Physics XII Reasoning Questions

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Physics XII Reasoning Questions for SSC.

Text of Physics XII Reasoning Questions

Chapter 11: Heat

Q1How do you distinguish between temperature and heat? Give example.Ans:Heat is the energy flowing between a body and the surrounding due to difference of temperature. But temperature is the measure of the average translational kinetic energy of the molecules of body. If we dip a red hot iron ball in the sea, heat will flow from the ball to the water (from high temperature to low temperature), although the amount of internal energy possessed by the ball is very small as compared to the immense amount contained in the sea.Q2Why 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 in various ways, e.g. re-radiation, conduction, convection and evaporation, compensating the heat transfer. Moreover, the earth and the sun are also not in physical contact with each other. The average temperature of the earth is about 300 K.Q3It is observed that when mercury thermometer is put in a flame, the column of mercury first descends then rises. Explain.

Ans:When a mercury thermometer is put in a flame, the glass bulb of thermometer expands; thus the column of mercury descends. But no sooner the heat reaches mercury in the bulb, which expands, and this expansion is greater than that of the glass bulb so the mercury column rises.Q4It is correct that unit for Specific Heat Capacity is m2.s2.C1.Ans:Yes. It is correct. The formula for Specific Heat Capacity is given by:

The S.I. unit of Q5When 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:The thermal expansion of homogenous 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 (becomes big).Q6A thermometer is placed in direct sunlight. Will it read the temperature of air, or of sun, or of something else?Ans:The thermometer will record the temperature of the surrounding.Q7One kilogram of hydrogen contains more atoms than one kilogram of lead. Explain.Ans:One kilogram of hydrogen contains 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 that of lead).Q8The pressure in gas cylinder containing hydrogen will leak more quickly than if it contained oxygen. Why?Ans:This is because the hydrogen molecules are lighter than oxygen molecules (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.Q9What are some factors that affect the efficiency of automobile engine?Ans:The efficiency of an automobile engine depends upon (i) temperature of hot reservoir, (ii) temperature of cold reservoir and (iii) friction and heat losses (dissipative effect).Q10What happens to the temperature of a room if 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 the 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 air-conditioner. The heat pumped out of the back of the air-conditioner and into the room is greater than the heat pulled into the front of the unit, as the work done to remove the heat puts into the room an additional amount of heat. Consequently, the air-conditioner warms the room.Q11When a sealed thermos of hot coffee is shaken, what are the changes, if any in: -a) The temperature of the coffee

b) The internal energy of the coffeeAns:(a) The temperature of the coffee increases due to shaking. (b) The internal energy of the coffee increases.

In fact 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 flask).Q12When an object is heated, not all the energy it absorbs goes into increasing the velocity of molecules. Explain where the remaining energy goes?Ans:When an object is heated, not all the energy it absorbs goes into increasing the velocity of molecules. Some goes into the rotational motion of the molecules and some into the internal vibration motion.Q13If a pendulum clock is to keep the correct time at different temperatures, will it be better to use aluminium or steel?Ans:It is better to use steel in a pendulum clock that has to keep the correct time at different temperatures because the coefficient of thermal expansion of steel is almost half that of aluminium.Q14Whys is the average velocity of the molecules of a gas zero but 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 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. However, the square of a negative component of the velocity is also positive. Hence the square of the molecular velocities is not zero.Q15Why does the pressure of a gas in an automobile tyre 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 tyres 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 tyre increases.Q16Under what conditions can heat be added to a system without changing its temperature?Ans:Heat can be added to a system without changing its temperature in the following ways:(i) For a gaseous system, it can be achieved in isothermal process.

(ii) For a liquid, it can be achieved at the melting point of the solid.Q17Is it possible to cool a room by keeping the door of the refrigerator open?

Ans:A room cannot be cooled by keeping the refrigerator door open. Whatever heat is removed from the air directly in front of the refrigerator is pumped back into the room at the rear of the refrigerator. Also the work done to remove heat from the room put an additional amount of heat into the room.Q18Is it possible, according to second law of thermodynamics, to construct an engine that is free from thermal pollution?Ans:It is not possible to construct an engine that is free from thermal pollution since heat rejected to sink is an essential requirement. The sink is the atmosphere to which the heat rejection results in thermal pollution. The small temperature changes have disruptive effect on the overall ecological balance. Thermal pollution is an inevitable consequence of the second law of thermodynamics.Q19When two systems are in thermal equilibrium, do they have same amount of kinetic energy?Ans:Temperature is the 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).Q20What do you mean by heat is energy in transit?Ans:Heat is not the energy a body contains. It refers to the amount of energy transferred from hot body to cold body.Q21Why is the specific heat of polyatomic gases higher than the monatomic gases?Ans:In a monatomic gas, the whole of the heat supplied is used up in increasing the translational kinetic energy, i.e. its temperature. But in a polyatomic gas, the heat energy supplied is wasted in increasing the rotational and vibration kinetic energy. Thus to obtain the same ranges of temperature, more heat is required for polyatomic gases.Q22Work can completely be converted into heat (). Can heat be completely converted into work?Ans:A given amount of heat cannot be completely converted into work as some of the heat energy is used in increasing the internal energy of the system. If heat is completely converted into work, then efficiency of heat engine will be 100%. This violates the second law of thermodynamics.Q23Entropy has often been called as time arrow. Explain.Ans:Entropy has often been called as time arrow because it tells us in which direction the time is going. The normal sequence of events is that in which disorder increases with time.Q24Why is the molar specific heat of a diatomic gas usually greater than that of monatomic gas?Ans:A monatomic gas contains only one atom in its molecule. When it is heated, the whole of the heat supplied is used in increasing the translational kinetic energy of the gas molecule, which determines the rise in temperature. In case of diatomic gas, each molecule contains two atoms. Thus its molecules may have vibratory as well as rotation motion in addition to translational kinetic motion, and partly increase vibration and rotational kinetic energies. Thus more heat is added or needed to increase the temperature of a diatomic gas by same amount.Q25Why must more work be done when gas is heated at constant pressure than at constant volume?Ans:At constant volume, no work is done by the gas and heat supplied simply increases the internal energy or the temperature of the gas. At constant pressure, the gas expands and work is done. Hence heat supplied is partially used in increasing the internal energy of the gas and partially to do work. Hence more heat is required to raise the temperature of the gas through the same amount as in case of heating at constant volume. It means that more work must be done on the gas heated at constant pressure.Q26Why does the temperature drop in an adiabatic expansion process?Ans:In an adiabatic process, heat neither