Magnetism Molecular Theory
The molecular theory of magnetism was given by Weber and modified later by Ewing. According to this theory.
(i) Every molecule of magnetic substance (whether magnetized or not) is a complete magnet in itself, having a north pole and a south pole of equal strength.
(ii) In an unmagnified substance the molecular magnets are randomly oriented such that they from closed chains the north pole of one molecular magnet cancel the effect of South Pole of the other so that the resultant magnetism of the unmagnified specimen is zero.
(iii) On magnetizing the substance the molecular magnets are realigned so that north poles of all molecular magnetism points in one direction and south poles of all molecular magnets points in the opposite direction.
(iv) The extern of magnetization of the specimen is the extent of realignment of the molecular magnets.
(v) When all the molecular magnets are fully aligned, the substance is said to the saturated with magnetism.
(vi) At all the stages the strengths of the two poles developed will always be equal.
(vii) On heating the magnetized specimen molecular magnets acquire some kinetic energy some of the molecules may get back to t6he closed chain arrangement. That is why magnetism of the specimen would reduce on heating.
Uniform and Non uniform magnetic filed
Magnetic field is defined as the space around a magnet or the space around a conductor carrying current in which the magnetic effect force of attract in and repulsion on a magnet) cane experienced.
In the last chapter we have already defined magnetic field strength (B) at a points as the force experienced by a unit charge moving with unit velocity in a direction perpendicular to the magnetic field we also define magnetic field strength at a point as the force experienced by an isolated hypothetical unit north pole placed at the point it is a vector quantity whose direction is the direction in which unit north pole would move if free to do so.
The S.I unit of magnetic field strength is tesla, which is already defined in the last chapter.
1 tesla (T) = 1 Newton ampere-1 metre-1 (NA-1m-1) = 1 Weber metre-2 (Wb m-2)
The C.G.S unit of magnetic field strength is 1 gauss (G) = 10-4 tesla (T).
Magnetic field in a region is said to be uniform if the magnetic field vector B has the same magnet due and the same direction at all the point in the region the only example of nearly uniform magnetic field over a wide area is that due to earth's magnetism magnetic field due to earth can be taken as fairly uniform over the surface of earth.
A infirm magnetic field acting in the plane of the paper is represented by equidistant parallel lines.
The number of magnetic lines of force per unit area normal to the area at a point is proportional to magnitude of the magnetic field at that point obviously crowded lines will represent a strong field and well spaced lines will represent a weak field.
A uniform magnetic fied acting perpendicular to the plane of the paper and outwards is represented by crosses spaced equally.
Different text books have used various names for the magnetic field B these are magnetic induction magnetic induction field magnetic flux density etc for the sake of simplicity and uniformity we shall use the name magnetic field strength for B raising that it means the same as magnetic flux density
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