Intermediate physics / by W. Watson.

  • Watson, W. (William), 1868-1919.
Date:
1912
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Licence: In copyright

Credit: Intermediate physics / by W. Watson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    however possesses potential energy exactly equal in amount to its original kinetic energy. 18. Principle of the Conservation of Energ-y.—In the previous section we saw that although the form of the energy of the stone altered, so that it existed sometimes as potential energy, some- times as kinetic energy, and sometimes as both together, yet the total quantity of energy was constant throughout. We shall in subsequent sections deal with many other forms in which energy can exist besides those already considered, and we shall find that these different forms of energy can be converted from one kind to the other. When the stone reaches the ground it is brought to rest by impact with the ground, when it will apparently have lost both its kinetic energy and its potential energy. It is, however, found that the energy has not been lost, but has been transformed into another form, namely, that of heat, so that both the stone and the part of the earth it struck are warmer than they were before the impact. Joule has indeed shown, as we shall see later, that in every case a given amount of work entirely spent in producing heat always produces the same quantity of heat, no matter how the conversion takes place. For instance, a given number of ergs of work which can be obtained from stopping a moving bullet, and hence destroying its kinetic energy, will produce a certain quantity of heat. The same number of ergs of work done in rubbing a button on a piece of wood will produce exactly the same quantity of heat. The energy in the form of heat possessed by a body is supposed to be due to the motion of its particles. If this be so, then the kinetic energy of the stone moving as a whole is converted, by impact with the ground, into kinetic energy of the -particles of the body and the earth near the point of impact, the particles moving (probably backwards and forwards with a vibratory motion) with, on the whole, a greater velocity than before. These observations may be generalised, for in every case, without exception, it is found that the sum total of all the energy within any given boundary, through which energy is not allowed to pass, remains constant, although the energy within the boundary may be transformed into any of the many forms in which it is capable of existing. The above statement amounts to an enunciation of a doctrine which Energy cannot practically the keystone of modern science, and is known be created or as the doctrine or -principle of tlie conservation of energy. destroyed. follows that if the boundary considered includes the universe, the principle of the conservation of energy amounts to a statement that the sum total of the energy of the universe is a fixed unalterable quantity. The principle of the conservation of energy also denies the possibility of perpetual motion. By perpetual motion is meant the devising