CBSE Class 8 Science Heat Chapter Notes

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Heat

Heat is a form of energy. It makes a substance hotter. Heat is measured by the temperature effect which it produces. When heat is given to a substance, its temperature increases (it becomes hotter); and when heat is removed from a substance, then its temperature decreases (it becomes cold). Actually, when we supply heat energy to a substance, the kinetic energy of its molecules increases and they move with greater speeds at higher temperatures. On the other hand, if we remove heat energy from a substance, then the kinetic energy of its molecules decreases and they move with lesser speeds at lower  temperatures. Thus, we can consider heat is energy of molecules motion. Heat flows from a hotter substance to a colder substance. Heat
energy is measured in the units of joules (J). We will now discuss the temperature. Temperature is the degree of hotness (or coldness) of a substance. A hot substance is said to have a high temperature whereas a cold substance is said to have a low temperature. The temperature of a substance is an indicator of the average kinetic energy of the molecules of the substance. In other words, we can also say that temperature is a measure of heat energy level. The temperature of a body governs the flow of heat. Heat always flows from a body at higher temperature to a body at a lower temperature. The heat keeps on flowing until both the bodies attain the same temperature. Temperature is measured in the units of degrees Celsius (°C) or kelvin (K).

We usually get confused between the terms ‘heat’ and ‘temperature’. Let us take an example to understand their difference. Suppose we have a vessel of boiling water and a red-hot spark from a fire. Now, the boiling water contains much more heat energy than the spark from a fire but its temperature is much less than the spark of fire. The high temperature spark from a fire will not burn our hand much (because it contains much less heat energy) but the boiling water will burn our hand badly (because it contains much more heat energy). We should remember the following points of different between heat and temperature.

Temperature Scales
Just as two fixed marks (of 0 cm and 100 cm) are required on a metre scale in the measurement of length, in the same way, two fixed temperatures are required for a temperature scale in the measurement of temperature. These two “fixed temperatures” are called “fixed points” of the temperature scale. The two fixed temperatures (or fixed points) which can be easily reproduced are:

i) The temperature at which pure ice melts under standard atmospheric pressure is taken as the lower fixed point of temperature scale. In other words, the melting point of pure ice is taken as the lower fixed temperature.

ii) The temperature at which pure water boils under standard atmospheric pressure is taken as the upper fixed point of the temperature scale. In other words, the boiling point of pure water is taken as the upper fixed temperature.

When water boils, then steam is formed. In fact, the boiling water and the steam formed from it are at the same temperature. So, we can also say that: The temperature of steam formed from water boiling under standard atmospheric pressure is taken as the upper fixed point of the temperature scale.

Please note that it is necessary to write standard atmospheric pressure in the definition of lower fixed point and upper fixed point because both, the melting point of ice and boiling point of water vary with the change in atmospheric pressure.

The distnace between the lower fixed point and the upper fixed point is called fundamental interval. The fundamental interval is divided into a number of equal divisions. Each division is called ‘one degree’.

There are two temperature scales which are commonly used in the measurement of temperature these days.
These are:
i) Celsius scale of temperature and
ii) Kelvin scale of temperature

The kelvin temperature scale is also called absolute temperature scale. We will now discuss both these temperature scales, one by one. Let us start with the Celsius scale of temperature.

Celsius Scale of Temperature
Celsius scale is a convenient, metric scale of temperature which was given by Celsius. On Celsius scale of temperature, the melting point of ice is given the value of 0° (zero degree), and the temperature of boiling water (or steam) is given the value of 100° (hundred degrees). In other words, on Celsius scale the lower fixed point of 0°C and the upper fixed point is 100°C, so that there are 100 – 0 = 100 equal divisions or 100 degrees between the
two fixed temperatures divisions or 100 degrees between the two fixed temperatures (see figure). So, in this case, the fundamental interval is divided into 100 equal divisions (or degrees). The two fixed temperatures of the Celsius scale have been given simple values of 0 and 100, and there are 100 degrees in it, so it is quite easy to do calculations by using Celsius temperatures. So, for convenience, the temperatures in laboratory and in everyday life are measured in ‘degrees Celsius’ (°C).

