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Why does heat flow from hot body to cold body?

Difficulty: Medium

Heat flows from the warm to the cold body because the energy state is higher.

         Heat flow moves energy from a higher temperature to a lower temperature. The bigger the difference in temperature between the two objects, the faster heat flows between them. When the temperature is the same there is no change in energy due to heat flow. Heat flows from a hot body to a cold body until thermal equilibrium is reached.

        Radiation and conduction are the two methods of heat transfer. Convection is a special type of conduction.

Define the terms heat and temperature?

Difficulty: Easy


         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 in temperature between them.


       The temperature of a body is the degree of hotness or coldness of the body.

What is meant by the internal energy of a body?

Difficulty: Easy

Internal energy:

              The sum of kinetic energy and potential energy associated with the atoms, molecules, and particles of a body is called internal energy.
              The internal energy of a body depends on many factors such as the mass of the body, kinetic and potential energy of molecules, etc.

                                    $\triangle U$ = Q – W

Where:     $\triangle U$ = internal energy,   Q = Heat and W = work done

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How does heating affect the motion of molecules of a gas?

Difficulty: Easy

The larger the temperature of gas the faster the molecules will move (temperature is proportional to the average kinetic energy of the particles) and the large the force they will exert, and the higher the pressure (pressure is the force exerted by the particles divided by the area).

                                                            $ T \: \alpha K.E \: \alpha \: P $

What is a thermometer? Why mercury is preferred as a thermometric substance?

Difficulty: Medium


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

Principle of thermometer:

          Mercury thermometer is based on the fact that material (in this case, the liquid mercury) expands when heated.

Basic properties of thermometric liquid:

 A thermometric liquid should have the following properties:

  • It should be visible.
  • It should have uniform thermal expansion.
  • I should have a low freezing point.
  • It should have a high boiling point.
  • It should not wet glass.
  • I should be a good conductor of heat.
  • It should have a small specific heat capacity

Preference for mercury:

         Mercury has a uniform thermal expansion, is easily visible, has a low freezing point, has a high boiling point, and has less specific heat.

Note:  Due to these properties’ mercury is used in a mercury thermometer. Since it is opaque, it is easy to see the capillary.

Explain the volumetric thermal expansion.


Derive the relation for volume thermal expansion in solids.


Show that   V =$ V_{0} (1+ \beta \triangle T)$?

Difficulty: Medium

Volumetric thermal expansion:

          The volume of a solid also changes with the temperature change and is called volume thermal expansion or cubical thermal expansion.
          Consider a solid of the initial volume$ V_{0}$ at a certain temperature $ T_{0}$. On heating, the solid to a temperature T, let its volume becomes V, then
Change in the volume of a solid $ \triangle V = V - V_{0}$
And change in temperature $ \triangle T$ =$ T - T_{0}$

         Like linear expansion, the change in $ \triangle V$ is found to be proportional to its original volume $ V_{0}$ and change in temperature $ \triangle T $. Thus 
 $ \triangle V \alpha V_0 \triangle T $

or                                                              $ \triangle V $ = $ \beta V_0 \triangle T $ ……….  (i)
or                                                            $ V - V_{0} =  〖\beta V〗_{0} \triangle T$
or                                                               $V = V_{0}+\beta V_{0} \triangle T $
or                                                              $ V = V_{0} (1+ \beta \triangle T)$ ……….  (ii) 

          where \beta is the temperature coefficient of volume thermal expansion, using equation (i), we get 
         $ \beta $ = $ (\triangle V )/(V_{0} \triangle T )$                           (iii)

Coefficient of volume expansion $ \beta$:

           Thus, we can define the temperature coefficient of volume expansion $ \beta $ as the fractional change in its volume per kelvin change in temperature. The coefficient of linear expansion and volume expansion are related by the equation:
   $\beta = $3 \alpha$ ……………..  (iv)

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Define specific heat. How would you find the specific heat of a solid?

Difficulty: Easy

Specific heat:
        Specific heat of a substance is the amount of heat required to raise the temperature of 1kg mass of that substance through 1K.
         It has been observed that the quantity of heat $\triangle Q$ required to raise the temperature $ \triangle T$ of a body is proportional to the mass of the body.

                    $\triangle Q \alpha m \triangle T$
                     $\triangle Q$ = c m \triangle T$   ……………….. (i)

           Here $ \triangle Q$ is the amount of heat absorbed by the body and c is the constant of proportionality called the specific heat capacity or simply specific heat.

   c =$\frac{\triangleQ}{(m\triangle T )}$              (ii)

Unit of specific heat:
    SI unit of specific heat is J〖kg〗$^{-1} K^{-1}$.

Define and explain the latent heat of fusion.

