Questions Answer (Q/A)
Chapter - 11
Q.1: How do you distinguish between temperature and heat? Give example?
Ans: Heat is the energy flowing between bodies and surroundings due to the difference of temperature. But temperature is a measure of the average translation kinetic energy of the molecules of a body. If we dip a red hot iron ball in the sea, heat will flow. The amount of internal energy possessed by the ball is very small as compared to the immense amount contained in the sea.
Q.2: Why is the earth not in thermal equilibrium with the sun?
Ans: The earth is not in thermal equilibrium with the sun; because while the earth is being warmed by the absorbed radiant energy, it is also losing heat in various ways (e.g. re-radiation, conduction, convection and evaporation). Moreover, they are not in perfect thermal contact with each other. The average temperature of the earth is about 300 K.
Q.3: Is temperature a macroscopic concept?
Ans: Yes, temperature is a macroscopic concept.
Q.4: It is observed that when mercury in a glass thermometer is put in a flame, the column of mercury first descends and then rises. Explain.
Ans: When mercury in a glass thermometer is put in a flame, the glass bulb expands. So the column of mercury descends. But no sooner the heat reaches the mercury in the bulb, it expands and this expansion is greater than that of the glass bulb. So, now, the mercury rises in the column.
Q.5: Is it correct that the unit for specific heat capacity (c) is m².s⁻².°C⁻¹?
Ans: Yes, it is correct. Because c = (ΔQ / mΔT)
The SI unit of c = Jkg⁻¹ °C⁻¹ = N.m.kg⁻¹ °C⁻¹ = kg m/sec² m.kg⁻¹ °C⁻¹ = m² s⁻² °C⁻¹
Q.6: What is the standard temperature?
Ans: The standard temperature is the ice-point at S.T.P. i.e. 0°C or 273 K.
Q.7: When a block with a hole in it is heated, why does not the material around the hole expand into the hole and make it small?
Ans: Thermal expansion of a homogeneous substance causes increase in all directions with the same linear thermal expansion coefficient. This increase in all directions causes an effective magnification of an object. So a hole in the block, on heating, expands outward (i.e. becomes big).
Q.8: A thermometer is placed in direct sunlight. Will it read the temperature of the air, or of the sun, or of something else?
Ans: This thermometer will record the temperature of the surroundings (thermometric substance).
Q.9: Will one kilogram of hydrogen contain more than one kilogram of lead? Explain.
Ans: Yes, one kilogram of hydrogen will contain more atoms than one kilogram of lead because hydrogen atoms are much lighter than lead atoms (the atomic mass of hydrogen is 207 times less than the atomic mass of lead).
Q.10: The pressure in a gas cylinder containing hydrogen will leak more quickly than if this containing oxygen. Why?
Ans: This is so because the hydrogen molecules are lighter than oxygen molecules (since the molecular mass of hydrogen is 16 times less than the molecular mass of oxygen). Molecular speed (and hence rate of diffusion) is inversely proportional to the molecular mass. Hence hydrogen will leak more quickly than oxygen.
Q.11: What are some factors that affect the efficiency of an automobile engine?
Ans: The efficiency of an automobile engine depends upon:
- Temperature of hot reservoir
- Temperature of cold reservoir
- Friction and heat losses (dissipative effect)
Q.12: What happens to the temperature of a room in which an air conditioner is left running on a table in the middle of the room?
Ans: When an air conditioner is left running on a table in the middle of a room, heat is removed from the room by the air conditioner. But, heat is radiated on the other side to the room by the coils (condenser) at the back of the refrigerator. The heat pumped out the back of the air conditioner and into the room is greater than the heat pulled into the front of the unit, as work done to remove the heat from cold hot puts into the room an additional amount of heat . Consequently, the air conditioner warms the room.
Q.13: When a sealed thermos bottle full of hot coffee is shaken? What are the changes, if any in?
Ans: The temperature of the coffee increase due to shaking.
- The internal energy of the coffee increases. Infect, the work done in shaking the coffee appears as increase in internal energy. Hence the temperature of the coffee increases (due to friction of walls of the flask).
Q.14: When an object is heated, not all the energy it absorbs goes into increasing the velocity of the molecules? Explain where does the remaining energy go?
Ans: When an object is heated, not all the energy it absorbs goes into increasing the velocity of the molecules. Some goes into the rotational motion of the molecules and some into the internal vibration motion.
Q.15: If a pendulum clock has to keep correct time at different temperatures, will it be better to use aluminum or steel?
