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Class 10, Science, Chapter-12, Lecture-1, Electromagnets (Notes)

MAGNETIC FIELD:

The region around a magnet, in which the force of attraction and repulsion due to the magnet can be detected, is called its magnetic field.


MAGNETIC FIELD LINES:

The path along which an isolated unit North pole moves in a magnetic field is termed as the magnetic field line or the magnetic line of force.
Characteristics of Magnetic Field Lines:  

  1. The arrowhead on the field lines indicates the direction of N-pole of an isolated magnet.
  2. The magnetic field is stronger where the field lines are more crowded.
  3. They are closed and continuous curves.
  4. The magnetic field lines cannot intersect one another.

The magnetic field lines cannot intersect one another.

Reason:

The arrowhead on the field line indicates the direction of N-pole of a magnet.

So, the N-pole of a magnet placed at the point of intersection of two field lines will be along two directions, which is not possible.
 

OERSTED’S EXPERIMENT:

Procedure: 

  • A magnetic needle compass is placed in North South direction. 
  • An insulated copper wire is placed over the magnetic needle and parallel to it.
  • The copper wire is connected to a battery.

Observation:  

  • When switch is off         magnet is in NS direction.
  • When switch is on         magnet gets deflected.
  • When current flows in North to South direction, the N-pole deflects towards East.
  • When current flows in South to North direction, the N-pole deflects towards West.

Conclusion:

  • A magnetic field develops around a current carrying conductor.

RIGHT-HAND THUMB RULE:

If a current carrying wire is held in the right hand such that the thumb points in the direction of the current, then the direction in which the fingers encircle the wire gives the direction of the magnetic field line.

FACTORS on which the strength of the magnetic field produced by a current passing through the wire depends:

  1. The strength of the current:
    The magnitude of the field is directly proportional to the current passing in the wire.
  2. Distance from the wire: 
    The magnitude of the field at a point is inversely proportional to the distance of the point from the wire.

FACTORS on which the strength of magnetic field produced at the centre of a current carrying circular wire depens:

  1. The strength of the current: 
    Magnitude of the field is directly proportional to the current passing in the wire.
  2. Radius of the circular loop: 
    Magnitude of the field at the centre is inversely proportional to the radius of the circular loop.

SOLENOID:

A coil of many circular turns of wire wrapped in the shape of a cylinder is called a solenoid.

FACTORS on which the strength of the magnetic field produced by a current-carrying solenoid depends:

  1. The strength of the current: 
    The magnitude of the field is directly proportional to the current passing in the wire.
  2. Radius of the circular loop:
    The magnitude of the field is inversely proportional to the radius of the circular loop.
  3. Number of turns:
    The magnitude of the field is directly proportional to the number of turns.
  4. The nature of the “Core material” used in making the solenoid.  (SOFT IRON– strongest)

ELECTROMAGNET:  

A long coil of insulated copper wire wound on a soft iron core is called an electromagnet.

Magnetic field lines are closed and continuous curves.
Reason:

Magnetic field lines represent a set of points representing the direction of deflection of unit N-pole. 
Outside the magnet, unit N-pole experiences a force in the direction North pole to the South pole of the magnet as like poles repel each other. But inside the magnet, it experiences a force in the South to North direction.
And there are no points around the magnet where the force experienced by unit N-pole is zero.
So, magnetic field lines are closed and continuous.