9th Class Physics: Chapter 8 Thermal Properties of Matter Short Questions Answers

Heat flow from hot body to cold body to attain the condition of thermal equilibrium.

Heat: Heat is the energy that is transferred from one body to the other in thermal contact with each other as a result of the difference of temperature between them.

Temperature: Temperature of a body is the degree of hotness or coldness of the body.

The sum of kinetic energy and potential energy associated with the atoms, molecules and particles of a body is called its internal energy.

By heating the gas, its molecules get high kinetic energy and start to collide more randomly. And motion of gas molecules is increased by heating. So pressure of gas molecules increase by heating.

A thermometer is a device which is used to measure the temperature of a body. Mercury is preferred as a thermometric substance due to following properties.

• It is easily visible.
• It has uniform thermal expansion.
• It has low freezing point and high boiling point.
• It has a small specific heat capacity.

Volume Thermal Expansion: The volume of a solid changes with the change in temperature and is called volume thermal expansion or cubical thermal expansion.

Explanation: Consider a solid of initial volume V₀ at certain temperature T₀. On heating, the solid of a temperature T, let its volume becomes V, then.

Original volume of the solid= V₀

Volume of the solid after heating = V

Increase in volume of the solid = ∆V =V-V₀

Original Temperature of the solid = T₀

Temperature of the solid after heating = T

Increase in Temperature of the solid = ∆T = T-T₀

Factors on which volume expansion depends:

Volume expansion depends on two factors.

Original volume: It is found that change in volume of a solid is directly proportional to its original volume V₀.

∆V∝ V₀ ————– (i)

Change in Temperature: It is found that change in volume of the solid is directly proportional to change in temperature ∆T.

∆V ∝ ∆T ————— (ii)

By Combining eq (i) and (ii), we have;

∆V ∝ V₀ ∆T ———- (iii)

To change the sign of proportionally into equality a constant is used;

So,

∆V = β V₀ ∆T ——- (iv)

Co-efficient of volume thermal expansion: In this equation β is called the co-efficient of volume thermal expansion of the substance.

Value of co-efficient of volume thermal expansion can be found out by using following relation.

Β = ∆V/V₀∆T

The co-efficient of volume thermal expansion β can be defined as the fractional change in its volume per Kelvin change in temperature.

Value of V: By using equation (iv) we can determine the value of V as following.

∆V = β V₀ ∆T

As we know;

∆V =V-V₀

So,

V-V₀ = β V₀ ∆T

V= V₀ + β V₀ ∆T

V = V₀ (1+ β ∆T).

The specific heat of a substance is the amount of heat required to raise the temperature of 1kg mass of that substance through 1K.

Specific heat of any substance can be found out by using following formula:

c = ∆Q/m∆T

c is the specific heat capacity

∆Q is the amount of heat absorbed by the body

m is the mass of the body.

∆T is the change of temperature

Latent heat of fusion: Heat energy required to change unit mass of a substance from solid to liquid state at its melting point without change in its temperature is called its latent heat of fusion denoted by H_f.

Explanation: For the explanation of latent heat of fusion following terms should be understood by us.

Melting of fusion: When a substance is changed from its solid state to liquid state by adding heat, the process is called melting or fusion.

Fusion point or melting point: The temperature at which a solid starts melting is called its fusion point or melting point.

Freezing point: When a liquid is cooled, it changes into solid state. The temperature at which a substance changes from liquid to solid is state is called its freezing point.

Melting points of different substances:

Different substances have different melting points. However, the freezing point of a substance is the same as its melting point.

Formula: H_{f ­}= ∆Q_f/.m or

∆Q_f  = mH_{f ­}

In this equation

H_f  is the latent heat of fusion ‘m’ is the mass of the substance.

Latent heat of fusion of ice:

Ice changes at 0^oC into water. Latent heat of fusion of ice is 3.36 x 10⁵jkg^-1. That is 3.36 x 10⁵ joule heat is required to melt 1kg of ice into water at 0^oC.

The quantity of heat that changes unit mass of a liquid completely into gas at its boiling point without any change in its temperature is called its latent heat of vaporization denoted by H_v.

H_v = ∆Qv/m

Evaporation: Evaporation is the changing of a liquid into vapours (gaseous state) form the surface of the liquid without heating it.

Factors which affect on evaporation:

The rate of evaporation is affected by the following factors.

• Temperature (ii) Surface area
• Nature of the liquid (iv) Wind

Evaporation causes cooling:

As evaporation takes place, fast moving molecules escape out from the surface of the liquid. Molecules that have lower kinetic energies are left behind.

This lowers the average kinetic energy of the liquids molecules and the temperature of the liquid. Since temperature of a substance depends on the average kinetic energy of its molecules, therefore the temperature of the liquid decreases.

Internal Energy: The sum of kinetic energy and potential energy associated with the atoms, molecules and particles of a body is called its internal energy.

Heat: Heat is the energy that is transferred from one body to the other in thermal contact with each other as a result of the difference of temperature between them.

Temperature: Temperature of a body is the degree of hotness or coldness of the body.

