Heat and Energy. The Kinetic Theory 201 



Just as potential energy may be transformed into 

 kinetic energy, and heat energy, as mentioned above, 

 may undergo a similar transformation, so may kinetic 

 energy be changed into heat. A piece of lead, struck 

 several times in rapid succession by a hammer, soon 

 becomes appreciably hot. Very careful measurements 

 of the quantity of heat that may thus be generated 

 by the transformation of kinetic energy have been 

 made by various investigators. The mean result of 

 their determinations shows that, to raise the tempera- 

 ture of 1 Ib. of water by 1 F. requires the expenditure 

 of about 775 ft.-lbs. of kinetic (or other form of) energy. 

 Conversely, 1 Ib. of water, in cooling through 1 F., 

 can perform 775 ft.-lbs. of work. Expressing this result 

 in terms of the Centigrade scale, the number of ft.-lbs. 

 becomes 775 x f = 1395. 



The magnitude of this quantity, which is termed 

 the " Mechanical Equivalent " of heat, may be grasped 

 if it be considered that the energy required to raise 

 the temperature of 1 gallon of water from 15 to the 

 boiling-point is equivalent to that which would raise a 

 weight of nearly 1 J tons from the pavement to the top 

 of St Paul's Cathedral. 



We have defined a calorie as the quantity of heat 

 needed to raise the temperature of 1 g. of water by 

 1 C. Since 1 foot = 30'48 cm., and 1 Ib. = 453'6g., 

 1 calorie is equivalent to 



(1395 x 30-48 x 453'6) * 453'6 = 42,500 

 gram-centimetres, i.e. to raise the temperature of 1 g. 

 of water by 1 C. requires the expenditure of as much 

 energy as would raise a weight of 425 g. to a height of 

 1 metre. 



134. We have already shown that hot bodies give 

 up their heat to their surroundings, if these be at 



