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Magnetism
What is a magnet?
• Any material that has the property of attracting Iron (or steel), Nickel or Cobalt
• Magnets exert a force on other magnets or particles with an electrical charge
• Magnets may be temporary or permanent • Electrons in a magnet’s atoms are arranged so that the
atoms are “lopsided” with one “side” being more negative than the other
• The sides are poles • Traditionally, because the poles are opposite, we call
them “North” and “South”
Magnetizing a substance
• Atoms with similar electron properties are usually found within domains in a material made of Iron, Nickel or Cobalt
• Applying an electrical current through the material can polarize the domains
• A material with polarized domains is a magnet
Early magnets
• Natural ores called magnetites contain Iron and are magnetic
• These are also called lodestones and are what the first compasses were made from
• Magnetites can be used to magnetize other iron-based substances
Domains and magnetism
• The domains can “sense” each other, and in a permanent magnet will resist going back to an unordered state
• The space around a magnetized object where the strength of the magnet can be detected is its magnetic field
• The diagram shows a magnet and its field, which is represented by lines of force
Recall: Force is any influence that causes the motion of an object to change.
Magnetic lines of force
• Magnetic lines of force are imaginary lines that indicate where the magnetic field exists and how it is orientated
• Demo – the magnetic field can be detected – and mapped – using a compass!
• Magnetic fields are much stronger at the poles of magnets
A magnet exerts a FORCE on the compass needle. We know there
was a force exerted because the needle
moved!
Final thoughts – magnetic lines of force
• Lines of force “flow” the same way in every magnet – from north to south
• Lines of force are parallel and repel other lines of force if they are travelling in the same direction
Pushing these together would
mean forcing the lines to intersect, so
they repel
Pushing these together would add the number of lines force
passing through the poles, which results in attraction
What do you get when you cut a magnet in half?
• Two magnets – each will still have a north pole and a south pole
• You could reduce a magnetic material down to individual atoms – and each of which would still be a very tiny magnet that would still have a north and south pole
• This is because the magnetic field comes from the motion of the material’s electrons
This shows why electrons can be
paired – if they are paired their
magnetic fields cancel out!
Elements that can magnetized are those that have
unpaired electrons
Where do we see magnetic lines of force?
Iron filings sprinkled around a bar magnet
Earth’s magnetic field
The Earth’s core is molten Iron and Nickel, and is
spinning inside the earth. This creates the same
effect as a very large and very powerful magnet,
the field extends far out into space
Which is necessary because of …
Sometimes a prominence detaches – and heads towards us!
But is safely deflected away by our planet’s magnetic field!
So all we see is this…
Aurora Borealis – the “Northern Lights”
The view from space
What would Earth look like without a magnetic field?
• Mars
• No magnetic field, so the solar wind has blasted away much of its atmosphere, and the surface of the planet is continually sterlized.
Earth and Mars (not to scale)
Earth is protected by its magnetic
field
Mars has no magnetic
field, so no protection
Magnetism versus electrical charge
• Electric charges and magnetism similar – Just as the positive (+) and negative (−) electrical charges
attract each other, the N and S poles of a magnet attract each other.
– In electricity like charges repel, and in magnetism like poles repel.
• Electric charges and magnetism different – The magnetic field is a dipole field. That means that every
magnet must have two poles. – On the other hand, a positive (+) or negative (−) electrical
charge can stand alone. Electrical charges are called monopoles, since they can exist without the opposite charge.
Bottom line – magnetism:
• Magnetism is a force that acts at a distance • The magnetic force strongly attracts the opposite pole
of another magnet – but strongly repels like poles of other magnets
• Magnetic fields come from the motions of electrons - and electrons are always moving (even at absolute zero!)
• What do we know to call the movement of lots of electrons moving together?
• So if we have electricity, what must we therefore also have?
• Magnetic fields
Got it – or naw?
Induced EMF, AC current and electromagnetism
Big words!
Demo – electricity and magnetism
• Not “discovered” but demonstrated by André-Marie Ampère in 1820, a DC circuit turned on will cause a compass’s needle to (momentarily) deflect
Demo – magnetism and electricity
• In 1831, Michael Faraday demonstrated that moving a magnet (a magnetic field) through a coil of wire in a closed circuit induced a current
• Detected with an Galvanometer
• (think Galvanic cell, which is the same thing as a voltaic cell)
• A Galvanometer measures current in a DC circuit
Induced EMF
• EMF is electromotive force, which is any force that causes electrons to move
• Faraday showed that because a magnet moving through a coil could induce a current, it would be possible to generate electricity without use of batteries
Faraday’s Law
• Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil
Alternating current
• Because the electron current moving through the coil depends on the direction the magnet moves in, a magnet moving moving back and forth through a coil will cause the electron flow to oscillate back and forth
DC versus AC circuit
An Alternating Current generator
A simple induction generator
A bicycle dynamo
Electromagnetism
• James Clerk Maxwell summarized electricity and magnetism as being not different things, but different aspects of the same force – A moving magnetic field can push electrons – Moving electrons create a magnetic field
• So a magnetic field is simply what you get when you have electrons!
• We call the combined effects of moving electrons and magnetic fields electromagnetism, which is considered one of the four fundamental forces of nature
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