Lecture 10 Magnetism

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Lecture 10Magnetism
Chapter 5.12 ? 5.20
Outline

• Magnets
• Magnetic Field
• The Electromagnetic Field
• Electromagnetic Induction

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Magnets
The simplest magnet is a somehow magnetized bar
of iron. It attracts and holds other pieces of
iron. Most of the force a magnet exerts from its
ends.
A magnets ends point north and south. The
north-pointing end is called the north pole. The
south-pointing end is called the south pole.
Like magnet poles repel one another, unlike poles
attract one another.
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Magnetic Poles
Positive and negative charges in neutral matter
can be separated from each other. The north and
south magnetic poles cannot be separated.
Every time we saw a magnet in half, it gets the 2
poles.
Magnetic properties have all substances, but to a
different extent. Iron is one of the best magnets.
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Magnetic Field
The presence of a magnet alters properties of
space near it.
The altered space is called a force field. We
cannot see a force field, but can detect its
presence by its effects. The form of the magnetic
field can be traced by a pattern of iron
filings. They line up in the direction in which a
piece of iron would move if put there (field
lines). They gather most thickly where the force
on the iron would be the greatest (larger field
line density).
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Origin of the Magnetic Field
Every electric current has a magnetic field
around it. This was first shown by Oersted in
1820.
The current and the field are perpendicular to
each other. The direction of the magnetic field
can be found by encircling the wire with the
fingers of the right hand. The direction of the
current along the wire is then shown by the thumb.
All magnetic fields originate from moving
electric charges.
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The Electromagnetic Field
An electric charge at rest is surrounded by only
an electric field. When it is moving, a magnetic
field around it appears.
The relative motion between the charge and
observer is needed to produce a magnetic field.
Both an electric and magnetic field are aspects
of a single electromagnetic field that surrounds
every electric charge.
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Electromagnetic Induction
If an electric current produces a magnetic field,
then a magnet should be able to generate an
electric current.
A current is produced in a wire when there is
relative motion between the wire and a magnetic
field.
Such a current is called an induced current. The
effect is called electromagnetic induction
(discovered by Michael Faraday in the XIX
century). The strength of the current depends on
the magnetic field strength and the wires speed.
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Summary

• Gravity, electric, and magnetic forces alter
  properties of the surrounding space.
• These properties are called force fields.

• Electromagnetic forces is one of the 4
  fundamental forces which exist in the Universe.

• A single electromagnetic field surrounds every
  moving electric charge.