Ans: It is better to use steel in a pendulum clock that has to keep correct time at different temperatures because the coefficient of thermal expansion of steel is almost half of that of aluminum.
Q.16: Why is the average velocity of the molecules of a gas zero but the average of the squares of the velocities is not zero?
Ans: Due to the random motion of the molecules in a gas, the number of molecules, on the average, moving with certain velocities along the positive x, y, and z axis is equal to the number of molecules moving along negative x, y, and z axis with the same velocities. Hence the average velocity of the molecules of a gas is zero. But however, the square of a negative component of velocity is also positive. Hence the square of the molecular velocities is not zero.
Q.17: (a) What is kilo-mole of 72g of water? (b) What is the value of universal gas constant in J.k mole⁻¹ k⁻¹?
Ans:
a) Kilo-mole is the mass of a substance in kilogram and is numerically equal to the molecular mass of a substance in kilogram.
Number of kilo-mole,
For water,
b)
Q.18: Why does the pressure of a gas in an automobile lyre increase if the automobile is driven for a while?
Ans: When an automobile is driven on road, it does work to overcome the friction between the tires and the road. So heat is produced. This, in turn, raises the temperature of the gas. Since pressure is directly proportional to temperature, hence pressure of the gas in the tire increases.
Q.19: Under what condition can heat be added to a system without changing its temperature?
Ans: Heat can be added without changing the temperature of the system.
- For a gaseous system, it can be achieved in an isothermal process.
- For a liquid, it can be achieved at the boiling point of the liquid.
- For a solid, it can be achieved at melting point of the solid.
Q.21: Is it possible to cool a room by keeping the refrigerator door open?
Ans: A room cannot be cooled by leaving the door of an electric refrigerator open. Whatever heat is removed from the air directly in front of the open refrigerator is deposited directly back into the room at the rear of the unit. Also, work done to remove the heat.
Q.22: When does the entropy of a system decrease?
Ans: When it rejects to the surroundings.
Q.23: Is it possible, according to the second law of thermodynamics, to construct an engine that is free from thermal pollution?
Ans: It is not possible to construct an engine free from thermal pollution since heat rejected to a sink is an essential requirement. This sink is the atmosphere to which the heat rejection results in thermal pollution. The small temperature changes have disruptive effects on the overall ecological balance. Thermal pollution is an invertible consequence of the second law of thermodynamics.
Q.24: When two systems are in thermal equilibrium, do they have the same amount of kinetic energy?
Ans: Temperature is a measure of the average translational kinetic energy of the molecules of a system. However, the systems with the same average translational kinetic energy have the same temperature, even if one has greater internal energy (due to greater rotational and vibration energy).
Q.25: What do you mean by “heat is energy in transit”?
Ans: Heat is not the energy that a body contains; it refers to the amount of energy transferred from a hot body to a cold body.
Q.26: What is the nature of the graph between the length and temperature of a heated metal rod?
Ans: The graph between length and temperature is a straight line (linear relationship).
Q.27: How much work should be done to produce 1 calorie of heat?
Ans: 4.2 J, since . When work is in Joule and quantity of heat is measured in calories, then Joules constant .
Q.28: Why are the values of molar heat capacities of substances (with a few exceptions) the same, i.e., almost mole⁻¹ k⁻¹?
Ans: In the case of molar heat capacity, we add heat energy to the same number of molecules, irrespective of the nature of the substance. Thus .
Q.29: Why is the specific heat of polyatomic gases higher than that of monatomic gases? Calculate specific heat ratio () of a monatomic gas.
Ans: In a monatomic gas, the whole of the supplied energy is used up in increasing the translational kinetic energy, i.e., its temperature. But in diatomic or polyatomic gases, the heat energy supplied is wasted in increasing the rotational kinetic energy and vibration kinetic energy. Thus to obtain the same range of temperature, more heat is required for polyatomic gases.
- Increase of KE of 1 molecule of monatomic gas =
- Increase of KE of 1 mole of monatomic gas =
- Increase of KE of 1 mole of monatomic gas per Kelvin =
- Increase of K.E. of 1 mole of monatomic gas per Kelvin =
Q.30: Work can be converted completely into heat (W a Q), so can heat be converted completely into work?
Ans: A given amount of heat cannot be completely converted into work, as some of the heat energy is used up in increasing the internal energy of the system. If heat is converted completely to work, then efficiency of heat engine will be 100%. This violates the second law of thermodynamics.
Q.31: Entropy has often been called as “time arrow”. Explain.
Ans: Entropy is called “time arrow,” because it tells us in which direction the time is going. The normal sequence of events is that in which disorder increase with time.
