Field around a bar magnet represented by lines of force ( Fig 1)
Practically all electrical motors and generators,ranging in size from fractional kilowatt power units found in domestic appliances to the gigantic several thousand KW motors used in heavy industries and several 100MW generators installed in generating stations ,depend upon the the electromagnetic field as the coupling device permitting interchange of energy between electrical system and mechanical system and vice-versa. Similarly ,static transformers provide the means for converting energy from one electrical system to another through medium of magnetic field . Though a transformer is not an electromechanical energy conversion device ,but the transformer analysis runs parallel to rotating machinery analysis and largely aids in understanding the later. Thus it is seen that all the electrical machinery including transformers make use of medium of magnetic field for energy conversion/transfer and study of such devices essentially involve electric and magnetic circuit analysis and their interaction . Other important devices like circuit brakers automatic switches , relays need the presence of a confined magnetic field for their proper operation.
Hence review of physical laws governing magnetic fields , production of emf and mechanical force and brief study of simple magnetic circuits and magnetic materials become essential
