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.

    460/502 page 440
    tivity, the third on account of leng-th, and the last on account of cross section. We can thus tell the relative distribution of heat in the various parts of a battery ; but in order to tell the whole heat- ing- effect produced from first to last, we must bear in mind the origin of the heat. This is, in fact, the burning of the fuel zinc, the potential energy of which is converted in the first instance into electricity in motion, and ultimately (let us sup)- pose) into heat. Now a certain quantity of zinc consumed will give us a certain definite quantity of heat, neither more nor less ; and it has been shown by Joule that if the same quantity of zinc be combined with acid in an ordinary vessel, it will give out the same amount of heat as if it were consumed by means of the voltaic arrangement. Thus the difference between dissolving zinc by acid in an ordinary vessel, and doing so by the voltaic arrangement, is not in the quantity of heat which it gives out, but in the dis- tribution of this heat. For in the voltaic arrangement heat may be developed many miles from the cells in which the combustion takes place, but in the ordinary case the heat is produced in the vessel in which the zinc is dissolved. 431. Joule’s Iiaw.—This relation between the electrical energy in a circuit and the heat produced is expressed by C-RT = JH ; where C is the strength of the current, R is the resistance of the circuit, T is the time the current passes, h the number of thermal-units produced, and j Joule’s equivalent. If the units used are the Ampere, Volt, Ohm, Second, and Gramme degree, then j = 4‘2. V ith the aid of this value the following example can be worked :— Example.—Find the heat produced in five minutes by a current of lo amperes passing through a resistance of 4 ohms. Auswe?'.—We have from Joule’s Law H C-RT J 10’- X 4 X 5 X 60 4’2 28571 432. Chemical Effects.—The electric current is capable ot decomposing certain compound bodies into their constituent
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image