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Heat

1. Heat:-

Heat is the form of energy the gives the sensation of warmth to the body.

It is produced due to molecular vibrations.

It is the such form of energy that always flows from hotter bodies to colder

bodies when they are kept in thermal contact.

According to molecular energy, “Heat is defined as the sum of kinetic energy of all the molecules present in a body.”

 

2. Instrument to measure Heat:-

Heat energy present in a body is measured by using a device called Calorimeter.

3. Units of Heat:-

After the measurement, heat is expressed in the unit of Joule (J) in SI system.

Where it is measured in calorie (cal.)unit in CGS system

The relation between Joule and CGS unit is:-

1 Calorie = 4.2 Joule

4. Good Conductors:-

Those substance that can transfer heat and electricity through them very easily are called good conductors.

For Example:-

Copper, Iron, Gold, Silver, Aluminum etc.

5. Bad Conductor or Insulator:-

Those substance that can not transfer heat and electricity through them are called bad conductors or insulators.

For Example:-

Plastic, Glass, Dry wood, etc.

6. Transformation of Heat:-

As we know that heat energy always flows from hotter bodies to colder bodies when they are kept in thermal contact so,

“The process of transformation of heat energy from hotter body to colder body when they are kept in thermal contact is called transformation of heat.”

Transformation of heat takes place from three different ways.

They are:-

(i) Conduction.

(ii) Convection

&

(iii) Radiation.

(i) Conduction:-

If the transmission of heat takes place from one end to another end on a solid body without the movement of molecules is called conduction.

In this method, molecules do not move but passes the energy to the neighbor molecules. This method of transmission of heat takes place in solids because molecules cannot move from one place to another in solids since they are very tightly arranged in solids.

(ii) Convection:-

If the transmission of heat energy takes place from one end to another end by the actual movement of molecules is called convection.

Molecules in the liquid and gas medium are not tightly packed and can move freely so this method is possible in liquid and gas medium.

(iii) Radiation:-

If the transmission of heat energy takes place in the absence any

material medium is called radiation.

By this method, transmission of heat energy takes place from the Sun to

Earth.

7. Sources of heat:-

Those substances that produce heat from their own surfaces are called as sources of heat.

For Examples:-

Heater, burning firewood, burning fuel, Burning coal. etc.

8. Uses or Importance of Heat:-

Some uses or importance of heat are as follows: -

· It is used to cook food in our daily life.

· It is used to run various types of machines and vehicles.

· It is used to dry different food grains during harvesting.

· It is used to keep our body warm.

· It is used to dry our wet clothes.

· It is used to obtain metals from their respective ores.

· It is used to refine crude oil to obtain petrol, diesel, kerosene etc.

9. Effects of Heat:-

Some important effects of heat on a body are as follows: -

· It changes the states of matter.

· It causes the expansion in volume of substance.

· It increases the temperature of the substance.

· It brings chemical change in the substances.

· It increases the rate of chemical reaction. Etc.

10. Temperature:-

Temperature of a body is defined as the degree of hotness or coldness of a body.

And according to kinetic theory of molecules*,* “Temperature is defined as the average kinetic energy of molecules present in a body.”

11. Instrument to measure temperature:-

Temperature of a body is measured by using a device called Thermometer.

12. Units of temperature:-

After the measurement, temperature is expressed in the various units like:-

· Degree Celsius or Degree Centigrade (0C).

· Degree Fahrenheit (0F).

· Kelvin (K).

But the SI unit of temperature is Kelvin (K).

13. Differences between heat and temperature:-

Heat Temperature
§ It is the form of energy that gives the sensation of warmth to the body. § It is the degree of hotness or coldness of the body.
§ It is measured by using a device called calorimeter. § It is measured by using a device called thermometer.
§ Its SI unit is joule (J). § Its SI unit is kelvin (K).
§ It is a cause in change in temperature. § It is an effect.

14. Thermometer:-

Thermometer is a device that is used to measure the temperature of different substances and living beings including human body.

15. Principle of Thermometer:-

Thermometer is constructed on a principle that is, “Liquid expands on heating and contracts oncooling.”

16. Thermometric liquids:-

Those liquids that are used within the thermometer to measure the temperature of different substances including human being are called thermometric liquids. Alcohol and Mercury are the examples.

17. Characteristics of thermometric liquids: -

Mercury and alcohol are the common thermometric liquids.

a. Mercury:-

· It issliver and shiny white in colour. So,it can be seen clearly in the glass.

