Note on Thermometry

  • Note
  • Things to remember

Heat and Temperature

Heat is a form of energy which stimulates our sense organs so that we feel hot or cold. The energy transferred from one object to another or within a single object from one place to another because of the temperature difference between source and destination is called heat. The internal energy of a substance is the sum of molecular kinetic energy, molecular potential energy.

The degree of hotness or coldness of a body is called temperature. It is the property which determines whether the body is in thermal equilibrium or not with a neighbouring system. If the systems are in thermal equilibrium, this common property of the system can be represented by a single numerical value called temperature.

HeatHeat

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Heat

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Temperature

1.

It is a form of energy.

1.

It is a degree of hotness or coldness.

2.

It is a cause.

2.

It is an effect.

3.

Its unit is joule in SI unit and calorie in CGS unit.

3.

Its unit is kelvin in SI unit and degree Centigrade in CGS unit.

4.

Two bodies can be in thermal equilibrium without having the same amount of heat.

4.

Two bodies cannot be in thermal equilibrium without having the same temperature.

5.

It is a measure of a total kinetic energy of all molecules of a body.

5.

It is a measure of an average kinetic energy of all molecules of a body.

6.

Flow of heat is independent of an amount of heat energy contained in the bodies in thermal contact.

6.

Flow of heat is dependent on the temperature of two bodies. It flows from a body at higher temperature to the body at a lower temperature.

Thermometry

Thermometry is a branch of physics which deals with the measurement of temperature. The device which is used to measure the temperature is called thermometer. Types of thermometer are:

  1. Liquid thermometer
  2. Gas thermometer
  3. Resistance thermometer
  4. Magnetic thermometer
  5. Radiation thermometer
  6. Thermoelectric thermometer

Liquid thermometer

The thermometric substance is liquid and it’s based on a principle of expansion of the liquid.

Gas thermometer

The thermometric substance used in this type of thermometer is in a gaseous state . It is based on a principle that for the constant volume the pressure of a gas depends on upon thermometer. It is more sensitive than the liquid thermometer. It is more sensitive than liquid thermometer because the expansivity of the liquid is greater than that of liquid.

Resistance thermometer

The thermometric substance is a resistor which consists of a length of fine wire. It is based on the principle that the resistance of substance changes with the change in temperature.

Magnetic thermometer

The thermometric substance is magnet based on the principle that magnetic substance depends on temperature. It is used to measure very low temperature up to 3 Kelvin.

Radiation thermometer

Radiation thermometer is used to measure very high temperature. These are based on the quantity of heat radiation emitted by a body. These are also called pyrometers. High temperatures can be measured using them.

Vapour pressure thermometer

These are based on the principle of change of vapour pressure with change in temperature. These are used to measure low temperature eg. helium vapour pressure thermometer etc.

Types of thermometer according to liquid used in the thermometer are:

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Mercury thermometer

In this thermometer, the thermometric liquid is mercury. At one end, mercury is sealed into a glass bulb that is connected to the fine capillary in a glass rod. When the temperature is increased, the mercury thermometer expands in a capillary tube. Though the volume of the glass bulb and mercury both increase, the amount of expansion of mercury is greater than that of glass and the mercury is forced to expand in the tube as the temperature increases.

Advantage of mercury thermometer
  1. Its boiling point is 357oC and freezing point -39oC.
  2. Its expansion is nearly uniform over the ordinary range of temperatures. This makes the calibration of thermometer easier.
  3. It can be easily seen in the glass as it is an opaque object.
  4. It does not wet the glass and can be found in a pure state.
  5. It has low specific heat, so it doesn’t take much heat the body whose temperature is to be measured.
Disadvantage of mercury thermometer

Mercury thermometer cannot measure the temperature -39oC and above 357 oC. The thermometer in the Artic and Antarctic region, this thermometer cannot be used.

Alcohol thermometer

The constriction of alcohol thermometer is same as the construction of alcohol thermometer. Alcohol is used as a thermometric liquid.

Advantages
  1. Alcohol thermometer is suitable in very cold places because it freezes at -117oC and boils at 78oC.
  2. It is more sensitive than mercury because its expansion is seven times that of mercury.
Disadvantages
  1. Its expansion is not uniform.
  2. It cannot be used to measure the temperature more than 78oC because its BP is 78oC.
  3. It is the colourless and bad conductor of heat.
  4. The specific gravity of alcohol is smaller than that of mercury.
  5. It sticks to the wall of the glass tube.

Reference

https://en.wikipedia.org

  • Heat is a form of energy which stimulates our sense organs so that we feel hot or cold.
  • The flow of heat is dependent on the  temperature of two bodies. It flows from a body at higher temperature to the body at a lower temperature.
  • Thermometry is a branch of physics which deals with the measurement of temperature. 
  • Gas is thermometric substance. It is based on a principle that for the constant volume the pressure of a gas depends on upon a thermometer. 
  • In resistance thermometer, the thermometric substance is a resistor. It is based on the principle that the resistance of substance depends on temperature.
  • In magnetic thermometer, the thermometric substance is magnet based on the principle that magnetic substance depends on temperature. 
  • Radiation thermometer is used to measure very high temperature. 
  • When the temperature is increased, the mercury thermometer expands in a capillary tube.
  • The construction of alcohol thermometer is same as the construction of alcohol thermometer. 
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maryam ibrahim

what are the advantage of using mercury over alcohol as a thermometric substances


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