Thermos Flask

Thermos Flask

A thermos flask is a device where all the three modes of transfer of heat are applied. It consists of a double walled glass bottle. The inner surface of the outer wall and the outer surface of the inner wall are highly polished. The space between the walls is evacuated and scaled. The whole system is enclosed with in a metal case which is provided with a cork at the bottom and a pad of felt at the neck for safety. Glass is a poor conductor of heat where as air, cork, felt etc. which are used in thermos flask are all had conductors of heat. Hence they prevent any loss of due to conduction. When a hot liquid is kept in the bottle, it remains hot for a long time. Any heat radiation coming from the hot liquid is reflected back from the inner surface of the outer wall. Also the heat from the liquid can not flow out through conduction or convection because of the empty space between the walls. Thus all the three processes of heat-transfer fail to transfer heat. Any heat energy through radiation conduction, convection or coming from outside cannot reach the liquid because of the highly polished surfaces and the empty space between them.

Boy’s Radio Micrometer

Boy’s Radio Micrometer

It is a very sensitive device and can detect heat radiation of very weak intensity from example radiation coming from a distant candle can be detected. IL is a combination of a moving coil galvanometer and a thermocouple. It consists of a single loop of silver or copper wire A. The lower ends of the wire are soldered to a copper disc which is coated with lamp black. The disc is exposed to heat radiation and as a result thermo-electric current of produced in the couple made of hismuth and antimony, say, begins to flow in the wire A. Hence we get a current in the galvanometer. The deflection produced in the galvanometer can be measured by using lamp and scale arrangement.

Applications of Heat Radiation

Applications of Heat Radiation

(1) Differential Air Thermo scope:

It is an important application of radiation of heat. It consists of two identical glass bulbs A & B which are connected by a narrow glass tubing have the shape of a U-tube. The tube consists of sulfuric acid (so as to absorb moisture of the air in. the tube). The space above the levels of the acid in the two arms of the tube contains air. When the bulbs are at the same temperature there is no difference in the level of the acid in the two limbs. The bulb A is coated with lamp black so that it may completely absorb the heat radiation falling on it. Now the bulb “A” is exposed to heat radiation. It absorbs whole of the radiation

falling of it. As a result the air in the bulb A gets heated, expands and presses down the acid in the limb. Thus we have a difference in the level of the liquid in the two limbs. This thermo scope is very sensitive and can detect radiation of very weak intensity for example the radiation coming from a distant candle.

Practical Applications of Convection of Heat

Practical Applications of Convection of Heat
Here we give a few applications of convection of heat.

(1) Ventilation
From health point of view every lining room of a building should be provided with ventilators near the ceiling. Due to respiration of the persons sitting or sleeping in the room the air in the room gets warmer and hence is less dense. It rises up and goes out side through the ventilator. Fresh air comes in the room through the doors and windows. Thus a convection current of air is maintain.

Practical Application of Conduction of Heat

Practical Application of Conduction of Heat

(1) Ice box has a double wall, made of tin or iron. The space between the two walls is filed with cork or felt which are poor conductors of heat. They prevent the flow of out side heat into the box, thus keeping the ice from melting.

(2) Woolen clothes have fine pores filled with air. Air and wool are bad conductors of heat. Thus the heat from the body does not flow out to the atmosphere. Thus the woolen clothes’ keep the body warm in winter.

(3) In cold countries windows are provided with double doors. The air in the space between the two doors forms a non conducting layer and so heat cannot flow out from inside the room.

Latent Heat

Latent Heat

In general when heat is added to a body, its temperature rises. For example when water is heated, its temperature rises. If we place a bowl of water in the sun for some time, its temperature rises because it absorbs heat-radian from the sun. But in certain situations even with the absorption of heat, temperature does not rise. This happens when the body changes its phase (from solid to liquid or from liquid to gas etc). During the change of phase (or state) the temperature of the body remains constant. With reference to the constancy of temperature, the added heat is called the latent heat or hidden heat (latent means hidden).

All solid substances require latent heat for fusing or melting. The latent heat of fusion depends on the nature of the substance. Here we give a table of melting and boiling points of certain substances.

Specific Heat Capacity of a Liquid

Specific Heat Capacity of a Liquid

The method adopted to determine, the specific heat capacity of a solid can be used for the determination of specific heat capacity of a liquid. Here the calorimeter is filled partially with the given liquid instead of water. A metal piece of specific heat capacity is heated in a hypsometer to a constant temperature and then dropped into the calorimeter containing the liquid. In this case the hot metal pieces loses heat whereas the liquid and the calorimeter gain heat. Using the law of heat exchange, the specific heat capacity of the liquid can be calculated.

Measurement of Specific Heat Capacity

Measurement of Specific Heat Capacity

The branch of heat which is concerned with the measurement of heat is called Calorimeter. A calorimeter consists of a copper vessel which is enclosed inside a wooden box. The space between these two is filled with insulating materials like paper, straw, wooden fittings, rubber, foam etc. The calorimeter is so designed as to minimize all types of heat losses. The top of the calorimeter is closed by a wooden lid which consists of two holes through which a thermometer and a stirrer are allowed to pass tightly. To measure the specific heat capacity of a solid substance like a metal piece the following procedure is adopted. First of all the mass of the calorimeter with stirrer is measured with a physical balance. After that nearly two-thirds (2/3) of the calorimeter is filled with water. The mass of the calorimeter with water is measured. The room temperature of the calorimeter with water is found by the thermometer. The metal piece is heated in a steam bath called “hypsometer” till it attains the temperature of the steam (generated at atmospheric pressure). It is then gently dropped into the water contained in the calorimeter which is now covered with the lid. The temperature of water begins to rise. The water is stirred so as to keep its temperature uniform throughout. The process of stirring is continued till the thermometer fitted in the lid of the calorimeter attains a final steady temperature. This temperature is noted. The calorimeter with water and metal piece (forming a mixture) is again weighed.