html
Electromagnetism
A question
Compass inside
a coil
Battery
Solid Iron ring
Inside two separate
coils
Switch
What would you expect to happen upon
closing the switch in this circuit?
For animation of circuit: http://micro.magnet.fsu.edu/electromag/java/faraday/
Electromagnetism
The solution
Coil 1
Coil 3
o When the switch is closed a current
passes through coil 1. That generates a
magnetic field in the coil.
o The iron ring, being ferromagnetic,
magnetizes, carrying the magnetic field
around it to coil 2. Now a change in
magnetic fields has occurred in coil 2.
Coil 2
Coil 2 was under the influence of the earth’s very weak magnetic field, but is now
influenced by the magnetic field generated by coil 1. The magnetic field increases
momentarily, then becomes steady. According to Faraday, a current must then be
generated through the wire in coil 2, but only while the magnetic field is changing.
Once the field is steady, no additional current flows in coil 2.
o Now, the same thing that happened in coil 1 is happening in coil 3. The
momentary current created in coil 3 creates a magnetic field that orients the needle
left-right. Once the current ceases, the compass returns to its original direction,
responding to earths field.
oYou will only see the compass change again when the switch is opened. The
magnetic field reduces, creating a momentary current in coil 2 but in the opposite
direction.
Magnetism
Electron orbitals:
An electron orbital:
• Is the region of space about the nucleus of an atom that an electron of
that atom has a finite probability of occurring.
• Is not analogous to planetary orbit! A simple view of the atom looks
similar to planetary bodies and you may have pictured the electrons as
orbiting around the nucleus, but to plot a path for something you need to
know exactly where the object is and be able to work out exactly where
it's going to be an instant later. You can't do this for electrons, so we say
that they occupy regions called orbitals.
• Is a region that has specific geometry based on the number of electrons
“belonging” to the atom. To see 3-d images of the shapes of electron
orbitals go to: http://winter.group.shef.ac.uk/orbitron/ and look
specifically at the 1s, 4s, and 4d orbitals.
Another useful website about atomic orbitals and the behavior of
electrons within them is :
http://www.chemguide.co.uk/atoms/properties/atomorbs.html
Magnetism
Magnetizable materials
On a macroscopic basis, we know that moving electric charges create a
magnetic field. This is true also on a quantum basis: moving electrons create a
magnetic field. Typically, we refer to an electron’s ’spin’ as creating the
magnetic field. However, the electrons do not spin, neither about their own
axes nor about the nucleus. To picture this, have a look at the typical orbitals of
electrons in a chemistry book. An electron can be on one side AND the other
side of a nucleus, but not in between. This weird characteristic arises from the
particle-wave duality of small particles (electrons, photons, neutrons).
Nonetheless, electrons create a magnetic field. Now, in materials where the
electrons are paired, each electron will possess equal and opposite magnetic
fields. Thus, many atoms are not inherently magnetic.
For iron, nickel, cobalt, and gadolinium, there are unpaired electrons in their dorbitals (remember s,p,d,f?). These ‘mini-magnets’ can be aligned to form a are
the four ferromagnetic elements http://en.wikipedia.org/wiki/Ferromagnetic) In
the presence of an external magnetic field, the electron spins will be influenced
and become the same. When the magnetic field is taken off, the electrons
continue spinning in that same direction because it requires energy to rerandomize them. Heating or dropping a magnet can re-randomize the electron
spins and thus cause demagnetization.
For a better understanding and an idea of what electron “spin” actually is go to:
http://www.wonderquest.com/physics-magnetism.htm It is useful!
Magnetizing materials
So what exactly happens when ferromagnetic materials become
‘magnetized’? Below is an example of the process of magnetization.
Imagine that the large rectangle is a piece of iron, and the smaller boxes
within it are its separate electron domains. The lines going through it
represent the magnetic field of the unmagnetized iron based on the
random alignment (i.e. magnetization) of the unpaired electrons within
those domains.
Magnetizing materials
Now the piece of iron has come into contact with a strong
permanent magnet. The electrons, being unpaired and thus having
random spins, are now lining up in accord with the magnet and are
aligning themselves in the same orientation throughout the rectangle.
Magnetizing materials
This image shows the magnetic field lines after complete
alignment of the electrons throughout the entire piece of iron. You can
see that it has a distinct north and south pole with a totally uninterrupted
magnetic field.
Electromagnetism
Eddy current
When a metal conductor (picture an iron metal ring or solid piece of copper foil)
is moved through a magnetic field, or a magnetic field is moved past a stationary metal
conductor, a current is generated in that conductor. We know this because changes in
magnetic fields generate current. This current travels in little loops called “eddies” and
are thus named eddy currents.
But what happens when a current flows through a continuous closed circuit?
According to Faraday a magnetic field must be generated, and in this case it will be
generated in the opposite direction of the magnetic field the conductor is passing through.
We all know that for every action there is an equal and opposite reaction, right? It’s the
same concept at work here: you expose the conductor to a force from a magnetic field and
so that conductor works to oppose that force. Using the right-hand rule we can determine
which direction the current must flow to generate an opposing magnetic field. Do not
think of this phenomenon as cause and effect, instead view the field and conductor as a
coupled system.
Remember the magnet falling much more slowly through a copper tube? This
was an example of eddy currents at work. The current generated by the magnet moving
through the copper pipe generated its own magnetic field to oppose it, causing the
magnet to travel slowly. The magnet traveling through an insulator (plastic pipe) does not
generate any current, and so drops as expected.
For a great illustration of what is happening here go to :
http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html
Magnetism
Diamagnetism: levitation
Quite simply, diamagnetism is the ability of an
object to repel magnetic fields completely.
Practically all matter on this earth is
diamagnetic, even you, but this property is only
exhibited in the presence of an externally
applied magnetic field. Diamagnetism is the
result of changes in the orbital motion of
electrons due to the application of a magnetic
field, which creates a magnetic force on moving
electrons. This force changes the centripetal
force on the electrons, causing them to either
speed up or slow down in their orbital motion.
This new speed modifies the magnetic moment
of the electron orbital in a direction against the
external field. The magnetic force of the floating
object is great enough that it can completely
oppose the force of gravity but small enough
that the magnetic force of the object below it can
suspend the object above itself. The result?
Levitation!
You can see more levitating objects here :
http://www.hfml.ru.nl/levitate.html
Levitating pyrolytic graphite
http://en.wikipedia.org/wiki/Diamagnetism