Laws of Magnetic Forces
Coulomb was the first to determine experimentally the quantitative expression for the magnetic force between two isolated point poles. It may be noted here that, in view of the fact that magnetic poles always exist in pairs, it is impossible, in practice, to get an isolated pole. The concept of an isolated pole is purely theoretical.
By using a torsion balance, he found that the force between two magnetic poles placed in a medium is
(i) directly proportional to their pole strengths
(ii) inversely proportional to the square of the distance between them and
(iii) inversely proportional to the absolute permeability of the surrounding medium.
Three laws of magnetic forces are stated as below :
(1) Like charges repel each other, and unlike charges attract. Thus, two negative or two positive charges repel one another, while a positive charge attracts a negative charge.
(2) The attraction or repulsion acts along the line between the two charges.
(3) The size of the force varies inversely as the square of the distance between the two charges.
Therefore, if the distance between the two charges is doubled, the attraction or repulsion becomes weaker, decreasing to one-fourth of the original value. If the charges come 10 times closer, the size of the force increases by a factor of 100.
AdBlock-2
For example, if m1 and m2 represent the magnetic strength of the two poles (its unit as yet being undefined), r the distance between them and μ the absolute permeability of the surrounding medium, then the force F is given by
where ^r is a unit vector to indicate direction of r.
In the S.I. system of units, the value of the constant k is = 1/4π.
In vector form,
If, in the above equation, m1 = m2 = m (say) ; r = 1 meter ;
Then m2 = 1 or m = ± 1 weber
Hence, a unit magnetic pole may be defined as that pole which when placed in vacuum at a distance of one meter from a similar and equal pole repels it with a force of 1/4π μ0 newtons.
Read article – Units of Resistivity
Visit NCERTplanet.com for NCERT solutions and Textbook downloads
