Magnetic fields can be represented using magnetic field lines starting at the North Pole and ending at the South Pole. Although the magnetic field of a permanent magnet is everywhere surrounding the magnet (in all three dimensions), we draw only some of the field lines to represent the field (usually only a two-dimensional cross-section is shown in drawings).
In areas where the magnetic field is strong, the field lines are closer together. Where the field is weaker, the field lines are drawn further apart. The number of field lines drawn crossing a given two-dimensional surface is referred to as the magnetic flux. The magnetic flux is used as a measure of the strength of the magnetic field through that surface. Magnetic field is a vector quantity that has both direction and magnitude.
One can map the magnetic field of a magnet by placing it underneath a piece of paper and sprinkling iron filings on top or by using a compass. The iron filings or needle of compass line themselves up parallel to the magnetic field as shown in following figures:
The field lines seem originated from North Pole and terminated at the South Pole, but inside the magnet, the direction of field lines is from its south pole to its north pole. Thus, the magnetic field lines form closed curves. No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
Important Conclusion for magnetic field line is:
i. The magnetic field lines never cross.
ii. Arrows drawn on the field lines indicate the direction of the field.
iii. A magnetic field points from the north to the South Pole of a magnet
iv. Inside the magnet, field points from south to North Pole.
v. Magnetic field lines are in closed curves.
Ferromagnetism is a phenomenon exhibited by materials like iron, nickel or cobalt. These materials are known as permanent magnets. They always magnetize so as to be attracted to a magnet, regardless of which magnetic pole is brought toward the unmagnetised iron.
Paramagnetic materials are materials like aluminum or platinum which become magnetized in an external magnetic field in a similar way to ferromagnetic materials but lose their magnetism when the external magnetic field is removed.
Diamagnetism is exhibited by materials like copper or bismuth which become magnetized in a magnetic field with a polarity opposite to the external magnetic field. Unlike iron, they are slightly repelled by a magnet.
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