Please note that if a Celsius thermometer is needed to measure the temperatures below the lower fixed point (below 0°C), then the scale markings are continued below 0°C mark. The temperatures below 0°C are written with a minus sign (like : – 10°C or –20°C, etc.). similarly, to measure temperatures higher than the upper fixed point (above 100°C) the

scale markings are contain and above the 100°C mark. The distance between consecutive marks on the Celsius scale remains the same. We will now discuss the kelvin scale of emperature.

Kelvin Scale of Temperature

The temperatures on the Celsius scale can be positive (above 0°C) as well as negative (below 0°C). We will now describe the kelvin scale of temprature on which all the temperatures are always in positive figures. There can be no minus temperatures on the kelvin scale. This point will become clear from the following discussion

It has been found that the lowest temperature which can be attained (or reached) in a laboratory is minus 273°C (–273°C). This lowest possible temperature is called ‘absolute zero’. Thus: The temperature of – 273°C (which is the lowest temperature that can be reached) is taken as the 0 (zero) of the kelvin scale (see figure). The size of each division on the kelvin scale is the same as that on the Celsius scale, so:

–273° celsius = 0 kelvin

or –273°C = 0 K

And hence, 0°C = 273 K

Thus, a temperature of 0° on celsius scale is equal to 273 on the kelvin scale. It should be noted that the world ‘degree’ or its sign (°) is not written while writing the kelvin temperatures. We just write K for kelvin. For example, we write 273 K, and not 273°K.

The temperatures measured on Celsius scale can be converted into kelvin temperatures just by adding the figure 273 to each reading on the celsius
scale. For example:

100° celsius = 100 + 273 kelvin

                   = 273 kelvin

or 100°C      = 273 K

We will now describe the lower fixed point and upper fixed point of the kelvin scale of temperature which was given by Kelvin.

The lower fixed point on the kelvin scale is also the temperature of pure melting ice under standard atmospheric pressure and its value if 273 K. This means that the melting point of ice has a value of 273 K on the kelvin scale of temperature.

The upper fixed point on the kelvin scale is also the temperature of boiling water (or steam) under standard atmospheric pressure and its value is 373K. This means that the boiling point of water (or steam)has a value of 373 K on the kelvin scale.
A comparison between the celsius and kelvin scales of
temperature is shown in figure.
It is clear from Figure that:
–273°C = 0 K
0°C = 273 K
and 100°C = 373 K

Please note that one of the advantages of the kelvin scale over the celsius scale is that all the temperatures on the kelvin scale are in positive figures. Another point to be noted is that kelvin is the SI unit of temperature which is denoted by the letter K. And since the fundamental intervals of celsius scale as well as kelvin scale have been divided into 100 divisions each, therefore, 1 degree Celsius is equal to 1 division of the kelvin scale. These days kelvin scale of temperature is used for all scientific work.
Relation between Celsius Scale and Kelvin Scale The relation between kelvin scale and celsius scale can be written as:
Temperature on kelvin scale = Temperature on celsius scale + 273
or K = C + 273
where K = Temperature on Kelvin Scale
and C = Temperature on Celsius scale
This relation can be used to convert a celsius temperature into kelvin temperature or a kelvin temperature
into Celsius temperature.
Conversion of Celsius of Fahrenheit Scale: On the Fahrenheit scale the ice-point and the steam-point are called 32°F and 212°F and their interval has been divided in 180 degrees. The following relation holds for conversion of celsius scale to Fahrenheit scale:
C/100 = F-32 /180

C/5 = F-32/9

Transfer of Heat: To carry heat from one part of an object to its other part, or from one object to another object is called transfer of heat. Heat can be transferred from a hot object to a cold object in three different ways:

i) by conduction

ii) by convection and

iii) by radiation

Conduction: Conduction is the transfer of heat from the hotter part of a material to its colder part (or from a hot material to a cold material in contact with it) without the movement of material as a whole.

In all the solids, heat is transferred by the process of conduction.

Examples

i) A cold metal spoon dipped in a hot cup of tea gets heated by conduction.

ii) A frying pan kept on a gas stove transfers the heat of gas through its metal bottom by the process of conduction.

Convection: Convection is transfer of heat from the hotter parts of a liquid (or gas) to its cable parts by the movements of the liquid (or gas) itself.
The transfer of heat by convection can take place only in liquids and gases and not in solids because particles in liquid and gases can move about freely.

