Tuesday, July 28, 2020

H C Verma solutions, HEAT TRANSFER, OBJECTIVE-II, Chapter-28, Concepts of Physics, Part-II

Heat Transfer

OBJECTIVE-II

   1. One end of a metal rod is dipped in boiling water and the other is dipped in melting ice.

(a) All parts of the rod are in thermal equilibrium with each other.

(b) We can assign a temperature to the rod.

(c) We can assign a temperature to the rod after a steady-state is reached.

(d) The state of the rod does not change after a steady-state is reached.  



Answer:  (d)    


Explanation:  After the rod is dipped with one end in boiling water and the other in melting ice the heat transfer begins from the hot end to the cold end. Thus the temperature of different parts begins to change and the parts are not in thermal equilibrium. Also due to the varying temperature, the rod can not be assigned a temperature. So the option (a) and (b) are not true. Even when a steady state is reached there is a heat current and temperature gradient in the rod. So a temperature still can not be assigned. Option (c) not true.

    When a state is reached in which the same amount of heat is transferred from one end to another in any fixed interval of time we say that a steady-state is reached. Now the state of the rod will remain the same and it will not change. Option (d) is correct. 





   2. A blackbody does not

(a) emit radiation

(b) absorb radiation

(c) reflect radiation

(d) refract radiation. 



Answer:  (c), (d).    


Explanation:  A blackbody is that which absorbs all the radiation falling on it. Since all the good absorbers of heat are also good radiators of heat, a blackbody also radiates heat. Since all radiation falling on it is absorbed there is no point of reflection or refraction of radiation. So the options (c) and (d). 





   3. In summer a mild wind is often found on the shore of a calm river. This is caused due to

(a) the difference in thermal conductivity of water and soil

(b) convection currents
(c) conduction between air and the soil
(d) radiation from the soil.



Answer:  (b)    


Explanation:  Due to the different specific heats of land and water the temperatures of them change to different levels during the day and also during the night. This makes the air above the hotter surface to rise and the cooler air above the other surface rushes to fill this vacant space. Thus a mild wind is set up which is due to these convection currents. Option (b) is correct.  





   4. A piece of charcoal and a piece of shining steel of the same area are kept for a long time in an open lawn in the bright sun.

(a) The steel will absorb more heat than charcoal.

(b) The temperature of the steel will be higher than that of the charcoal.

(c) If both are picked up by bare hands, the steel will be felt hotter than the charcoal

(d) If the two are picked up from the lawn and kept in a cold chamber, the charcoal will lose heat at a faster rate than the steel.  



Answer:  (c), (d).    


Explanation:  Since both are in the same condition the temperature of both the objects will be the same. The amount of heat absorbed depends not only upon temperature difference but also on the mass and specific heat of the objects. Hence having the same surface area does not mean that both will absorb the same amount of heat. The options (a) and (b) are not true.

     Being metal steel is a better conductor of heat than charcoal. Thus when they are picked up by bare hands the heat current from the steel to the hand is greater than charcoal to the hand. Hence the steel will be felt hotter than charcoal. Option (c) is true.

       When both are kept in a cold chamber most of the heat will be lost through radiation. The black charcoal will radiate heat faster than the shining steel, hence the charcoal will lose heat faster than the steel. Option (d) is correct.





   5. A heated body emits radiation which has maximum intensity near the frequency πœˆβ‚’. The emissivity of the material is 0.5. If the absolute temperature of the body is doubled,

(a) the maximum intensity of the radiation will be near the frequency 2πœˆβ‚’.

(b) the maximum intensity of radiation will be near the frequency πœˆβ‚’/2.

(c) the total energy emitted will increase by a factor 16.

(d) the total energy emitted will increase by a factor 8. 



Answer:  (a), (c).    


Explanation:  From Wien's displacement law the wavelength around the maximum intensity is inversely proportional to the temperature. But the frequency of radiation is also inversely proportional to the wavelength. i.e.

𝞴 ∝ 1/T

and 𝞴 ∝ 1/𝜈

So, 1/𝜈 ∝ 1/T

→𝜈 ∝ T

So when the absolute temperature T is double in this case the maximum intensity of radiation will be near 2πœˆβ‚’. Option (a) is true.

       The energy emitted by radiation is proportional to the fourth power of the absolute temperature. Hence when the temperature is increased by factor 2 the total energy emitted will increase by factor 2⁴ = 16. Option (c) is true.





   6. A solid sphere and a hollow sphere of the same material and of equal radii are heated to the same temperature.

(a) Both will emit an equal amount of radiation per unit time in the beginning.

(b) Both will absorb an equal amount of radiation from the surrounding in the beginning.

(c) The initial rate of cooling (dT/dt) will be the same for the two spheres.

(d) The two spheres will have equal temperatures at any instant. 



Answer:  (a), (b).    


Explanation:  From the Stefan-Boltzmann law the energy of thermal radiation emitted per unit time is proportional to the area and fourth power of the absolute temperature of the object for the same material. Hence option (a).

     Since the emissive power and the absorptive power of a body has the same value, the option (b) is correct.

    From Newton's law of cooling, the rate of cooling 

dT/dt = -bA(T-Tβ‚’)

        Though for both the spheres A and initial (T-Tβ‚’) are the same, b will be different because it depends upon the mass of the object. The option (c) is not correct.

       Since the rate of cooling of both the spheres is different they will not have equal temperatures at any time. The option (d) is not correct.   

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Links to the Chapters











CHAPTER- 23 - Heat and Temperature



CHAPTER- 20 - Dispersion and Spectra


CHAPTER- 19 - Optical Instruments

CHAPTER- 18 - Geometrical Optics



CHAPTER- 17 - Light Waves




CHAPTER- 14 - Fluid Mechanics



CHAPTER- 13 - Fluid Mechanics


CHAPTER- 12 - Simple Harmonic Motion








CHAPTER- 11 - Gravitation




CHAPTER- 10 - Rotational Mechanics




CHAPTER- 9 - Center of Mass, Linear Momentum, Collision


CHAPTER- 8 - Work and Energy

Click here for → Question for Short Answers

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → Exercises (1-10)

Click here for → Exercises (11-20)

Click here for → Exercises (21-30)

Click here for → Exercises (31-42)

Click here for → Exercise(43-54)

CHAPTER- 7 - Circular Motion

Click here for → Questions for Short Answer 

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → EXERCISES (11-20)

Click here for → EXERCISES (21-30)

CHAPTER- 6 - Friction

Click here for → Questions for Short Answer

Click here for → OBJECTIVE-I

Click here for → Friction - OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → Exercises (11-20)

Click here for → EXERCISES (21-31)

For more practice on problems on friction solve these- "New Questions on Friction".

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CHAPTER- 5 - Newton's Laws of Motion


Click here for → QUESTIONS FOR SHORT ANSWER

Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)

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CHAPTER- 4 - The Forces

The Forces-

"Questions for short Answers"    


Click here for "The Forces" - OBJECTIVE-I


Click here for "The Forces" - OBJECTIVE-II


Click here for "The Forces" - Exercises


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CHAPTER- 3 - Kinematics - Rest and Motion

Click here for "Questions for short Answers"


Click here for "OBJECTIVE-I"


Click here for EXERCISES (Question number 1 to 10)


Click here for EXERCISES (Question number 11 to 20)


Click here for EXERCISES (Question number 21 to 30)


Click here for EXERCISES (Question number 31 to 40)


Click here for EXERCISES (Question number 41 to 52)


CHAPTER- 2 - "Physics and Mathematics"

Click here for "Questions for Short Answers"


Click here for "OBJECTIVE-II"

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