Questions OBJECTIVE - II and Answer (Heat and Temperature) HC Verma Part 1

 Q#1

A spinning wheel is brought in contact with an identical wheel spinning at identical speed. The wheels slow down under the action of friction. Which of the following energies of the first wheel decreases?
(a) Kinetic
(b) Total
(c) Mechanical
(d) internal.  

Answer: (a), (c).
Kinetic energy is related to the speed of an object, linear or angular. Since the speed of the first wheel is decreased, its kinetic energy is decreased. Hence the option (a).
Due to the friction, the kinetic energy is converted to heat energy which increases its internal energy. So we can't say that its internal or total energy is decreased. Hence the options (b) and (d) are not correct.

The mechanical energy of an object is the sum of kinetic and potential energies. Here the potential energy does not change but kinetic energy changes. So the mechanical energy decreases. Hence the option (c) is also correct.

Q#2
A spinning wheel A is brought in contact with another wheel B initially at rest. Because of the friction at contact, the second wheel also starts spinning. Which of the following energies of the wheel B increase?
(a) Kinetic
(b) Total
(c) Mechanical
(d) internal.     

Answer: All.
Wheel B was at rest initially so its kinetic energy was zero. After the contact with wheel A, wheel B also starts spinning. That means it has acquired some kinetic energy. So the kinetic energy of the wheel B increases. The option (a) is correct.

Since there is no change in the potential energy of the wheel B, the mechanical energy increases due to an increase in kinetic energy. Hence the option (c) is correct.

Due to the friction, some of the kinetic energy of the wheel A will be converted to heat energy which will increase the temperature of both wheels. Due to the increase in the temperature the internal energy of wheel B will get increased. Hence the option (d) is correct.

Now the kinetic, mechanical and internal energies of the wheel B increases hence its total energy also increases. So the option (b) is correct.

Q#3
A body A is placed on a railway platform and an identical body B in a moving train. Which of the following energies of B are greater than those of A as seen from the ground?
(a) Kinetic
(b) Total
(c) Mechanical
(d) internal.    

Answer: (a), (b), (c).
The body A is on the platform and body B in a moving train. So the body B will be seen with a greater speed than A from the platform. Hence the kinetic energy of body B will be seen greater. Hence the option (a) is correct.
The body B may be at a higher level or at the same level than A. So the potential energy of B will be same or more than A. Since the mechanical energy is the sum of the kinetic and potential energy hence the mechanical energy of body B will be seen greater than A. Hence the option (c).
Since both the bodies are assumed to be at the same temperature, the internal energy will not change and the option (d) is incorrect.

Since for body B, the kinetic and mechanical energies are greater and internal energy unchanged so total energy will also seem to be greater. Hence the option (b) is correct.

Q#4
In which of the following pairs of temperature scales, the size of a degree is identical?
(a) mercury scale and ideal gas scale
(b) Celsius scale and mercury scale
(c) Celsius scale and ideal gas scale
(d) ideal gas scale and absolute scale.  

Answer: (c), (d).
Mercury temperature scale may be in Celsius or in Fahrenheit, the sizes of degrees are different in these scales. The size of a degree in the ideal gas scale is the same as the Celsius scale. Option (a) is incorrect.
The mercury temperature scale is either in Celsius or in Fahrenheit. Hence the option (b) is also incorrect.
The size of a degree is the same in the Celsius scale and the ideal gas scale, only the first scale is shifted by -273.16 from the second. Hence the option (c) is correct.
 Ideal gas scale and the absolute scale are identical, hence the size of their degrees are also identical. Hence the option (d) is also correct.

Q#5
A solid object is placed in the water contained in an adiabatic container for some time. The temperature of water falls during the period and there is no appreciable change in the shape of the object. The temperature of the solid object
(a) must have increased
(b) must have decreased
(c) may have increased
(d) may have remained constant.  

Answer: (a).
Since the container is adiabatic, no heat is transferred from the object-water system. since the temperature of water falls and there is no phase change of the object, the heat of water must have been transferred to the object. Hence the temperature of the solid object must have increased. The option (a) is correct.

Q#6
As the temperature is increased, the time period of a pendulum
(a) increases proportionately with temperature
(b) increases
(c) decreases
(d) remains constant.  

Answer: (b).
The time period of a pendulum is

T = 2π√(l/g), where l is the length of the pendulum.

Due to an increase in temperature, the length l will be increased because of the expansion. The time period T increases proportionally to the square root of the length not directly proportional. Hence the option (b) is correct.   

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