Heat transfer MCQ Quiz - Objective Question with Answer for Heat transfer - Download Free PDF

Let \(A_2\) be the area when length and breadth are reduced by half. Thus the area will be \(\dfrac {1}{4}\)th of \(A_1\)

\(\therefore A_2=\dfrac {1}{4} \times 32=8\)

Given \(T_1=127^\circ \text{C}=400 \, \text{K}\)

Given \(T_2=327^\circ \text{C}=600 \, \text{K}\)

From Stefan's law \(E=\sigma AT^4\)

\(\therefore \dfrac{E_1}{E_2}= \dfrac {A_1{T_1}^4}{A_2{T_2}^4}=\dfrac {32 \times (400)^4}{8 \times (600)^4}=\dfrac{64}{81}\)

\(\therefore \dfrac{E_2}{E_1}= \dfrac{81}{64}\)

Calculation:

Energy emitted by the transmitter per sec, \(E_t = 10\)kW

Energy of the wave, \(E_w = \dfrac{hc}{\lambda}\)

where \(\lambda= 300\)m

Plugging the values gives \(E_w = 6.6 \times 10^{-28}\)

No of photons emitted per sec,

\(n = \dfrac{E_t}{E_w} = \dfrac{10000}{6.6 \times 10^{-28}} = 1.5 \times10^{31}\)

CONCEPT:

Newton's Law of Cooling

EXPLANATION:

Newton's Law of Cooling states that the rate of change of temperature of an object is directly proportional to the difference in temperature between the object and its surroundings. This law is typically applicable to situations where an object is cooling or heating in the presence of a temperature difference with its environment.

1. Ice is melting in a glass full of water:
   • The melting of ice is more related to phase change and the absorption of latent heat rather than simple cooling or heating, and thus Newton's Law of Cooling is not directly applicable.
2. Water is boiling in an open container:
   • The boiling of water involves phase change and vaporization, which includes the absorption of latent heat and is not merely a simple cooling or heating process. Thus, Newton's Law of Cooling is not directly applicable.
3. A metal rod is heated in a furnace:
   • While heating in a furnace involves heat transfer, it is typically conducted at high temperatures and might involve various heat transfer mechanisms beyond simple cooling or heating to room temperature. Thus, Newton's Law of Cooling is not typically applicable here.
4. A cup of coffee is allowed to cool on a table:
   • This situation is a classic example of Newton's Law of Cooling, where the coffee cools by transferring heat to the surrounding air. The rate of cooling is proportional to the temperature difference between the coffee and the surroundings.

Therefore, the case where Newton's Law of Cooling is applicable is: (2) 4 only.

CONCEPT:

Heat

• Heat is a form of energy transfer between systems or objects with different temperatures (thermal energy in transit).
• It always flows from the hotter object to the cooler one, until thermal equilibrium is reached.
• The SI unit of heat is the joule (J), but it is also commonly measured in calories (cal).

EXPLANATION:

• Among the given options:
  • Option 1: "The transformation of energy from one form to another" is not specific to heat, as it applies to energy in general.
  • Option 2: "The conversion of energy into mass and vice-versa due to temperature difference" refers to a relativistic concept (E=mc²), not heat transfer.
  • Option 3: "The transfer of energy due to temperature difference" accurately describes heat.
  • Option 4: "The change in volume of a substance with temperature" refers to thermal expansion, not heat itself.

Therefore, the best definition for heat among the options provided is The transfer of energy due to temperature difference.

Concept Used:

The emissivity (e) of a surface is related to its reflectivity (r) and absorptivity (a) based on Kirchhoff's Law of Radiation:

e = a

Since energy conservation applies, we use the relation:

r + a = 1

where:

r = Reflectivity of the surface

a = Absorptivity of the surface

e = Emissivity of the surface

Calculation:

Given that the reflectivity of the surface is r = 0.7 , we calculate emissivity:

⇒ a = 1 - r

⇒ a = 1 - 0.7

⇒ a = 0.3

From Kirchhoff's Law:

⇒ e = a = 0.3

Looking at Column-3 , the corresponding value for 0.3 is (R).

From Column-1 and Column-2 , Kirchhoff's Law states that:

⇒ Absorptivity equals emissivity in thermal equilibrium → (III) (iii)

Thus, the correct match is (III) (iii) (R) .

Correct Option: Option 4

CONCEPT:

• Heat Transfer: Heat is transferred from one place to another in substance due to temperature difference. 

• Evaporation: In this process the liquid form a substance tends to convert in the gaseous phase on the uppermost surface due to heat and pressure.
• Condensation: In this process the gaseous phase of an object tends to convert into liquid form due to pressure and temperature.

EXPLANATION:

• Once the lower molecules of pot get heated comes at the top.
• And the upper molecules are heavy therefore goes down due to gravity.
• This cyclic process continues heating occurs.

While heating a fluid in pot Convection takes place.

• The good absorbers of heat are good-emitters of heat.

Key Points

• Concept
  • The fact that 'good absorbers of heat are good emitters' is based on Kirchoff's radiation law.
  • The Kirchoff's radiation law states that at a given temperature the coefficient of absorption of a body is equal to its coefficient of emission.
• Explanation
  • A black body is an example of a good absorber of heat as well as a good emitter of heat.
  • The ease with which a black body can absorb a photon is the reverse process of emitting the one.
  • This entire cycle takes place because of the number of transitions that are associated with the EM field.

Additional Information

• Application of Kirchoff's Radiation Law,
  • Sand is rough black, so it is a good absorber and hence, in deserts, days will be very hot.
  • A person with black skin experiences more heat and cold as compared to white skin person.
  • When a green glass is heated in a furnace and taken out, it is found to glow with red light, because red and green are complementary colours.

