Chapter
Magnetism
Topics Covered in Chapter 13
13-1: The Magnetic Field
13-2: Magnetic Flux Φ
13-3: Flux Density B
13-4: Induction by the Magnetic Field
13-5: Air Gap of a Magnet
13
Topics Covered in Chapter 13
 13-6: Types of Magnets
 13-7: Ferrites
 13-8: Magnetic Shielding
McGraw-Hill
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13-1: The Magnetic Field
 Magnetic Field Lines
 Every magnet has two
poles (north and south).
 The magnetic field, or
strength of the magnet, is
concentrated at the poles.
 The field exists in all
directions but decreases
in strength as distance
from the poles increases.
Fig. 13-2b: Field indicated by lines of force.
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13-1: The Magnetic Field
 Field Lines
 Magnets have an invisible field (made up of lines of
force).
 These lines of force are from the north to the south pole
of the magnet (external field).
 Field lines are unaffected by nonmagnetic materials, but
become more concentrated when a magnetic substance
(like iron) is placed in the field.
13-1: The Magnetic Field
 North and South Magnetic Poles
 Earth is a huge natural magnet.
 The north pole of a magnet is the one that seeks the
earth’s magnetic north pole.
 The south pole is the one that is opposite the north
pole.
13-1: The Magnetic Field
 Like magnetic poles repel one another. (Fig. 13-4 b)
 Unlike poles attract one another. (Fig. 13-4 a)
Fig. 13-4
13-1: The Magnetic Field
 North and South Magnetic
Poles
 If a bar magnet is free to
rotate, it will align itself
with the earth’s field.
 North-seeking pole of the
bar is simply called the
north pole.
Fig. 13-1a: The north pole on a bar magnet points to the geographic north pole of the Earth.
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13-2: Magnetic Flux Φ
 Magnetic flux is defined as the number of lines of
force flowing outward from a magnet’s north pole.
 Symbol: Φ
 Units:
 maxwell (Mx) equals one field line
 weber (Wb) One weber (Wb) = 1 x 108 lines or Mx
13-2: Magnetic Flux Φ
Fig. 13-5: Total flux Φ is 6 lines or 6 Mx. Flux density B at point P is 2 lines
per square centimeter or 2 G.
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13-2: Magnetic Flux Φ
 Systems of Magnetic Units
 CGS system: Centimeter-Gram-Second. This system
defines small units.
 Mx and μWb (100 Mx) are cgs units.
 MKS system: meter-kilogram-second. This system
defines larger units of a more practical size.
 Wb (1 × 108 Mx) is an MKS unit.
 SI: Systeme Internationale. Basically another name for
the metric system. SI units provide a worldwide
standard in mks dimensions; values are based on one
ampere of current.
13-3: Flux Density B
 Flux density is the number of lines per unit area of a
section perpendicular to the direction of flux.
 Symbol: B
 Equation: B = Φ / area
 Flux Density Units
 Gauss (G) = 1 Mx/cm2 (cgs unit)
 Tesla (T) = 1 Wb/meter2 (SI unit)
13-3: Flux Density B
 Nikola Tesla was a scientist around the turn of the
century, and the designation for magnetic field is
named after him.
 Scientists measure magnetic strength with two units,
tesla and gauss; one tesla equals 10,000 gauss.
 Your average fridge magnet is 10 gauss.
 The Earth's magnetic field is about 0.5 gauss (or
0.00005 tesla).
 A typical MRI scanner features a 1.5 tesla magnet.
 The world’s most powerful magnet is 45 tesla
Source: http://www.magnet.fsu.edu/mediacenter/features/meetthemagnets/index.html#menu
13-4: Induction by
the Magnetic Field
 Induction is the electric or magnetic effect of one body
on another without any contact between them.
 When an iron bar is placed in the field of a magnet,
poles are induced in the iron bar.
 The induced poles in the iron have polarity opposite
from the poles of the magnet.
13-4: Induction by
the Magnetic Field
Fig. 13-7: Magnetizing an iron bar by induction.
