Joule's Law Of Heating - Practice Questions & MCQ

Joule's experiments conclusively established that heat is a form of energy. Spending a given amount of mechanical work, always the same amount of heat will be produced. It does not depend on what type of arrangement is used for doing mechanical work.

• The main aim of his investigations was to determine exactly the ratio between the work done and the quantity of heat produced.

Apparatus used in Joule's experiment - 

• An insulated container has vanes
• One paddle submerged in the liquid
• A mass attached to the paddle through a pully and rope
• A thermometer

Assumption - 

The friction between the pulley and the rope is negligible.

Procedure - 

• A paddlewheel 2 was submerged in the heat-insulated vessel 1 to the walls of which vanes 3 were fastened, the vanes interfering with the motion of water due to rotation of the paddle.

• Now, when block G falls down, it will cause the rotation motion of the paddle, the block is connected to the paddle by means of a rope and pulley 5.

• As the block falls through a distance Δh, the work done by it is equal to the decrease in the potential energy of the block.

• Because of this work, the kinetic energy of the molecules of the fluid gets increased and the temperature rises. The heat liberated in the water-filled vessel is calculated from the rise in water temperature, measured with the help of a thermometer.

Importance of Joule’s Experiments:

• This experiment shows that heat is a form of energy.

• Always the same amount of heat was produced by spending a given amount of mechanical work. It does not depend on what type of arrangement is used for doing mechanical work.

• It develops the relationship between Joule and Calories.

Whenever heat is converted into mechanical work or mechanical work is converted into heat, then the ratio of work done to the heat produced always remains constant. i.e. $J=\frac{W}{Q}$

This is Joule’s law and J is called the mechanical equivalent of heat.