Transcript Magnetism - TeacherWeb

Magnetism
Intro to magnetism and
Electromagnetism.
“the basic laws governing
magnetism and how they relate to
electricity”
Objectives:
At the completion of this unit you will be able to:
• Understand the properties of permanent magnets
• Utilize terms used to describe magnetism and
magnetic quantities
• Understand the operation of electromagnets
• Determine the polarity of an electromagnet
Introduction to Magnetism
• One of the most important phenomena in
the study of electricity
• The force of magnetism has been known
for over 2000 years (Greeks)
• Stones with magnetic qualities (magnetite)
were first found in Magnesia in Asia Minor
• In the Dark Ages magnetism was thought
to be caused by evil spirits
The Earth is a Magnet
• Geographic poles
vs. magnetic poles
• lodestones
• Angle of declination
THE FIRST COMPASS
Permanent Magnets:
Magnets that do not require any power or
force to maintain their field
One of the basic laws of
magnetism states:
“Energy is required to create a
magnetic field, but no energy is
required to maintain a magnetic field”
The Electron Theory of
Magnetism
What makes materials such as iron, nickel, or cobalt,
a natural magnetic material
• Electron spin theory
• Electrons become tiny
permanent magnets
• Electrons in most materials
spin in opposite directions
• Once “paired” they tend to
cancel out any magnetic
properties
The Electron Theory of
Magnetism
What makes materials like iron different than
most materials:
• An Atom of iron contains 26 electrons
• 22 of the electrons are “paired” and cancel
each other out
• The remaining 4 electrons located in the next
to the outermost shell do not become “paired”
and spin in the same direction
• These 4 electrons account for the magnetic
properties of iron
The Electron Theory of
Magnetism
What makes materials like iron different than
most materials:
• As atoms combine to form molecules
• They arrange themselves to form a total of 8 valence
electrons
• In most materials the electrons cancel each other out
• In materials such as iron, the magnetic fields “add”
rather than cancel
• This “additive” effect forms regions in the molecular
structure of the metal called: Magnetic Domains or
Magnetic Molecules
Magnetic Materials
The three basic classifications:
• Ferromagnetic - metals that are easily
magnetized; iron, nickel, cobalt, manganese
• Paramagnetic - metals that can be magnetized but
not as easily as ferromagnetic;
platinum,titanium,and chromium
• Diamagnetic - metal or non-metallic materials that
cannot be magnetized; copper,brass, and antimony
Note: Alloys often make the best permanent magnet materials.
Combinations of such metals as aluminum, nickel, cobalt, copper, and
iron (Alnico 5) are commonly used in the production of permanent
magnets
Magnetic Lines of Force
• Magnetic lines of force are
called “flux”
• Magnetic “lines of flux” repel
each other & never cross
• Lines of flux do not “flow” but
it is assumed that they run
North to South
One of the basic laws of magnetism:
Unlike poles attract - Like poles repel
Electromagnetics
A basic law of physics: Whenever an electric
current flows through a conductor, a magnetic
field is formed around the conductor
• Electromagnets depend on electrical
current flow
• They do not retain their magnetic field
once current flow stops
Electromagnetics
Terms to remember:
• Permeability - a measure of a material’s
willingness to become magnetized
• Reluctance - a material’s resistance to
magnetism
• Saturation - the point when an increase in
current fails to significantly increase the
magnetic field
• Residual Magnetism - the amount of magnetism
left once current flow has stopped
Electromagnetics
Factors that effect electromagnetic fields:
• Winding the conductor into a coil increases the
strength of the magnetic field
• A coil with 10 turns of wire will increase the
magnetic field around the conductor by 10
• The strength of the magnetic field is proportional to
the amount of current flowing through the
conductor
“Ampere-Turns”
is determined by multiplying
the number of turns of wire
by the current flow
Electromagnetics
Core Materials: magnetic/non-magnetic
materials around which the conductor is coiled
Two Types:
• “Air core” electromagnets
>Core is made of Non-magnetic
material
• “Iron core” electromagnets
>Made of magnetic materials
>Greatly increases strength of the
magnetic field by increasing the
number of flux lines
Magnetic Polarity
Determining the Polarity of an Electromagnet
• The “Right-Hand Rule”
Magnetic Devices
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Electromagnets
Measuring instruments
Inductors
Transformers
Motors
Generators;Alternators
A Speaker uses both a permanent
magnet and an electromagnet
Summary
• Energy is required to create a magnetic field, but
no energy is required to maintain a magnetic field
• Unlike poles attract, like poles repel
• Whenever current flows through a conductor a
magnetic field is formed around the conductor
• The direction of current flow through a conductor
determines the polarity of the magnetic field