Liquid-in-Glass Thermometers

Liquid-in-Glass Thermometers

We discuss liquid-in-glass thermometers in some detail. Commonly there are two main types of liquid-in-glass thermometers. They are

(a) Mercury-rn-glass thermometer

(b) Alcohol-in-glass thermometer

Here we give a list of main features of mercury and alcohol.

Mercury

It is a good conductor of heat. It expands uniformally on increasing its temperature and contracts on decreasing its temperature. Hence mercury is used in a liquid-in-glass thermometer

1. Freezing and boiling point

Mercury freezes at —39°C and boils at 360° C. Hence it can be used to measure a relatively long range of temperature.

2. Colour of Mercury

It has a shining silvery colour. Hence no colouring is needed to read the temperature in the thermometer.

3. Non-wetting Property

Mercury does not wet the glass surface of the thermometer tube. Hence the reading can easily taken. Since the meniscus is convex, the reading is taken at the top of the meniscus.

4. Thermal Conductivity

Since mercury has high thermal conductivity, it responds quickly to the change of temperature.

5. Thermal Expansion

Although mercury is a good conductor of heat, its thermal expansion is only one fourth of that of alcohol. Hence alcohol expands 4 times more than mercury for the same rise in temperature.

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Alcohol

Alcohol

It has temperature dependent property. It expands on increasing the temperature by heating and contracts on decreasing the temperature by cooling. Hence alcohol is used as a thermometric substance. It has the following main features:

1. Freezing and Boiling Point

Alcohol freezes at —115°C and boils at 78°C. Hence alcohol thermometers are not suitable for laboratory uses as the temperature encountered are usually alone the boiling point of alcohol. It can be used in polar regions where the temperatures are usually in the neighborhood of —40°C.

2. Color of Alcohol

It is a colorless liquid. It is usually colored red when used in a thermometer.

3. Wetting Property

Alcohol wets the internal glass surface. When it contracts, it leaves same alcohol sticking to the capillary tube. Due to this property reading, cannot be taken clearly.

4. Thermal Conductivity

It has a low thermal conductivity. Hence it takes a long time as compared to mercury to attain the temperature of the surroundings.

5. Thermal Expansion

Its expansion is nearly four times the expansion of mercury for the same rise of temperature. Hence, the scale divisions can be made larger in size, making it easier to read.

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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.

In hot countries as well, the windows are made with double doors. Heat can not flow in from outside because of the presence of air between the two doors.

(4) When a stopper, fitted tightly to the bottle is to be remained, the neck of the bottle is gently heated. It expands slightly on heating. Since glass is a bad conductor of heat, the heat does not reach the stopper. Thus it can be removed easily.

(5) Davy’s Safety Lamp: It is one of the most important applications of conduction of heat. The principle of Davy’s safety lamp can be understood from a simple example in which a wire gauze is placed over a Bunsen burner. The gas coming from the burner is lit above the wire gauze.

A flame appears at the top surface of the wire gauze. The gas coming out from the burner below the wire gauze does not get sufficiently heat for ignition. The reason is that the wire gauze conducts away the heat of the flame above it and so the temperature at the lower surface of the gauze does not reach the ignition point.

In Davy’s safety lamp, a cylindrical metal gauze of high thermal conductivity surround the flame. When this lamp is taken inside a mine, the explosive gases present in the mine are not ignited because the wire gauze in the form a cylinder conducts away the heat of the flame of the lamp. The result is that the temperature outside the gauze remains below the ignition point of the gases. In absence of the wire gauze the gases outside can explode.

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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.

2. Trade Wings

At the equator the surface of the earth gets heated more than at the poles. This results in the movement of the warm air from the equator to the poles of the cold air moves towards the equator. Because of the rotation of the earth (from west to east) the air in the Northern Hemisphere seems to be coming from north-east instead from north. In the South Hemisphere the air from the south hole appear to be coming from south west. These winds are called trade winds because in olden days these winds were used by traders for sailing their ships.

3. Land and Sea Breezes

Land is a better conductor of heat than water. Hence in day-time the land gets hotter than water in the sea. The air above the land becomes warm and rises up being lighter and some what cold air above sea surface moves towards the sea short. This is known as seabreeze. Thus convection currents fair our set-up. During night land becomes cooler than water and so the warm air over the surface of water in. the sea rises up. The air on the land near the sea shore begins to move towards sea side and is called land breeze.

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