Examples i) A room heater heats all the air in a room by setting up convection currents in air.

ii) Water is a poor conductor of heat, so it cannot transfer heat by conduction. Water transfers heat by the process of convection and gets heated up.
Radiation: Radiation is the transfer of heat energy from a hot body to a cold body by means of heat rays, without any material medium between them.

Examples i) The sun’s heat reaches the earth by the process of radiation.

ii) When we stand next to a burning fire, we can feel the heat of the fire. This heat, transferred from the fire to our face obey the process.

Thermodynamic Process

The operation by which a thermodynamic system changes from one state to another is called a thermodynamic process.

i) Isothermal process: A process in which although heat enters or leaves the system yet temperature of the system remains constant throughout the process is called an isothermal process. For an isothermal process, change in temperature (T) = 0. Change of state (e.g. freezing, melting, evaporation and condensation) are all examples of isothermal process.

ii) Adiabatic Process: A process during which no heat enters or leaves the system during any step of theprocess is known as adiabatic process. A reaction carried out in an isolated system is an example of adiabatic process. For an adiabatic process, change on heat (q) = 0 or q remain constant.

iii) Isobaric Process: A process during which pressure of the system remains constant throughout the reaction is called as isobaric process. For example, heating of water to its boiling point, and its vaporisation taking place at the same atmospheric pressure. Expansion of a gas in an open system is an example of isobaric process. For an isobaric process P = 0

iv) Isochoric Process: A process during which volume of the system remains constant throughout the reaction is known as isochoric process. The heating of a substance in a non-expanding chamber or change taking place in a closed system are examples of isochoric process. For an isochoric process, V = 0.

Units of Heat

Like other forms of energy, the S.I. unit of heat is the joule (J).

The other most commonly used unit of heat is the calorie (symbol: cal). It is defined as follows:

One calorie of heat is the quantity of heat required to raise the temperature of 1 g of water through 1°C.

In this definition of calorie, it has been assumed that the heat required to raise 1g of water through 1°C at any temperature is the same. However this is not true. The reaction is that the thermal expansion of water is not uniform near 4°C. Water shows uniform and smooth expansion only beyond 14°C. Hence the correct definition of calorie is given as follows:

One calorie of heat is the quantity of heat required to raise the temperature of 1t of water from 14.5°C
to 15.5°C.

The unit calorie is related to the S.I. unit joule as follows:

scale markings are contain and above the 100°C mark. The distance between consecutive marks on the Celsius scale remains the same. We will now discuss the kelvin scale of emperature.

Kelvin Scale of Temperature

The temperatures on the Celsius scale can be positive (above 0°C) as well as negative (below 0°C). We will now describe the kelvin scale of temprature on which all the temperatures are always in positive figures. There can be no minus temperatures on the kelvin scale. This point will become clear from the following discussion.

It has been found that the lowest temperature which can be attained (or reached) in a laboratory is minus 273°C (–273°C). This lowest possible temperature is called ‘absolute zero’. Thus: The temperature of – 273°C (which is the lowest temperature that can be reached) is taken as the 0 (zero) of the kelvin scale (see figure). The size of each division on the kelvin scale is the same as that on the Celsius scale, so:

–273° celsius = 0 kelvin

or –273°C = 0 K

And hence, 0°C = 273 K

Thus, a temperature of 0° on celsius scale is equal to 273 on the kelvin scale. It should be noted that the world ‘degree’ or its sign (°) is not written while writing the kelvin temperatures. We just write K for kelvin. For example, we write 273 K, and not 273°K.

The temperatures measured on Celsius scale can be converted into kelvin temperatures just by adding the figure 273 to each reading on the celsius
scale. For example:

100° celsius = 100 + 273 kelvin

                   = 273 kelvin

or 100°C      = 273 K

We will now describe the lower fixed point and upper fixed point of the kelvin scale of temperature which was given by Kelvin.

The lower fixed point on the kelvin scale is also the temperature of pure melting ice under standard atmospheric pressure and its value if 273 K. This means that the melting point of ice has a value of 273 K on the kelvin scale of temperature.

The upper fixed point on the kelvin scale is also the temperature of boiling water (or steam) under standard atmospheric pressure and its value is 373K. This means that the boiling point of water (or steam)has a value of 373 K on the kelvin scale.

