Question

Explain Joule’s Experiment of the mechanical equivalent of heat.

Answer 1

Joule’s mechanical equivalent of heat: The temperature of an object can be increased by heating it or by doing some work on it.

In the eighteenth century, James Prescott Joule showed that mechanical energy can be converted into internal energy and vice versa.

In his experiment, two masses were attached with a rope and a paddlewheel as shown in Figure. When these masses fall through a distance of h due to gravity, both the masses lose potential energy equal to 2 mgh. When the masses fall, the paddlewheel turns. Due to the turning of the wheel inside the water, the frictional force comes in between the water and the paddlewheel.

Joule’s experiment for determining the mechanical equivalent of heat energy

This causes a rise in the temperature of the water. This implies that gravitational potential energy is converted to the internal energy of water. The temperature of water increases due to the work done by the masses. In fact, Joule was able to show that the mechanical work has the same effect as giving heat. He found that to raise 1 g of an object by 1°C, 4.186 J of energy is required. In earlier days the heat was measured in calories.

1 cal = 4.186 J

This is called Joule’s mechanical equivalent of heat.