20 magnetism
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TRANSCRIPT
MAGNETISM
A Strangely Attractive Topic
HISTORY Term comes from the ancient Greek city of
Magnesia, at which many natural magnets were found.
We now refer to these natural magnets as lodestones (lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.
HISTORY Pliny the Elder (23-79 AD Roman) wrote of a
hill near the river Indus that was made entirely of a stone that attracted iron.
HISTORY Chinese as early as 121 AD knew that an iron
rod which had been brought near one of these natural magnets would acquire and retain the magnetic property, and that such a rod when suspended from a string would align itself in a north-south direction.
Use of magnets to aid in navigation can be traced back to at least the eleventh century.
HISTORY
Basically, we knew the phenomenon existed
and we learned useful applications for it.
We did not understand it.
Not until its connection to electrical charges and currents was discovered.
What is magnetism? Magnetism is the force
of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.
What causes magnetism? Atoms themselves have magnetic properties
due to the spin of the atom’s electrons. Groups of atoms join so that their magnetic
fields are all going in the same direction. These areas of atoms are called “domains”.
What causes magnetism? When an unmagnetized substance is placed in
a magnetic field, the substance can become magnetized.
This happens when the spinning electrons line up in the same direction.
What causes magnetism? An unmagnetized substance looks like this…
While a magnetized substance looks like this…
How to “break” a magnet? Drop it.
Heat it.
This causes the domains to become random again!
A big natural magnet.. It exerts magnetic
forces and is surrounded by a magnetic field that is strongest near the North and South magnetic poles.
Magnetic South Pole
Magnetic North Pole
Geographic North Pole
Geographic South Pole
PROPERTIES OF MAGNETS There are north poles and south poles. Magnets set up a magnetic field around it. Like poles repel, unlike poles attract. Magnetic forces attract only magnetic
materials. Magnetic forces act at a distance.
1. North and South Poles Every magnet has at least one north pole and
one south pole. If you take a bar magnet and break it into two
pieces, each piece will again have a north pole and a south pole.
No matter how small the pieces of the magnet become, each piece will have a north pole and a south pole.
1. North and South Poles, cont. It has not been shown to be possible to end up
with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).
2. Magnetic Fields Michael Faraday realized that a magnet has
a ‘magnetic field’ distributed throughout the surrounding space.
This field exerts a force on any charge/magnetic material on it.
2. Magnetic Fields, cont. Field lines converge where the magnetic force
is strong, and spread out where it is weak. In a compact bar magnet or dipole, field lines spread out from one pole and converge towards the other.
3. Like poles, unlike poles LAW of MAGNETISM: Like poles repel, unlike
poles attract.
4. Attract only magnetic materials Magnets only attract certain types of metals, other
materials such as glass, plastic and wood aren't attracted.
Metals such as iron, nickel and cobalt are attracted to magnets.
Most metals however are not attracted to magnets, these include copper, silver, gold, magnesium, platinum, aluminium and more. They may however magnetize a small amount while placed in a magnetic field.
Magnetism can attract magnetic objects or push them away.
5. Action at a Distance Although two
magnets may not be touching, they still interact through their magnetic fields.
This explains the ‘action at a distance’, say of a compass.
Electricity to MagnetismHans Oersted
first observed that a current in a wire affects a nearby compass needle
Implication: an electric current creates a magnetic field
Electromagnets- Magnets created using a current
Magnetic strength increases with: more current in the wire more coils of wire bigger ferromagnetic core
Uses of Electromagnetism1. Electric motors Converts electrical energy to mechanical
energy Anytime electricity is converted into a motion
its through an electric motor
Uses of Electromagnetism2. Current meters Galvanometer – used to measure small currents Ammeter – used to measure currents Voltmeter – used to measure voltage
Magnetism to ElectricityMichael Faraday
if an electric current can create a magnetic field, then maybe a magnetic field can create a electric current
this led to his Law of Electromagnetic Induction
Law of Electromagnetic Induction Any change in the magnetic field of a coil of
wire will cause a voltage to be induced in the coil, called the induced voltage.
If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.
How to increase induced voltage/current Increase number of coils Increase magnetic field Increase speed of relative motion between coil
and magnetic field
Uses of Electromagnetic Induction1. Generator Converts mechanical energy to electrical energy Used in most power plants to create electricity In coal, oil, natural gas, nuclear or geothermal
power plants, the heat is used to boil water into steam which is then used to rotate the turbines
In wind, hydroelectric (dam/falls) and tidal power plants, the turbines are rotated by the wind/water
The only sources of electricity that does not use generators are solar, chemical (batteries), piezoelectric (crystals) and thermoelectric (heat)
Uses of Electromagnetic Induction2. Transformer Used to increase or
decrease voltage/current Step-up or Step-down High voltage electricity is
used in long distance transmission
Low voltage is used in homes