A comparison between the celsius and kelvin scales of
temperature is shown in figure.
It is clear from Figure that:
–273°C = 0 K
0°C = 273 K
and 100°C = 373 K

Please note that one of the advantages of the kelvin scale over the celsius scale is that all the temperatures on the kelvin scale are in positive figures. Another point to be noted is that kelvin is the SI unit of temperature which is denoted by the letter K. And since the fundamental intervals of celsius scale as well as kelvin scale have been divided into 100 divisions each, therefore, 1 degree Celsius is equal to 1 division of the kelvin scale. These days kelvin scale of temperature is used for all scientific work.
Relation between Celsius Scale and Kelvin Scale The relation between kelvin scale and celsius scale can be written as:
Temperature on kelvin scale = Temperature on celsius scale + 273
or K = C + 273
where K = Temperature on Kelvin Scale
and C = Temperature on Celsius scale
This relation can be used to convert a celsius temperature into kelvin temperature or a kelvin temperature
into Celsius temperature.
Conversion of Celsius of Fahrenheit Scale: On the Fahrenheit scale the ice-point and the steam-point are called 32°F and 212°F and their interval has been divided in 180 degrees. The following relation holds for conversion of celsius scale to Fahrenheit scale:
C/100 = F-32 /180

C/5 = F-32/9

Transfer of Heat: To carry heat from one part of an object to its other part, or from one object to another object is called transfer of heat. Heat can be transferred from a hot object to a cold object in three different ways:

i) by conduction

ii) by convection and

iii) by radiation

Conduction: Conduction is the transfer of heat from the hotter part of a material to its colder part (or from a hot material to a cold material in contact with it) without the movement of material as a whole.

In all the solids, heat is transferred by the process of conduction.

Examples

i) A cold metal spoon dipped in a hot cup of tea gets heated by conduction.

ii) A frying pan kept on a gas stove transfers the heat of gas through its metal bottom by the process of conduction.

Convection: Convection is transfer of heat from the hotter parts of a liquid (or gas) to its cable parts by the movements of the liquid (or gas) itself.
The transfer of heat by convection can take place only in liquids and gases and not in solids because particles in liquid and gases can move about freely.

Examples i) A room heater heats all the air in a room by setting up convection currents in air.

ii) Water is a poor conductor of heat, so it cannot transfer heat by conduction. Water transfers heat by the process of convection and gets heated up.
Radiation: Radiation is the transfer of heat energy from a hot body to a cold body by means of heat rays, without any material medium between them.

Examples i) The sun’s heat reaches the earth by the process of radiation.

ii) When we stand next to a burning fire, we can feel the heat of the fire. This heat, transferred from the fire to our face obey the process.

Thermodynamic Process

The operation by which a thermodynamic system changes from one state to another is called a thermodynamic process.

i) Isothermal process: A process in which although heat enters or leaves the system yet temperature of the system remains constant throughout the process is called an isothermal process. For an isothermal process, change in temperature (T) = 0. Change of state (e.g. freezing, melting, evaporation and condensation) are all examples of isothermal process.

ii) Adiabatic Process: A process during which no heat enters or leaves the system during any step of theprocess is known as adiabatic process. A reaction carried out in an isolated system is an example of adiabatic process. For an adiabatic process, change on heat (q) = 0 or q remain constant.

iii) Isobaric Process: A process during which pressure of the system remains constant throughout the reaction is called as isobaric process. For example, heating of water to its boiling point, and its vaporisation taking place at the same atmospheric pressure. Expansion of a gas in an open system is an example of isobaric process. For an isobaric process P = 0

iv) Isochoric Process: A process during which volume of the system remains constant throughout the reaction is known as isochoric process. The heating of a substance in a non-expanding chamber or change taking place in a closed system are examples of isochoric process. For an isochoric process, V = 0.

Units of Heat

Like other forms of energy, the S.I. unit of heat is the joule (J).

The other most commonly used unit of heat is the calorie (symbol: cal). It is defined as follows:

One calorie of heat is the quantity of heat required to raise the temperature of 1 g of water through 1°C.

In this definition of calorie, it has been assumed that the heat required to raise 1g of water through 1°C at any temperature is the same. However this is not true. The reaction is that the thermal expansion of water is not uniform near 4°C. Water shows uniform and smooth expansion only beyond 14°C. Hence the correct definition of calorie is given as follows:

One calorie of heat is the quantity of heat required to raise the temperature of 1t of water from 14.5°C
to 15.5°C.

The unit calorie is related to the S.I. unit joule as follows:

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