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ELIZABETHTOWN COMMUNITY AND TECHNICAL COLLEGE
BEX 100 BASIC ELECTRICITY FOR NON MAJORS
LECTURE GUIDE – UNIT 4
Instructor: Jerry Brown
Reference: Delmar’s Standard Textbook of Electricity, Third Edition
MAGNETISM – basic laws governing magnetism and how they relate to electricity
OBJECTIVES:
At the completion of the lesson, the student should be able to:
1. Understand the properties of permanent magnets
2. Understand the operation of electromagnets
3. Determine the polarity of an electromagnet
4. Define terms used to describe magnetism and magnetic quantities
INTRODUCTION
Magnetism is
electricity
one
of
the
most
important
phenomena
in
the
study
of
Magnetic force has been known for over 2000 years (the Greeks again!)
Stones with magnetic qualities (magnetite) were first found in Magnesia
(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 magnetic field.
A Basic Law of Magnetism – Energy is required to create a magnetic field, but no
energy is required to maintain a magnetic field.
Electron theory of magnetism – what makes materials such as iron, nickel and
cobalt a natural magnetic material
Electron spin theory
Electrons become tiny permanent magnets
Pairs of electrons in most materials spin in opposite directions
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Once paired, they tend to cancel out any magnetic forces
What makes materials like iron different
An atom of iron has 26 electrons
22 of the 26 are paired and cancel out each other’s magnetic force
There are 4 electrons in the next to the outer shell that do not become
paired and spin in the same direction
These 4 electrons account for the magnetic properties of iron
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’s magnetic fields
In materials such as iron, the magnetic fields add instead of canceling
This additive effect forms regions in the molecular structure of the metal
called “Magnetic Domains” or “Magnetic Molecules”
Nuisance static charges
Ferromagnetic – metals that are easily magnetized, such as iron, nickel,
cobalt and manganese
Paramagnetic – metals that can be magnetized, but not as easily as
ferromagnetic, such as platinum, titanium and chromium
Diamagnetic – metal or non-metallic materials that cannot be magnetized,
such as copper, brass and antimony
Magnetic lines of force
-
Magnetic lines of force are called “flux”
-
Magnetic “lines of flux” repel each other and never cross
-
Lines of flux do not flow, but it is assumed that they run from north
to south
Another basic Law 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 that conductor.
Electromagnets depend on current flow
They DO NOT retain their magnetic field once the current stops
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 a factor of 10
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ELIZABETHTOWN COMMUNITY AND TECHNICAL COLLEGE
BEX 100 BASIC ELECTRICITY FOR NON MAJORS
LECTURE GUIDE – UNIT 4
The strength of the magnetic field is proportional to the amount of
current flowing through the conductor
“Ampere-Turns” – determined by multiplying the number of turns of wire by
the current flow
Core Materials
Magnetic/non-magnetic materials around which the conductor is coiled – 2
types
“Air Core” electromagnets – Core is made up of non-magnetic material
“Iron Core” electromagnets – Core is made of a magnetic material (much
stronger magnet by increasing the number of flux lines
Terms to remember
Permeability – a measure of a material’s ability to become magnetized
Reluctance – a material’s resistance to magnetism
Saturation – the point when an increase
significantly increase the magnetic field
in
the
current
fails
to
Residual Magnetism – the amount of magnetism remaining once the current
flow has stopped
MAGNETIC MEASUREMENT
Three different systems are used to measure the strength of a magnetic
field (flux density)
The English System – Number of lines per square inch
The CGS System – Centimeter-gram-second
The MKS System – meter-kilogram-second
MAGNETIC POLARITY
Determining the polarity of an electromagnet
“The Left Hand Rule” – fingers in the direction of current flow,
thumb points toward North Pole
Demagnetizing – disarranging the molecules in a magnet
Striking, heating, placing in an AC magnetic field and pulling it
away
MAGNETIC DEVICES
Electromagnets
Measuring Instruments
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Inductors
Transformers
Motors
Generators/alternators
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
REVIEW QUESTIONS
1. Is the north magnetic pole of the earth a north or south polarity?
2. The south pole of one magnet is brought close to the south pole of
another magnet. Will the two magnets repel or attract?
3. How can the polarity of an electromagnet be determined if the direction
of current is known?
4. Define the following:
Flux density
Permeability
Reluctance
Saturation
Residual Magnetism
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