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مقدمه ای بر الکتریسیته بخش ششم

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Magnetism

The principles of magnetism are an integral part of electricity.

In fact, magnetism can be used to produce electric current and

vice versa.

When we think of a permanent magnet, we often envision a

horse-shoe or bar magnet or a compass needle, but permanent

magnets come in many shapes. However, all magnets have two

characteristics. They attract iron and, if free to move (like the

compass needle), a magnet assumes a north-south orientation.

Magnetic Lines of Flux Every magnet has two poles, one north pole and one south

pole. Invisible magnetic lines of flux leave the north pole and

enter the south pole. While the lines of flux are invisible, the

effects of magnetic fields can be made visible. When a sheet

of paper is placed on a magnet and iron filings loosely scattered

over it, the filings arrange themselves along the invisible lines of

flux.

The density of these lines of flux is greatest inside the magnet

and where the lines of flux enter and leave the magnet.

The greater the density of the lines of flux, the stronger the

magnetic field.

Interaction Between When two magnets are brought together, the magnetic flux

Magnets around the magnets causes some form of interaction. When

two unlike poles are brought together the magnets attract each

other. When two like poles brought together the magnets repel

each other.

Electromagnetism An electromagnetic field is a magnetic field generated by

current flow in a conductor. Every electric current generates a

magnetic field and a relationship exists between the direction

of current flow and the direction of the magnetic field.

The left-hand rule for conductors demonstrates this

relationship. If a current-carrying conductor is grasped with the

left hand with the thumb pointing in the direction of electron

flow, the fingers point in the direction of the magnetic lines of

flux.

Current-Carrying Coil A coil of wire carrying a current, acts like a magnet. Individual

loops of wire act as small magnets. The individual fields add

together to form one magnet. The strength of the field can

be increased by adding more turns to the coil, increasing the

amount of current, or winding the coil around a material such as

iron that conducts magnetic flux more easily than air.

The left-hand rule for coils states that, if the fingers of the left

hand are wrapped around the coil in the direction of electron

flow, the thumb points to the north pole of the electromagnet.

An electromagnet is usually wound around a core of soft iron

or some other material that easily conducts magnetic lines of

force.

A large variety of electrical devices such as motors,

circuit breakers, contactors, relays and motor starters use

electromagnetic principles.

Review 4

1. The two characteristics of all magnets are: they attract

and hold ________, and, if free to move, they assume

roughly a ________ position.

2. Lines of flux always leave the ________ pole and enter

the _________ pole.

3. The left-hand rule for conductors states that, when the

left hand is placed around a current-carrying conductor

with the thumb pointing in the direction of ________

flow, the fingers point in the direction of ________.


نوشته شده در : سه شنبه 21 آذر 1391  توسط : جواد خانی.    نظرات() .

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شنبه 14 مرداد 1396 02:09 ق.ظ
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