Magnetism Topics Covered in Chapter 13 13-1: The Magnetic Field 13-2: Magnetic Flux Φ 13-3: Flux...

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Magnetism 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 Chapter 13 © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

Transcript of Magnetism Topics Covered in Chapter 13 13-1: The Magnetic Field 13-2: Magnetic Flux Φ 13-3: Flux...

Page 1: 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:

MagnetismMagnetism

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

ChapterChapter1313

© 2007 The McGraw-Hill Companies, Inc. All rights reserved.

Page 2: 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:

Topics Covered in Chapter 13Topics Covered in Chapter 13

13-6: Types of Magnets 13-7: Ferrites 13-8: Magnetic Shielding 13-9: The Hall Effect

McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

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13-1: The Magnetic Field13-1: 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 Field13-1: 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.

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13-1: The Magnetic Field13-1: The Magnetic Field

Like magnetic poles repel one another.

Unlike poles attract one another.

Fig. 13-4

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13-1: The Magnetic Field13-1: 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.

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13-1: The Magnetic Field13-1: 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 13-2: Magnetic Flux ΦΦ

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

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13-2: Magnetic Flux 13-2: Magnetic Flux ΦΦ

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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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13-2: Magnetic Flux 13-2: Magnetic Flux ΦΦ

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.

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Who is Maxwell?Who is Maxwell?

Scottish mathematician and physicist who published physical and mathematical theories of the electromagnetic field.

Maxwell proved that electromagnetic phenomena travel in waves at the speed of light

http://scienceworld.wolfram.com/biography/Maxwell.html

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How about Weber?How about Weber?

German physicist who devised a logical system of units for electricity. Weber wanted to unify electricity and magnetism into a fundamental force law.

He invented the electrodynamometer, an instrument for measuring small currents

http://scienceworld.wolfram.com/biography/WeberWilhelm.html

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13-3: Flux Density 13-3: Flux Density BB

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)

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Who is Gauss?Who is Gauss?

German mathematician who is sometimes called the “prince of mathematics”.

Set up the first telegraph in Germany http://scienceworld.wolfram.com/biography/Gauss.ht

ml

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How about Tesla?How about Tesla?

Eccentric Serbian-American engineer who made many contributions to the invention of electromagnetic devices.

Tesla’s ac power became the worldwide power standard

http://scienceworld.wolfram.com/biography/Tesla.html

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13-4: Induction by 13-4: Induction by the Magnetic Fieldthe 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.

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13-4: Induction by 13-4: Induction by the Magnetic Fieldthe Magnetic Field

Fig. 13-7: Magnetizing an iron bar by induction.

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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)

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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.

Magneticshield

around ameter

movement.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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13-5: Air Gap of a Magnet13-5: 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 Magnet13-5: 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 Magnet13-5: 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.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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13-6: Types of Magnets13-6: 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.

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13-6: Types of Magnets13-6: Types of Magnets

An electromagnet produces a field via current flow.

The direction of current determines the field direction.

Left-hand rule: Thumb points toward N if hand is curled around coil in direction of current

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 13-11: Electromagnet holding nail when switch S is closed for current in coil.

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13-6: Types of Magnets13-6: 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.

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13-6: Types of Magnets13-6: 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

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13-7: Ferrites13-7: Ferrites

Ferrites are nonmetallic materials that have the ferromagnetic properties of iron. They have high permeability. However, a ferrite is a nonconducting 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.

http://www.mag-inc.com/ferrites/ferrites.asp

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13-8: Magnetic Shielding13-8: 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 cathode-

ray tube.

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How does Magnetic Shielding How does Magnetic Shielding Work?Work?

When magnetic lines of flux encounter high permeability material, the magnetic forces are both absorbed by the material and redirected away from its target.

The most effective shields are constructed as enclosures such as boxes or better yet, cylinders with end caps.

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What is EMI?What is EMI?

EMI is the abbreviation for Electro Magnetic Interference.

EMI is an electrical or magnetic disturbance that causes unwanted interference.

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13-9: The Hall Effect13-9: The Hall Effect

A small voltage is generated across a conductor carrying current in an external magnetic field. This is known as the Hall effect.

The amount of Hall voltage VH is directly proportional to the value of flux density B.

To develop Hall effect voltage, the current in the conductor and the external flux must be at right angles to each other.

Some gaussmeters use indium arsenide sensors that operate by generating a Hall voltage.

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13-9: The Hall Effect13-9: The Hall Effect

Additional Applications for MagnetismB

VH B

Hall effect sensor

B

VH B

Hall effect sensor

VH B

Hall effect sensor

Ferrite beadFerrite bead

The ferrite bead concentrates the magnetic field of the current in the wire. This construction serves as a simple RF choke that will reduce the current just for an undesired radio frequency.

The semiconductor material indium arsenide is generally used as a Hall effect sensor.