Electromagnetism and the transformer

Abstract

When an electric current flows in a conductor, magnetic forces are produced in the area around the conductor, a magnetic field being established because of the current in the conductor. The direction in which the magnetic field acts at any point may be defined as the direction in which a small isolated north pole, imagined to be removed from a permanent magnet, would tend to move if placed in the field at that point. A line of force is then the imaginary line that would be followed by the pole if free to move under the action of the force due to the magnetic field. An isolated north pole cannot, of course, have any physical existence, unlike an isolated positive charge of electricity, and the concept of the direction of a magnetic field being defined in terms of the force on an isolated north pole is simply to emphasise the similarities between the electric field discussed in chapter 1, and the magnetic field now being dealt with. More practically, magnetic fields may be mapped out using iron filings, which do not, however, give the direction in which the magnetic field is defined as acting. If, though, a small compass needle, which may be regarded as an iron filing capable of rotating about a pivot, is placed in the field, the direction in which the needle points is defined as the direction in which the field acts. This direction is dependent upon the direction of the current in the conductor. The use of iron filings to map out the field round a current-carrying conductor shows that magnetic flux lines surrounding the conductor take the form of concentric circles, and that the intensity of the magnetic field decreases as the distance from the conductor increases.