electromagnetism topics covered in chapter 14: 14-1: ampere-turns of magnetomotive force (mmf) 14-2:...

ElectromagnetismElectromagnetism

Topics Covered in Chapter 14:14-1: Ampere-turns of Magnetomotive Force (mmf)

14-2: Field Intensity (H)

14-3: B-H Magnetization Curve

14-4: Magnetic Hysteresis

14-5: Magnetic Field around an Electric Current

ChapterChapter1414

Topics Covered in Chapter 14Topics Covered in Chapter 14

14-6: Magnetic Polarity of a Coil 14-7: Motor Action between Two Magnetic Fields 14-8: Induced Current 14-9: Generating an Induced Voltage 14-10: Relays

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

14-1: Ampere-turns of 14-1: Ampere-turns of Magnetomotive Force (mmf)Magnetomotive Force (mmf)

The strength of a coil’s magnetic field is proportional to the amount of current flowing through the coil and the number of turns per given length of coil.

Ampere-turns = I × N = mmf I is the amount of current flowing through N turns of

wire. This formula specifies the amount of magnetizing force

or magnetic potential (mmf).

14-1: Ampere-turns of 14-1: Ampere-turns of Magnetomotive Force (mmf)Magnetomotive Force (mmf)

The SI abbreviation for ampere-turn is A · t.

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

Fig. 14-1: Two examples of equal ampere-turns for the same mmf. (a) IN is 2 × 5 = 10. (b) IN is 1 × 10 = 10.

14-2: Field Intensity (14-2: Field Intensity (HH))

The length of a coil influences the intensity of a magnetic field. Intensity is different from mmf.

Equation: H = mmf/length Units: A·t/m

ampere-turns per meter H is the mks unit

Shorter magnetic circuits produce a greater field intensity


How well current can go through a wire?

V

I

Conductance (G)


H

B

Permeability ():

How easily magnetic flux can be set up inside a piece of material?


Permeability () is a measure of the ability to concentrate magnetic fields.

Materials with high permeability can concentrate flux, and produce large values of flux density B for a specified H.

The amount of flux produced by H depends on the material in the field.

These factors are reflected in the formulas: B = × H μ = B / H

The unit is teslas per ampere-turn per meter: A · t/m

T


Ferromagnetic materials have high values of permeability (as high as 10,000).

Paramagnetic materials The permeability is slightly more than 1.

Diamagnetic materials The permeability is less than 1.

Non-magnetic materials The permeability is approximately 1.

14-3: 14-3: B-H B-H Magnetization CurveMagnetization Curve

The B-H magnetization curve shows how much flux density B results from increasing field intensity H.

Saturation is the effect of little change in flux density when the field intensity increases.


Fig. 14-3: B-H magnetization curve for soft iron. No values are shown near zero, where μ may vary with previous magnetization.


Permeability drops drastically at saturation.

- H + H

+ B

- B

+ Bmax

- Bmax

Slope ofB/H is small

Slope ofB/H is large

Permeability drops drastically at saturation.

- H + H

+ B

- B

+ Bmax

- Bmax







Hysteresis Loop BR is due to retentivity

(memory), which is the flux density remaining after the magnetizing force is reduced to zero.

Note that H = 0 but B > 0. HC is the coercive force

(needed to make B = 0)


Fig. 14-4: Hysteresis loop for magnetic materials. This graph is a B-H curve like Fig. 14-3, but H alternates in polarity with alternating current.


http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html

14-4: Magnetic Hysteresis14-4: Magnetic Hysteresis

Hysteresis refers to a situation where the magnetic flux lags the increases or decreases in magnetizing force.

Hysteresis loss is energy wasted in the form of heat when alternating current reverses rapidly and molecular dipoles lag the magnetizing force.

For steel and other hard magnetic materials, hysteresis losses are much higher than in soft magnetic materials like iron.


Demagnetization: To demagnetize a magnetic material completely, the retentivity BR must be reduced to zero.

To demagnetize a magnetic material completely, the retentivity BR must be reduced to zero.