Internal energy depends upon the following factors.

1. Mass of the body.
2. E. of molecules of a body
3. E. of molecules of a body

Heat: Heat is the energy that is transferred from one body to the other in thermal contact with each other as a result of the difference of temperature between them.

Internal energy: The sum of kinetic energy and potential energy associated with the atoms, molecules and particles of a body is called its internal energy.

C = 20^oC

As T = 273 +C

T = 273+ 20 = 293 K

F =100^oF

Since 1.8C = F-32

1.8C=100-32

1.8 C = 68

C=68/1.8

C = 37.8^oC

Thus 100^oF is equal to 37.8^oC.

1. Celsius Scale
2. Fahrenheit Scale

A thermometer is a device which is used to measure the temperature of a body.

T = 300K

C = T(K) – 273

C = T(300– 273)

C =27^oC

C = 50^oC

F = (1.8 x C + 32)

F = (1.8 x 50 + 32)

F = (1.8 x 82)

F = 122^oF

Thus, 50^oC on Celsius scale is 122^oF on Fahrenheit scale.

Conversion of temperature from Celsius scale to Kelvin scale: The temperature T on Kelvin scale can be obtained by adding 273 in temperature C on Celsius scale, thus;

T(k) = 273 + C

Conversion of temperature from Celsius scale to Fahrenheit scale: Since 100 divisions on Celsius scale are equal to 180 divisions on Fahrenheit scale. Therefore, each division on Celsius scale is equal to 1.8 division on Fahrenheit scale.

Moreover, 0^oC corresponding to 32^oF.

F = 1.8C + 32

Thermometer: A thermometer is a device which is used to measure the temperature of a body.

Mercury is preferred as a thermometric substance due to following properties.

• It is easily visible.
• It has uniform thermal expansion.
• It has low freezing point and high boiling point.
• It has a small specific heat capacity.

C = 60^oC

F = 1.8C +32

F = 1.8(60) + 32

F = 1.8(60 + 32)

F = 1.8(92)

F= 140^oF

Lower Fixed Points: The lower fixed point is marked to show the position of liquid in the thermometer when it is placed in ice.

Upper fixed point: Upper fixed point is marked to show the position of liquid in the thermometer when it is placed in steam at standard pressure above boiling water.

Temperature: Temperature of a body is the degree of hotness or coldness of the body.

Thermometer: A device that is used to measure the temperature of a body is called thermometer.

Properties of thermometric liquid:

A thermometric liquid should have the following properties.

• It is easily visible.
• It has uniform thermal expansion.
• It has low freezing point and high boiling point.

Conversion of temperature from Celsius scale to Kelvin scale: The temperature T on Kelvin scale can be obtained by adding 273 in temperature C on Celsius scale, thus;

T(k) = 273 + C

Fahrenheit scale: On Fahrenheit scale, the interval between lower and upper fixed points is divided into 180 equal parts.

Kelvin scale: On Kelvin scale, the interval between the lower and upper fixed points is divided into 100 equal parts.

A clinical thermometer is used to measure the temperature of human body. It has a narrow range from 35^oC to 42^oC. It has a construction that prevents the mercury to return. Thus, its reading does not change until rest.

• Mercury freezes at -39^oC and boils at 357^o
• Mercury has all thermometric properties, which are necessary for thermometric material.

C = 20^oC

T= 273+C

T= 273 + 20= 293K

Specific Heat: The specific heat of a substance is the amount of heat required to raise the temperature of 1kg mass of that substance through 1K.

Formula: Specific heat of any substance can be found out by using following formula:

c = ∆Q/m∆T

Specific Heat: The specific heat of a substance is the amount of heat required to raise the temperature of 1k mass of that substance through 1K.

Heat Capacity: Heat capacity of a body is the quantity of thermal energy absorbed by it for one Kelvin increase in its temperature.

The cooling system of automobiles uses water to carry away unwanted thermal energy. In an automobile, large amount of heat is produced by its engine due to which its temperature goes on increasing. The engine would cease it is not cooled down.

Heat Capacity: Heat capacity of a body is the quantity of thermal energy absorbed by it for one Kelvin increase in its temperature.

Formula: Heat Capacity = mc

How much heat is required to increase the temperature of 0.5 kg of water from 10^oC to 65^oC?

Mass = m = 0.5 kg

Increase in temperature = T₂ = 65^oC

= 65 + 273 = 338 K

Original temperature = T₁ =10^oC

= T₁ = 10 + 273 = 283K

Specific heat of water = c = 4200 Jkg^-1K^-1

Required: Amount of heat absorbed by the body

= ∆Q =?

Formula: ∆Q = mc∆T

By putting the values, the amount of heat absorbed can be found out;

∆Q = (0.5kg) (4200 Jkg^-1K^-1) (T₂ – T₁)

∆Q = (0.5kg) (4200) (338 – 273)

∆Q = (0.5kg) (4200) (55)

∆Q = (2100) (55)

∆Q = 115,500 J

The required heat to increase the temperature is 115,500 J.

Relation between heat capacity and quantity of substance: Mass and heat capacity are directly proportional to each other larger is the quantity of a substance, higher will be its heat.

