Q.4.18 Joule of work is equivalent to ............. calories of heat.

In the eighteenth century, James Prescott Joule demonstrated experimentally that mechanical energy can be converted into internal energy and vice versa, disproving the earlier "caloric fluid" theory (which wrongly treated heat as a substance that hot objects contained more of). In his classic experiment, two masses attached by a rope over pulleys to a paddle wheel immersed in an insulated container of water are allowed to fall through a height $h$; as they fall, they lose gravitational potential energy $2mgh$, which turns the paddle wheel, and friction between the paddle and the water converts this mechanical energy into internal energy of the water, raising its temperature. Joule found that to raise the temperature of 1 g of water by 1°C requires 4.186 J of mechanical energy -- so 1 calorie = 4.186 J …