A practical way to do so is to magnetize and demagnetize the material with a decreasing hysteresis loop.

This method of demagnetization is called degaussing. Applications of degaussing include:

Metal electrodes in a color picture tube Erasing the recorded signal on magnetic tape.


A magnetic field is produced by alternating current. The magnetic field and the magnetic material are moved

away from each other, or the current amplitude is reduced. The hysteresis loop then becomes smaller and smaller until it

effectively collapses.

14-5: Magnetic Field around an 14-5: Magnetic Field around an Electric CurrentElectric Current

The left-hand rule for conductors: Grasp the conductor with your left hand so the thumb points in the direction of electron flow. Your fingers will encircle the conductor in the same direction as the circular magnetic field lines.

14-6: Magnetic Polarity of a 14-6: Magnetic Polarity of a CoilCoil

Bending a straight conductor into the form of a loop produces two effects: The magnetic field lines are more dense inside the

loop. All the lines inside the loop aid in the same direction.

This makes the loop field the same as a bar magnet, with opposite poles at opposite faces of the loop.

14-6: Magnetic Polarity of a 14-6: Magnetic Polarity of a CoilCoil


Fig. 14-10: Left-hand rule for north pole of a coil with current I. The I is electron flow.

14-7: Motor Action between Two 14-7: Motor Action between Two Magnetic FieldsMagnetic Fields


Fig. 14-13: Motor action of current in a straight conductor when it is in an external magnetic field. The HI is the circular field of the current The HM indicates field lines between the north and south poles of the external magnet.

14-8: Induced Current14-8: Induced Current

Generator Action When a moving conductor cuts across flux lines, a

voltage is induced. The amount of induced voltage is proportional to:

The conductor velocity The amount of flux The number of turns of wire

14-8: Induced Current14-8: Induced Current


Fig. 14-15: Induced current produced by magnetic flux cutting across turns of wire in a coil. Direction of I here is for electron flow.

14-9: Generating an 14-9: Generating an Induced VoltageInduced Voltage

The motion of flux across a conductor in an open circuit forces free electrons to move.

Since the ends are open, electrons accumulate at them, creating a potential difference.

The potential difference is an electromotive force (emf), generated by the work of cutting across the flux.

Induced emf increases with the number of turns in a coil.

The polarity of the induced voltage follows from the direction of induced current.


Faraday’s Law of Induced Voltage The amount of induced voltage can be calculated by

Faraday’s law:

N = number of turns dΦ/dt = how fast the flux cuts across the conductor.

vind = NdΦ (webers)dt (seconds)


Consider a magnetic flux cutting a conductor that is not in a closed circuit, as shown in Figure 14-16. The motion of flux across the conductor forces free electrons to move. The potential difference is an electromotive force (emf) generated and only present while the motion of flux is cutting across the conductor.



Faraday’s Law of Induced Voltage The amount of induced voltage is calculated by

Faraday’s law:

The induced voltage is directly proportional to the number of turns times d/dt.

To generate more voltage: Increase the number of turns Increase the flux (stronger magnet) Decrease the time (faster movement of the coil)

vind = N d(Wb)

dt(seconds)


The polarity of induced voltage is determined by Lenz’s law. The induced voltage has the polarity that opposes the

change causing the induction????????? Movement of a wire/coil in the presence of magnetic field

induces current/voltage. Induced current/voltage interacts with the magnetic field to

produce a force. The direction of the force depends on polarity of the induces

current/voltage. The direction of the force is opposite to the direction of the

movement of the wire/coil. It will try to slow down the wire/coil.

14-10: Relays14-10: Relays

A relay is an electromechanical device that operates on the basis of electromagnetic induction.

It uses an electromagnet to open or close one or more sets of contacts.

Relays, like switches, have poles and throws. Relays can switch or control high power loads with a

low amount of input power. In remote-control applications, relays can control high

power loads long distances away more efficiently than can mechanical switches.

electromagnetism topics covered in chapter 14: 14-1: ampere-turns of magnetomotive force (mmf) 14-2:...

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