Heat required to changes unit mass of a substance from solid to liquid state at its melting point without any change in its temperature is called its latent heat of vaporization denoted by H.

Latent heat of fusion of ice is 3.36 x 10⁵jkg^-1

Latent Heat of Fusion: Heat energy required to changes unit mass of a substance from solid to liquid state at its melting point without any change in its temperature is called its latent heat of vaporization denoted by H.

There are two types of latent heat:

• Latent heat of fusion.
• Latent heat of vaporization.

Latent heat of fusion: Latent Heat of Fusion: Heat energy required to changes unit mass of a substance from solid to liquid state at its melting point without any change in its temperature is called its latent heat of vaporization denoted by H_f.

Mathematical form:

∆Q_f = mH_f

Latent Heat of Vaporization: The quantity of heat that changes unit mass of a liquid completely into gas at its boiling point without any change in its temperature is called its latent heat of vaporization denoted by H_v.

H_v = ∆Qv/m

Co-efficient of Linear thermal expansion:

Co-efficient of linear thermal expansion ∝ of a substance can be defined as fractional increase in its length per Kelvin rise in temperature.

Formula: ∝ = ∆L/L_o∆T

Evaporation: Evaporation is the changing of a liquid into vapours (gaseous state) from the surface of the liquid without heating it.

Vaporization: Vaporization is the changing of a liquid into vapours (gaseous state) from the surface of liquid with heating it.

Evaporation of liquid depends upon the following factors:

1. Temperature
2. Surface Area
3. Wind
4. Nature of Liquid

• Evaporation of perspiration helps to cool our bodies.
• Cooling is produced in refrigerators by evaporation of liquefied gas, which produces cooling effects.

Liquid differ in the rate at which they evaporate.

Example: Put a few drops of ether or spirit on the palm, it evaporates rapidly while for drops of water on the palm does not evaporate rapidly.

Effect of temperature on evaporation:

• Evaporation takes place at all temperature from the surface of a liquid.
• Evaporation is faster at high temperature, more molecules of liquid are moving with high velocities. Thus, more molecules are escaping from its surface.

Evaporation is faster at high temperature than at low temperature thus, is summer, at higher temperature, more molecules of a liquid are moving with high velocities and wet clothes dry up more quickly in summer then in winter.

Liquid differ in the rate at which they evaporate. If we put a few drops of ether or spirit on the palm, it evaporates rapidly while for drops of water on the palm does not evaporate rapidly.

Evaporation: Evaporation is the changing of a liquid into vapours (gaseous state) form the surface of the liquid without heating it.

Co-efficient of Linear thermal expansion:

Co-efficient of linear thermal expansion ∝ of a substance can be defined as fractional increase in its length per Kelvin rise in temperature.

Water on cooling below 4^oC begins to expand until it reaches 0^oC. On further cooling its volume increases suddenly as it changes into ice at 0^oC. When ice is cooled below 0^oC, it contracts i.e. its volume decreases like solids. This unusual expansion of water is called the anomalous expansion of water.

For the measurement of temperature: In thermometers, thermal expansion is used in temperature measurements.

Opening the metallic cap of bottle:

To open the cap of a bottle that is tight enough, immerse it in hot water for a minute or so. Metal cap expands and becomes loose. It would now be easy to turn it to open.

Gold: α = 1.3 x 10^-5k^-1 Silver: α = 1.93 x 10^-5k^-1

Uses of bimetal strips:

Bimetal strips are used for various purposes.

• Bimetal strips are used in thermometer to measure temperatures especially in furnaces and ovens.
• Bimetal strips are also used in thermostats. Bimetal thermostat switch is used to control the temperature of heater coil in an electric iron.

Thermal Equilibrium: When two bodies of different temperatures are brought close to each other, the heat is released by hot body and is absorbed by cold body. So thermal equilibrium is the condition in which two bodies attain same temperature.

Thermal expansion: Expansion due to heating is called thermal expansion.

Co-efficient of volume thermal expansion: The co-efficient of volume thermal expansion β can be defined as the fractional change in its volume per Kelvin change in temperature.

Volume thermal expansion: The volume of solid changes with the change in temperature and is called volume thermal expansion or cubical thermal expansion.

Equation: V = V_o(1+ β∆T)

Types of thermal volume expansion for liquids: When a liquid is heated, both liquid and the container undergo a change in their volume. So there are two types of thermal volume expansion for liquid.

• Apparent volume expansion.
• Real volume expansion.

Process of liquid expansion:

The molecules of liquids are free to move in all directions within the liquid. On heating a liquid, the average amplitude of vibration of its molecules increases. The molecules push each other and need more space to occupy. This accounts for the expansion of the liquid when heated.

The expansion of solids may damage the bridges, railway tracks and roads as they are constantly subjected to temperature changes. So provision is made during construction for expansion and contraction with temperature.

Expansion of Railway track:

Railway tracks buckled on a hot summer day due to expansion if gaps are not left between sections.

Gaps are left in railway tracks to compensate thermal expansion during hot season.