Physics · Ch 8 — Heat and Thermodynamics
Joule's Mechanical Equivalent of Heat
Joule's Mechanical Equivalent of Heat
James Prescott Joule demonstrated experimentally, in the eighteenth century, that mechanical energy can be converted into internal energy and back again, disproving the earlier "caloric fluid" theory that wrongly treated heat as a substance a hot object simply contained more of. In his classic paddle-wheel experiment (Figure 8.19), two falling masses connected by a rope over pulleys to a paddle wheel submerged in an insulated container of water lose gravitational potential energy as they descend through height ; this turns the paddle wheel, and friction between the paddle and the water converts the resulting rotational kinetic energy into internal energy, raising the water's measured temperature. Joule found that 4.186 J of mechanical energy raises 1 g of water by 1°C, giving -- historically called "Joule's mechanical equivalent of heat," though since what is really converted is mechanical energy into internal energy (via the paddle' …
What this figure shows. An insulated container of water with a paddle wheel submerged inside it, connected by a rope running over one or more pulleys to two hanging masses m outside the container. As the two masses m fall through a measured height h under gravity, the rope turns the paddle wheel inside the water; a thermometer inserted into the container reads the resulting rise in the water's temperature. The figure is the classic setup used to measure the mechanical equivalent of heat: the loss in gravitational potential energy of the falling masses (2mgh) is compared against the measured temperature rise of the water to establish tha …