Difficulty: Medium

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

It is denoted by H_f.
                                      H_f  = $\frac{( \triangle Q_{f})}{(m)}$  
   Or                   $\triangle Q_{f}$=  $m H_{f}$   …………   (i)

Ice changes at 0°C into water. Latent heat of fusion of ice is:
$3.36\times 10^{5}$  $Jkg^{-1}$.  That is, $3.36 \times 10^{5}$  joule heat is required to melt 1kg of ice into the water at 0°C.

Define latent heat of vaporization.

Difficulty: Easy

Latent heat of vaporization:

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

It is denoted by $H_{v}$.
                                      $H_{v} = \frac{ (\triangle Q_{v})}{m}$
Or                    $\triangle Q_{v} = m H_{v}$   …………   (i)
       When water is heated, it boils at 100°C under standard pressure. Its temperature remains 100°C until it is changed completely into steam. Its latent heat of vaporization is $2.26 \times 10^{6}$  $Jkg^{-1}$  . That is; one kilogram of water requires $2.26 \times10^{6}$   joule heat to change it completely into a gas (steam) at its boiling point.
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What is meant by evaporation? On what factors the evaporation of liquid depends? Explain how cooling is produced by evaporation.

Difficulty: Hard

The evaporation:

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


Evaporation causes cooling:

Evaporation plays an important role in our daily life. Ours clothe dry up rapidly when spread. During evaporation; fast-moving molecules escape out from the surface of the liquid. Molecules that have lower kinetic energies-l are left behind. This lowers the average kinetic energy of the liquid molecules and the temperature of the liquid. Since the temperature of a substance depends on the average kinetic energy of its molecules. Evaporation of perspiration helps to cool our bodies.


$T \alpha K.E$

Evaporation takes place at all temperatures from the surface of a liquid. The rate of evaporation is affected by various factors.


Factors affecting the rate of evaporation:

1. Temperature:

Why do wet clothes dry up more quickly in summer than in winter? At a higher temperature, more molecules of a liquid are moving with high velocities. Thus, evaporation is faster at high temperatures than at low temperatures.


2. Surface area:

Why does water evaporate faster when spread over a large area? The larger the surface area of a liquid, the greater number of molecules have the chance to escape from its surface.


3. Wind:

Wind blowing over the surface of liquid sweeps away the liquid molecules that have just escaped out, this increases the chance for more liquid molecules to escape out.


4. Nature of the liquid:

Do spirit and water evaporate at the same rate? Liquids differ in the rate at which they evaporate and spread a few drops of ether or spirit on your palm. You feel cold, why?


Mini Exercise

1. How does specific heat differ from heat capacity?

Specific Heat

Heat Capacity

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

The heat capacity of a body is the quantity of thermal energy absorbed y it for one Kelvin (1K) increase in its temperature. 

Specific heat can be found by the relation.

            c =$ \frac{\triangle Q}{(m \triangle T )}$

Heat capacity can be found out by the relation

Heat capacity = mc

SI unit of specific heat is  $ J〖kg〗^{-1} K^{-1}$

Unit of heat capacity is $JK^{-1}$


2. Give two uses of the cooling effect of evaporation.

Ans:   Uses of cooling effect by evaporation:

  1. During hot summers, the water is usually kept in the earthen pot to keep it cool. Water is cooled in the pot since the surface of the pot contains large pores and water seeps via their pores to the outside of the pot. This water evaporates and takes the latent heat for vaporization hence retaining the water inside the pot to be cooled.
  2. Especially in villages, people often sprinkle water on the round in front of their homes during hot summers.
  3. Water vaporization from leaves of trees also cools the surroundings.
  4. A desert cooler cools better on a hot and dry day.
  5. It is a common observation that we can sip hot tea (or milk) faster from a saucer than from a cup.
  6. Wearing cotton clothes on summer days to keep the body cool and comfortable.
  7. Put a little spirit on your hand and wave around, the spirit evaporates rapidly and our hands feel cooler.


3. How does evaporation differ from vaporization?

Ans:    Difference between vaporization and evaporation:



Vaporization is a transitional phase of an element or compound from a solid phase or liquid phase to a gas phase. It changes matter from one state or phase into another without changing its chemical composition.

Vaporization has three types:

(i) Boiling ii.    Evaporation iii.   Sublimation



Evaporation, wherein the transition from the liquid phase to the gas phase takes place below the boiling temperature at a given pressure and occurs on the surface.



Cooling is produced in refrigerators through evaporation of liquefied gas. This produces a cooling effect. Freon, a CFC, was used as a refrigerator gas. But its use has been forbidden when it was known that CFC is the cause of ozone depletion in the upper atmosphere which increases the number of UV rays from the Sun. The rays are harmful to all living matter. Freon gas is now replaced by Ammonia and other substances which are not harmful to the environment.

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