· It is good conductor of heat. So, it transmits heat equally in all directions.

· It does not make the wall of glass tube wet. So, its rise and fall in the tube is clear.

· Its freezing point is -39°C so it cannot measure temperature below its freezing point.

· Its boiling point is 357°C. Therefore, it measures the temperature up to 357°C.

· The expansion and contraction.

b.Alcohol:-

· It is colourless liquid.So, colour should be providedbefore its use.

· It is bad conductor of heat so it does not expand and contract uniformly.

· It makes the wall of glass tube wet. Therefore, its movement within the tube is not clear.

· Its freezing point is -117°C so it can measure very cold temperature up to -117°C.

· Its boiling point is 78°C. Therefore, we cannot measure the temperature above 78°C.

· The expansion rate of alcohol is six times more than that of mercury.

18. Differences between mercury and alcohol:-

Some differences between mercury and alcohol are as follows.

Mercury Alcohol
§ It is good conductor of heat. § It is bad conductor of heat.
§ It is shiny white in colour. § It is colourless liquid
§ It does not make the walls of glass tube wet. § It makes the wall of glass tube wet.
§ Its boiling point is 3570C. § Its boiling point is 780C.
§ Its freezing point is – 390C. § Its freezing point is -1170C.

19. Types of Thermometer:-

On the basis of liquid used within it, there are two types of thermometer.

They are:-

(i) Mercury Thermometer.

&

(ii) Alcoholic Thermometer.

(i) Mercury Thermometer:-

The thermometer inside which mercury is used as a thermometric liquid is called mercury thermometer.

(ii) Alcoholic Thermometer:-

The thermometer inside which alcohol is used as a thermometric liquid is called alcoholic thermometer.

20. Types of Thermometer:-

On the basis of function, there are three types of thermometer.

They are:-

(i) Clinical Thermometer.

(ii) Laboratory Thermometer.

&

(iii) Maximum- Minimum Thermometer.

(i) Clinical Thermometer:-

The thermometer that is used to measure human body temperature in the clinic or hospital is called clinical thermometer.

It is also called as doctors’ thermometer.

image-20200901093008362

Figure:- Clinical Thermometer.

*Some features of this thermometer are as follows: -*

· It is prismatic in shape.

· It has short scale range i.e. from 350C to 420C or 940F to 1080F.

· It consists of mercury within it as a thermometric liquid.

· It has constriction or kink near the bulb.

(ii) Laboratory Thermometer: -

The thermometer that is used to record the melting and boiling point of various substances in the lab during different experiments is called laboratory thermometer.

image-20200901093032926

Figure:- Laboratory Thermometer.

*Some features of laboratory thermometer are as follows: -*

· It is cylindrical in shape.

· It has long scale range i.e. from -100C to 1100C.

· Some of them consist alcohol whereas some of them consist mercury as a thermometric liquid within it as thermometric liquid.

· It has no constriction or kink near the bulb.

(iii) Maximum – minimum thermometer:-

The thermometer that is used to measure the maximum

and minimum temperature of a particular place within 24 hours is called maximum and minimum thermometer.

image-20200901093051746

Figure: - Maximum- Minimum Thermometer.

*Some features of maximum- minimum thermometer are as follows: -*

· Both thermometric liquids (mercury & alcohol) are used in the same

thermometer.

· It is U shaped in structure.

· Two movable metal indexes are kept within it to separate alcohol and

mercury.

· Partial vacuum is left in one of the arms.

21. Differences between clinical and laboratory Thermometer: -

Clinical thermometer. Laboratory thermometer.
§ It is used to record human body temperature in the hospitals. § It is used to record temperature of different substances in the lab.
§ It has kink or construction near the bulb § It has no kink or constriction near the bulb.
§ It has short scale range. § It has long scale range.
§ It has mercury inside it. § It has either mercury of alcohol inside it.
§ It is prismatic in shape. § It is cylindrical in shape.

22. Upper fixed point: -

The temperature of boiling point of pure water at standard atmospheric Pressure is called upper fixed point.

It is taken 100 in degree Celsius.

23. Lower fixed point: -

The temperature of melting point of pure ice at standard atmospheric pressure is called lower fixed point.

It is taken as 0 in degree Celsius.

24. Calibration of Thermometer: -

The process of division of distance between upper fixed point and lower fixed point in to 100 or 180 equal parts for different temperature scales is called calibration of thermometer.

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This content is contributed by:

Dipak Dangi