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13-4: Induction by
the Magnetic Field
V
I
Conductance (G):
13-4: Induction by
the Magnetic Field
H
B
Permeability ():
13-4: Induction by
the Magnetic Field
 Magnetic Permeability
 Magnetic permeability is the ability to concentrate
lines of magnetic force.
 Ferromagnetic materials have high permeability.
 Magnetic shields are made of materials having high
permeability.
 Symbol: r (no units; r is a comparison of two
densities)
13-4: Induction by
the Magnetic Field
 Permeability () is the ability of a material to support
magnetic flux.
 Relative permeability (r) compares a material with air.
Ferromagnetic values range from 100 to 9000.
 Magnetic shields use highly permeable materials to
prevent external fields from interfering with the
operation of a device or instrument.
Magnetic
shield
around a
meter
movement.
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13-5: Air Gap of a Magnet
 The air space between the
poles of a magnet is its air
gap.
 The shorter the air gap, the
stronger the field in the gap
for a given pole strength.
Fig. 13-8: The horseshoe magnet in (a) has a
smaller air gap than the bar magnet in (b).
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13-5: Air Gap of a Magnet
 The shorter the air gap, the more
intense the field. Eliminating the air
gap eliminates the external field.
This concentrates the lines within
the field.
 Magnets are sometimes stored with
“keepers” that eliminate the
external field.
Fig. 13-9: Example of a closed magnetic ring without any
air gap. (a) Two PM horseshoe magnets with opposite
poles touching.
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13-5: Air Gap of a Magnet
 A toroid coil has very little
external field.
 Toroid cores (doughnut
shaped) are used to greatly
reduce unwanted magnetic
induction.
Fig. 13-9b: Toroid magnet.
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13-6: Types of Magnets
 There are two main classes of magnets:
 An electromagnet is made up of coils of wire, and
must have an external source of current to maintain a
magnetic field.
 Applications: buzzers, chimes, relays (switches whose contacts
open or close by electromagnetism), tape recording.
 A permanent magnet retains its magnetic field
indefinitely.
13-6: Types of Magnets
 An electromagnet produces
a field via current flow.
 The direction of current
determines the field
direction.
Fig. 13-11: Electromagnet holding nail
where switch S is closed for current in coil.
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13-6: Types of Magnets
 Classification of Magnetic and Nonmagnetic Materials
 Magnetic materials:
 Ferromagnetic materials include iron, steel, nickel,
cobalt, and certain alloys. They become strongly
magnetized in the same direction as the magnetizing
field, with high values of permeability.
 Paramagnetic materials include aluminum,
platinum, manganese, and chromium. They become
weakly magnetized in the same direction as the
magnetizing field. The permeability is slightly more
than 1.
13-6: Types of Magnets
 Classification of Magnetic and Nonmagnetic Materials
 Diamagnetic materials include copper, zinc,
mercury, gold, silver, and others. They become
weakly magnetized in the opposite direction from the
magnetizing field. The permeability is less than 1.
 Nonmagnetic materials: air, paper, wood, and
plastics. The permeability is approximately 1.
13-6: Types of Magnets
 The basis of magnetic effects is the magnetic field
associated with electric charges in motion.
 There are two kinds of electron motion in the atom:
 Electron revolving in its orbit. This produces a weak
diamagnetic effect.
 Electron spinning on its axis. The spinning electron
serves as a tiny permanent magnet.
13-7: Ferrites
 Ferrites are nonmetallic materials that have the
ferromagnetic properties of iron.
 They have high permeability.
 However, a ferrite is a non-conducting ceramic material.
 Common applications include ferrite cores in the coils
for RF transformers, and ferrite beads, which
concentrate the magnetic field of the wire on which they
are strung.
13-8: Magnetic Shielding
 Shielding is the act of preventing one component from
affecting another through their common electric or
magnetic fields.
 Examples:
 The braided copper wire shield around the inner
conductor of a coaxial cable
 A shield of magnetic material enclosing a cathoderay tube.
13-8: Magnetic Shielding
 A good conductor is best for two shielding functions:
 Preventing induction of static electric charges
 Shielding against the induction of a varying magnetic
field.
 The best shield for a steady magnetic field is a good
magnetic material of high permeability.