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GROUP 4

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A long coil of wire made up of many loops.

When current moves along the wire of thesolenoid, the current produces a magnetic field.

The direction of the magnetic field is definedby the right-hand rule

If you grasp the coil with your four fingers, the

direction where your thumb points shows thedirection of the magnetic field.

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1. Directly Proportional to the number of turns inthe solenoid;

2.Directly proportional to the strength of the

current in the solenoid;3.Dependent on the nature of the core material

used in making the solenoid. The use of softiron rod as core in a solenoid produces thestrongest magnetism.

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Magnetic Field of a Solenoid A solenoidis a tightly wound helical coil of

wire whose diameter is small compared to itslength. The magnetic field generated in the

centre, or core, of a current carrying solenoid isessentially uniform, and is directed along theaxis of the solenoid. Outside the solenoid, themagnetic field is far weaker. Figure 27shows(rather schematically) the magnetic field

generated by a typical solenoid. The solenoidis wound from a single helical wire whichcarries a current . The winding is sufficientlytight that each turn of the solenoid is wellapproximated as a circular wire loop, lying inthe plane perpendicular to the axis of the

solenoid, which carries a current . Supposethat there are such turns per unit axial lengthof the solenoid. What is the magnitude of themagnetic field in the core of the solenoid?

http://farside.ph.utexas.edu/teaching/302l/lectures/node76.htmlhttp://farside.ph.utexas.edu/teaching/302l/lectures/node76.htmlhttp://farside.ph.utexas.edu/teaching/302l/lectures/node76.html

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FIG.27

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In order to answer this question, let us apply Ampre's

circuital law to the rectangular loop . We must first find the line

integral of the magnetic field around . Along and the magneticfield is essentially perpendicular to the loop, so there is no

contribution to the line integral from these sections of the loop.

Along the magnetic field is approximately uniform, of

magnitude , say, and is directed parallel to the loop. Thus, thecontribution to the line integral from this section of the loop is ,

where is the length of . Along the magnetic field-strength is

essentially negligible, so this section of the loop makes no

contribution to the line integral. It follows that the line integral of

the magnetic field around is simply

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(178)

By Ampre's circuital law, this line integral is equal to times the algebraic sum of

the currents which flow through the loop . Since the length of the loop along the

axis of the solenoid is , the loop intersects turns of the solenoid, each carryinga current . Thus, the total current which flows through the loop is . This

current counts as a positive current since if we look against the direction of the

currents flowing in each turn (i.e., into the page in the figure), the

loop circulates these currents in an anti-clockwise direction. Ampre's circuital

law yields(179)

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which reduces to

(180)

Thus, the magnetic field in the core of a solenoid is directly proportional to the

product of the current flowing around the solenoid and the number of turns per unit

length of the solenoid. This, result is exactin the limit in which the length of thesolenoid is very much greater than its diameter.