Chapter 15, Problem 1CQ

Describe the photo of the tea kettle at the beginning of this section in terms of heat transfer, work done, and internal energy. How is heat being transferred? What is the work done and what is doing it? How does use kettle maintain its internal energy?

To determine

(i) How the heat is transferred to the kettle.

(ii) The amount of work done and by what agents

(iii) How the kettle maintains its internal energy.

Answer to Problem 1CQ

(i) The heat is transferred from the stove to the kettle.

(ii) Work is done from the evaporation of the water to the whistling of the kettle.

(iii)The kettle maintains its internal energy by heating the water inside it.

Explanation of Solution

Introduction:

The kettle on the stove takes thermal energy from the stove and uses it to convert water into steam at constant temperature. As the steam escapes, the kettle sounds a whistle.

The kettle in the picture is placed on the stove. The stove burns fuel and converts the chemical energy stored in the fuel into thermal energy. The base of the kettle is a good conductor of heat. The base of the kettle absorbs heat and transfers it to water inside the kettle. Thus the heat energy provided by the kettle is transferred to the water inside the kettle.

Water absorbs energy and its temperature increases. The molecules of water vibrate with increasing speeds when the temperature of water increases. When the temperature of water reaches the boiling point, water starts to change its state to steam. The molecules in steam are farther apart when compared to the molecules of water. Work is done against the intermolecular forces between the molecules of water, when water converts into steam. The energy for this purpose is taken from the heat energy supplied to the kettle.

As heat is continuously provided to water, more and more water vaporizes and the pressure inside the kettle increases. The spout of the kettle has two thin plates separated by a small gap, which allows the steam to escape. As the steam escapes through the narrow gap, it expands and in the process generates small vortices at the gap, which produces its characteristic whistle. When steam expands, the intermolecular separation further increases hence work is done. The energy of the steam is also converted to audible energy in the form of the whistle.

When water changes state, it does so at constant temperature. Internal energy of a body is proportional to its Kelvin temperature. Thus, the internal energy of the kettle remains constant. According to the first law of thermodynamics,

  $\Delta U=Q-W$

Here the change in the internal energy is $\Delta U$ , the heat given to the kettle is Q and W is the total work done by the system.

As the entire system gets hotter, work is done from the evaporation of the water to the whistling of the kettle.

As the base of the kettle absorbs heat and transfers it to water inside the kettle and this is how the kettle maintains its internal energy by heating the water inside it.