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SS3: PHYSICS - 1ST TERM

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Lesson 10, Topic 1
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Magnetic Field

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Magnetic field is a region under the influence of a magnetic force or a region where the influence of a magnet can be felt through a force.

Any magnetic substance placed in the region surrounding a magnet can easily be attracted or repelled.

Magnetic Lines of Forces

This is an imaginary line which a free N-pole would tend to move if placed in the field. The magnetic lines of forces do not really exist, hence, they are called imaginary lines

Patterns of Magnetic Field

With iron fillings, magnetic lines can easily be observed. When a magnet is placed on a paper and iron fillings are sprinkled on the paper lightly, if the paper is tapped gently, the iron fillings will give a definite pattern. 

Magnetic field lines around a straight conductor carrying current
Magnetic field lines around a straight conductor carrying current 
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  • pass a straight conductors through a cardboard 
  • sprinkle iron filling on the cardboard uniformly
  • connect that to a battery and switch on the circuit 
  • open the switch then place a compass needle and switch on the current, observe the direction of the compass needle

The compass needle shows that the magnetic field lines of force are observed to be arranged in concentric circles around the wires. The direction of the field is in the direction in which a free north pole will move. The direction of the field lines depends on the direction of the current. If the direction of the current charges i.e. flows downward, the field lines also change in the opposite direction 

Direction of Magnetic Field Lines

The direction of the magnetic field lines around a straight current carrying wire can be identified using Maxwell’s corkscrew or clenched first rule or right hand grip rule.

Maxwell’s Corkscrew Rule

It states that the direction of the magnetic field around a straight conductor carrying current is the direction of a right handed corkscrew will turn in order to move the corkscrew in the direction of the current.

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Current = Thumb Direction

Field Direction – Direction of fingers

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Magnetic Field around a Solenoid

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A solenoid is an insulated copper wire of many turns wound close together on a long cylindrical coil. When current flows through the solenoid, the magnetic field lines through an outside the solenoid is similar to that of a bar magnet with north and south poles.

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