Concept:

• Heat Transfer: Heat is transferred from one place to another in substance due to temperature difference. 

• Evaporation: In this process, the liquid form a substance tends to convert in the gaseous phase on the uppermost surface due to heat and pressure.
• Condensation: In this process, the gaseous phase of an object tends to convert into liquid form due to pressure and temperature.

EXPLANATION:

• Once the lower molecules of pot get heated comes at the top.
• And the upper molecules are heavy therefore goes down due to gravity.
• This cyclic process continues heating occurs.

While heating the water Convection takes place.

CONCEPT:

EXPLANATION:

• In the presence of oxygen, the filament of the light bulb would burn up as a result of the high temperature. So the vacuum is maintained in the light bulb.
• Since there is no medium present in the light bulb, so the heat transfer takes place by radiation because the conduction and the convection require a material medium to transfer heat.
• Hence, option 3 is correct.

Concept:

• Weightlessness is experienced in absence of gravity.
• Heat is transferred through the material in three forms

1. conduction
2. convection
3. Radiation 

.

Explanation:

• In weightlessness, there is no interaction between molecules.
• Convection: In convection, gravity plays an important role.
• When liquid is heating the density of the lower molecules decreases.
• More dense molecules come down.
• In this example, there is no gravity so the molecule won't perform this motion.
• Radiation: It is the transfer of thermal energy in form of an electromagnetic wave.
• It does not require a medium to transfer heat.
• Here heat is transferred by conduction.
• the vessel will conduct heat and transfer it to liquid.
• Conduction: In this process, heat is transferred due to temperature differences in neighbouring molecules
• Therefore option 1 is correct.

CONCEPT: 

• The movement of energy from one place to another is called energy transfer. Heat transfer mainly takes place due to temperature differences.
• There are three modes of Heat transfer.
  1.Conduction: The mode of heat transfer in solids where heat transfer takes place without the movement of medium particles is called conduction.
  For example:  By heating one end of a metal rod we can feel hotness at another end.
  2.Convection: The mode of heat transfer in fluids where heat transfer takes place due to the movement of particles of the medium is called convection.
  For example: Heating of water in a pot
  3. Radiation: The mode of heat transfer where heat is transferred from one place to another without affecting the medium particles is called radiation.
  For example: When we place our hand nearby the burning gas stove then we can feel the hotness. This is due to radiation.

EXPLANATION:

• From above it is clear that the process of the transfer of heat from one place to another place without heating the intervening medium is called radiation.
• It is possible to even in vacuum e.g. the heat from the sun reaches the earth through radiation. Therefore option 3 is correct.

Concept:

There are three methods of heat transfer between the two systems. They are conduction, convection, and radiation.

• Conduction is a method of heat transfer in solids and heat transfer takes place without the movement of particles.
• Convection is a method of heat transfer in fluids (gases and liquids) and heat transfer takes place due to the movement of particles.
• Radiation is a method of heat transfer where heat is transferred from one place to another without affecting the medium of heat transfer.

Explanation:

• Both are at the same temperature, the metal will feel colder than the wood because of the thermal conductivity of the metal, compared to the wood is more.

Metal will conduct the heat faster than wood to your hand.

The specific heat capacity of wood is much higher than iron. Hence it takes time to get cooler or hotter compared to metal.

Concept:

Density:

• The density of a substance is its mass per unit volume.
• It means that density is inversely proportional to volume. 
• The SI unit of the density is kg/m³.

The graph between the specific volume and temperature of water:

• The volume of a given amount of water decreases between 0°C to 4°C.
• The volume starts to decrease when water is heated from 0°C because the heating causes the density of water to increase.

Explanation:

• Though liquids expand on heating, water does not expand between 0°C to 4°C instead it contracts as water has maximum density at 4°C.
• It expands above 4°C.
• Hence, the volume of a given amount of water decreases when its temperature is increased from 0°C to 4°C.

The Correct Answer is option 2 i.e. During the Day

Key Points

• A sea breeze is any wind that blows from a large body of water toward or onto a landmasses
• Sea breezes occur during hot, summer days because of the unequal heating rates of land and water.
• During the day, the land surface heats up faster than the water surface hence creating a high temperature and low-pressure zone on land.
• The wind blows from the higher pressure over the water to the lower pressure over the land causing the sea breeze
• At night, the air over the ocean is now warmer than the air over the land. The land loses heat quickly after the sun goes down and the air above it cools too

Concept:

Conduction: 

• The heat transfer through direct physical contact is known as conduction.
• Heat transferred from one end to other end is called conduction. 
• Heat transfer through conduction can only happen in conductors of heat like iron, copper etc. 
• Heat transfer do not take place across insulators like wood, plastic etc. 

Radiation: 

• When electromagnetic radiation is emitted or absorbed to transfer the heat or When heat is transferred in the form of electromagnetic waves is known as radiation.
• Electromagnetic radiation includes microwaves, radio waves, infrared radiation, visible light, ultraviolet radiation, gamma rays, and X-rays.
• The method does not require any medium so heat from the sun reaches us by this method.

Convection

• It is heat transfer by the movement of its molecules.
• It occurs only in fluids (liquid and gas) not in solids because solid particles can not move from one place to another.
• For example, in a pot boiling water on the stove, or in thunderstorms, where hot air rises up to the base of the clouds.

Conclusion

The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. 

In convection, the heat transfer take place through transfer of molecules while radiation takes place through electromagnetic waves.