ALP Stage-2 exam: Physics notes- MAGNETIC EFFECTS OF ELECTRIC CURRENT(PART I)



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MAGNETIC FIELD AND FIELD LINES

The area around a magnet where a magnetic force is experienced is called a magnetic field. It is a quantity that has both direction & magnitude.


A bar magnet showing magnetic field

Characteristics of Magnetic field lines :

(a) The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole.

(b) The strength of magnetic field is expressed by the closeness of magnetic field lines. Closer the lines, more will be the strength and farther the lines, less will be the magnetic field strength.

(c) No two field lines will intersect each other. If they intersects, then at point of intersection the compass needle will show two directions of magnetic field which is not possible.


MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR

  • A current carrying straight conductor has magnetic field in the form of concentric circles around it. 
  • The magnitude of the magnetic field produced at a given point increases as the current through the wire increases. 
  • The magnetic field produced by a given current in the conductor decreases as the distance from it increases. 


RIGHT HAND THUMB RULE:
A convenient way of finding the direction of magnetic field associated with a current-carrying conductor is the Right hand thumb rule. If a current carrying straight conductor is held in your right hand such that the thumb points towards the direction of current, then the wrapped fingers show the direction of magnetic field lines.


MAGNETIC FIELD DUE TO A CURRENT THROUGH A CIRCULAR LOOP

  • At every point of a current-carrying circular loop, the concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire.
  • Every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the center of the loop.

The strength of Magnetic field depends on-

  • The radius of the coil: The strength of the magnetic field is inversely proportional to the radius of the coil.If the radius increases, the magnetic strength at the centre decreases.
  • The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus the field due to each turn adds up.
  • The strength of the current flowing in the coil: As the strength of the current increases, the strength of thee magnetic field also increases.

MAGNETIC FIELD DUE TO A CURRENT IN A SOLENOID 



  • A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
  • The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
  • A strong magnetic field produced inside a solenoid can be used to magnetize a piece of magnetic material, like soft iron, when placed inside the coil. The magnet so formed is called an electromagnet.
  • Magnetic field produced by a solenoid similar to bar magnet. Strength of magnetic field is proportional to number of turns and magnitude of current.

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