Electromagnetism 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 Chapter 14

Topics 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 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 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 (H)  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

14-2: Field Intensity (H) How well current can go through a wire? V I Conductance ( G )

14-2: Field Intensity (H) H B Permeability (  ): How easily magnetic flux can be set up inside a piece of material?

14-2: Field Intensity (H)  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

14-2: Field Intensity (H)  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: B-H Magnetization 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 14-3: B-H magnetization curve for soft iron. No values are shown near zero, where μ may vary with previous magnetization.

14-2: Field Intensity (H) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

14-4: Magnetic Hysteresis  Hysteresis Loop  B R 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.  H C is the coercive force (needed to make B = 0) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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.

14-2: Field Intensity (H)

14-4: 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.

14-4: Magnetic Hysteresis  Demagnetization: To demagnetize a magnetic material completely, the retentivity B R must be reduced to zero.  To demagnetize a magnetic material completely, the retentivity B R 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.

14-4: Magnetic Hysteresis  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 Electric 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 Coil  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 Coil Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig : Left-hand rule for north pole of a coil with current I. The I is electron flow.

14-7: Motor Action between Two Magnetic Fields Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig : Motor action of current in a straight conductor when it is in an external magnetic field. The H I is the circular field of the current The H M indicates field lines between the north and south poles of the external magnet.

14-8: 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 Current Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig : 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 Induced 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.

14-9: Generating an Induced Voltage  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. v ind =N dΦ (webers) dt (seconds)

14-9: Generating an Induced Voltage  Consider a magnetic flux cutting a conductor that is not in a closed circuit, as shown in Figure  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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

14-9: Generating an Induced Voltage  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) v ind = N d  (Wb) dt(seconds)

14-9: Generating an Induced Voltage  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: 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.