Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    446/502 page 426
    work. From what source, therefore, do we derive the energy of these currents? 4-10. The Law of Lenz.—Let us see what really takes place. As we approach the coil to the magnet a current is generated in it contrary to that of the magnet. There will thus be a repulsion between the coil and the magnet (Art. 400), and we shall be spending energy in bringing them together against this repulsive force. Again, when the coil is withdrawn from the magnet the currents produced in it are in the same direction as those of the magnet, and hence (Art. 400) the two will attract each other, so that we shall separate the two bodies against an attractive force, and thus energy is spent in the separation. Thus both in the approach and in the withdrawal of the coil from the magnet energy is spent, and it is this energy which produces the currents in the coil. If the coil itself were in oscillatory motion in the neighbour- hood of the magnet, the energy of this motion would be soon stopped by the influence of the magnet, for as the coil ap- proached or receded from the magnet it would experience a resistance to its motion. This fact is known as the Law of Lenz. The visible energy of motion would thus be lost, being con- verted, in the first place, into electric currents, but ultimately into heat. By means of a suitable apparatus this conversion of mechanical energy may be very clearly shown. Thus, if we have a very powerful electro-magnet, and ar- range a thick copper disc so as to rotate between its poles, we shall experience intense difficult}' in producing this rotation of the disc, and after an enormous expenditure of energy we shall only be able to produce a very slow rotation, just as if the disc were moving in thick honey or treacle. Meanwhile the disc will have become heated, because the energy we have spent upon it ultimately takes the form of heat. This fact was discovered by Joule, who made use of this experiment, among others, to obtain the mechanical equivalent of heat. 410a. Use of Lines of Force in Studying Induction.— The experiments of Faraday may be conveniently summarised by considering the lines of force produced when a current flows round a coil. The direction of the.se lines may be ob- tained by placing a piece of cardboard, on which iron filings
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