Principles & Applications Electricity Principles & Applications Sixth Edition Richard J. Fowler Chapter 7 Magnetism and Electromagnetism ©2003 Glencoe/McGraw-Hill

INTRODUCTION Magnetic Forces Electromagnetism Induced Voltage Magnetic Domains Magnetic Forces Electromagnetism Induced Voltage Reluctance Magnetic Units

Facts About Magnetism Magnetism is an invisible force field. Most magnets have a north pole and a south pole. Flux leaves the north pole and enters the south pole. Like poles repel; unlike poles attract. Magnetic materials have magnetic domains. Current-carrying conductors produce magnetic fields. Magnetic flux follows the path of least reluctance.

Magnetic Domains in a Magnetic Field (case 1) S N S N S N Domains are randomly arranged in this unmagnetized temporary magnetic material. When subjected to a magnetic force field, the domains are aligned with the field. When the magnetic force field is removed, the domains return to a random arrangement.

Magnetic Domains in a Magnetic Field (case 2) S N S N S N Domains are randomly arranged in this unmagnetized permanent magnetic material. When subjected to a magnetic force field, the domains are aligned with the field. When the magnetic force field is removed, the domains remain aligned.

Reaction of Like Poles S N Like poles produce a repelling force on these held-in-place magnets. The flux loops are distorted.

Reaction of Like Poles S N S N Like poles produce a repelling force on these held-in-place magnets. When freed, the magnets move apart and the distortion decreases.

Reaction of Like Poles S N S N Like poles produce a repelling force. The magnets move apart and the distortion decreases.

Reaction of Like Poles S N S N Like poles produce a repelling force. The magnets move apart and the distortion decreases.

Magnetism Quiz In non-magnetized magnetic materials the_____ are randomly arranged. domains Most magnets have ____ poles. two Flux leaves the ____ pole of a magnet. north Like magnetic poles ____ each other. repel Magnetic materials have magnetic ____. domains A magnetic force field contains ____. flux

Electromagnetism A larger current produces more flux. Current in a conductor produces flux around the conductor. A larger current produces more flux. ¬ ¬ ¬ ¬ Conductor Direction of current Use the left-hand rule to determine the direction of the flux.

Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. ®

Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. ® ® ® ® ® ®

Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. ® ® ® ® ® ®

Flux Around Parallel Conductors The conductors repel each other when the currents are in opposite directions. ® ® ® ® ® ®

Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. ¬ ®

Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. ® ® ® ¬

Flux Around Parallel Conductors The conductors attract each other when the currents are in the same direction. ® ® ® ¬

Repulsion of Conductors When Current is in Opposite Directions = direction of current for first ½ of each cycle = direction of current for alternate ½ cycles As shown in the next slide, the conductors repel each other when power is applied

Click on the image to run the display. To rerun the display, click again on the image.

Attraction of Conductors When Current is in the Same Direction Two-conductor cables = direction of instantaneous current As shown in the next slide, the two conductors in each cable will be pulled together when power is applied.

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Electromagnetism Quiz Current-carrying conductors produce magnetic ____. flux The flux around a conductor ____ as the current increases. increases The ____ rule is used to determine the flux direction around a conductor. left-hand Parallel conductors ____ each other when the currents are in the same direction. attract

Low Reluctance Flux Path Iron Flux Flux bends to follow a low reluctance path.

Electromagnet With Extended Center Leg The next slide shows the results of dropping three different rings on the rod. The iron ring is both magnetic and a conductor. Notice it is hard to remove from the center leg where the field is strongest. This ring also has an induced current that creates a repelling field that is too weak to over- come the magnetic attraction. The aluminum ring produces a “floating action”. It is non-magnetic and its induced current creates an opposing field that counteracts gravity. The plastic ring produces no reaction because it is both nonmagnetic and an insulator. Iron rod setting on the center leg of the core Iron core with I laminations removed Coil connected to an ac source

Click on the image to run the display. To rerun the display, click again on the image. The next slide shows the behavior of a larger aluminum ring.

Click on the image to run the display. To rerun the display, click again on the image.

Adding a Low-Reluctance Path in a Magnetic Field S Opposite magnetic poles produce a flux in the air between the poles. When a low-reluctance path is provided, the flux increases and is concentrated in the low-reluctance path.

Induced Voltage X · Voltage is induced when conductors cut lines of flux. Green arrow shows direction of conductor movement. Dot and X show direction of current caused by voltage. No voltage is induced when conductors move parallel to the to the lines of flux.

Magnetic Quantities and Units magnetomotive force (mmf) ampere-turn (A×t) magnetic field strength (H) ampere-turn/meter (A×t/m) flux (f) weber (Wb) flux density (B) tesla (T) permeability (m) Wb/(A×t×m) relative permeability unitless

Magnetic Field Strength mmf = 3 A x 8 t = 24 A· t 8 t 3 A Core length = 0.4 m mmf = 3 A x 4 t = 12 A· t 24 A· t H = = 60 A· t / m Core length = 0.2 m 0.4 m 12 A· t H = = 60 A· t / m 3 A 0.2 m The magnetic field strength is the same for the two circuits.

A B Flux Density Material B carries twice as much flux as material A. However, the flux density (B) is the same in both materials because the cross-sectional area of B is two times larger than that of A.

Quantities and Units Quiz The opposition to flux is called ____. reluctance The unit of flux is the ____. weber Flux density is specified in ____. tesla Wb/(A×t×m) is the unit for ____. permeability The symbol for magnetic field strength is ____. H The symbol for flux density is ____. B The value of H is ____ when I = 4A , N = 12t , and l = 0.2m. 240 A×t/m

REVIEW Magnetic Domains Magnetic Forces Electromagnetism Induced Voltage Reluctance Magnetic Units