Question 1

Is heat a conserved quantity?

Accepted Answer

Yes, heat is a conserved quantity. Suppose two bodies A and B are at different temperatures, wherein A is higher and B is lower. When they are brought in contact, the heat given by A is equal to the heat gained by B. Thus, the heat is conserved in the system.Here, we have ignored the heat exchanged with the surroundings. If we consider the surroundings, the heat of the system will be conserved.

Question 2

The calorie is defined as 1 cal = 4.186 joule. Why not as 1 cal = 4 J to make the conversions easy?

Accepted Answer

One calorie is defined as the heat required to raise the temperature of 1 g of water from 14.50C  to 15.50C . Also, exactly 4.186 J of work is to be done on heating 1 g of water from 14.5o C to 15.5o C. So, we cannot round-off 4.186 J to 4 J as it will give significant difference in work done required to raise the temperature and heat required to raise the temperature. Since work and heat are equivalent here, so on taking 1 calorie = 4 J, the validity of the equivalence of work and heat will get defied. Thus, just to make conversion easy, 1 calorie cannot be taken equal to 4 J.

Question 3

A calorimeter is kept in a wooden box to insulate it thermally from the surroundings. Why is it necessary?

Accepted Answer

A calorimeter is kept in a wooden box to insulate it thermally from the surroundings because in order to determine the specific heat capacity, the total heat transferred must be known. Heat must not be exchanged with the surroundings, otherwise the principle of calorimeter would not stand valid.

Question 4

In a calorimeter, the heat given by the hot object is assumed to be equal to the heat taken by the cold object. Does it mean that heat of the two objects taken together remains constant?

Accepted Answer

Yes, heat of the two objects taken together remains constant. If no heat is lost to the surroundings, the heat of the two bodies taken together actually remains conserved.

Question 5

In Regnault's apparatus for measuring specific heat capacity of a solid, there is an inlet and an outlet in the steam chamber. The inlet is near the top and the outlet is near the bottom. Why is it better than the opposite choice where the inlet is near the bottom and the outlet is near the top?

Accepted Answer

The inlet is near the top and the outlet is near the bottom because there is a loss of heat from the steam as it passes through the chamber. As the steam loses heat, a part of it condenses back to water and the cold steam gets denser and moves down towards the bottom.But when done the other way round, the used steam does not pass through the chamber correctly and gets mixed up. This can result in discrepancy in the desired results.

Question 6

When a solid melts or a liquid boils, the temperature does not increase even when heat is supplied. Where does the energy go?

Accepted Answer

When a solid melts or a liquid boils, the heat supplied is actually used to break the bond forces between the molecules and bring them apart till the body changes its state completely. Thus, the energy is transferred to the molecules as kinetic energy and the temperature of body remains constant in the process.

Question 7

What is the specific heat capacity of 
(a) melting ice?
(b) boiling water?

Accepted Answer

(a) The specific heat capacity of melting ice is 0.50 cal/g-oC or 2093 J/kg-K.
(b) The specific heat capacity of boiling water is 0.46 cal/g-oC or 1926 J/kg-K.

Question 8

A person's skin is more severely burnt when put in contact with 1 g of steam at 100°C than when put in contact with 1 g of water at 100°C. Explain

Accepted Answer

Steam has greater energy than boiling water due to latent heat of vapourisation. The internal energy of the vapour at 1000C  is greater than the internal energy of the boiling water at the same temperature. Thus, steam burns the skin more severely than boiling water.

Question 9

The atmospheric temperature in the cities on sea-coast change very little. Explain

Accepted Answer

The atmospheric temperature in the cities on the sea-coast changes very little due to the following reasons:-1) The specific heat capacity of water is four times greater than land. This means water heats and cools more slowly than land. Also, once the ocean has warmed, it gives up its heat much slower than the land.2) The moisture content of the air over coastal locations is very high. Moisture in the form of water vapour is the predominate greenhouse gas in the troposphere. Water vapour prevents the loss of heat into space at night. Thus, the temperature during day and night is almost the same.

Question 10

Should a thermometer bulb have large heat capacity or small heat capacity?

Accepted Answer

The thermometer bulb should have large heat capacity because if it has lower heat capacity, it may expand at high temperatures resulting in false readings. Large heat capacity of the thermometer bulb ensures the correct reading of temperature by restricting the bulb from expanding.

Question 11

The specific heat capacity of a body depends on _____________ .

Accepted Answer

The material of the body Heat capacity of a body is due to their material properties. Due to different molecular structures, different bodies have a different capacity to absorb heat. Therefore, specific heat of a body depends on the material of the body.

Question 12

Water equivalent of a body is measured in ___________ .

Accepted Answer

Kg Since water equivalent of a body is the mass of the water having the same heat capacity as the given body, the water equivalent is measured in kilogram.

Question 13

When a hot liquid is mixed with a cold liquid, the temperature of the mixture ____________ .

Accepted Answer

Is undefined for some time and then becomes nearly constantWhen hot liquid is mixed with cold liquid, the molecules collide and transfer heat. When heat is transferred, the temperature is undefined. Once the heat energy is shared by the molecules, the system reaches equilibrium and the temperature becomes nearly constant.

Question 14

Which of the following pairs represent units of the same physical quantity?

Accepted Answer

Joule and calorie One calorie is defined as the amount of heat needed to raise the temperature of 1 g of water from 14.5° to 15.5° at the pressure of 1 atm. Heat is a form of energy and the unit of energy is joule.Therefore, joule and calorie represent energy.

Question 15

Which of the following pairs of physical quantities may be represented in the same unit?

Accepted Answer

Heat and WorkAs work done in raising temperature of a body is actually the heat supplied to the body, heat and work may be represented in the same unit.

Question 16

Two bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved?

Accepted Answer

Total internal energy of the two bodiesWhen two bodies at different temperatures are mixed in the calorimeter, heat flows from one body to the other due to the temperature difference.This results in change in the internal energy of the individual bodies. There is no exchange of heat with the surrounding in the calorimeter. Thus, the total internal energy of the bodies remain conserved as no external work is done on them.

Question 17

The mechanical equivalent of heat ____________ .

Accepted Answer

Is dimensionless If the mechanical work done (W) produces the same temperature change as heat (H),then the mechanical equivalent of heat (J) is equal to W/H. Thus,J = W/HSince the unit of work and heat is the same, mechanical equivalent of heat is dimensionless.

Question 18

The heat capacity of a body depends on:

Accepted Answer

(c) The mass of the body(d) The material of the body The bigger the body, the larger is its capacity to absorb heat. Therefore, the heat capacity of a body depends on the mass of the body. Also, different bodies have different heat capacities due to their material properties, i.e. due to their molecular structure, the heat capacity of a body depends on the material of the body.

Question 19

The ratio of specific heat capacity to molar heat capacity of a body _____________ .

Accepted Answer

Depends of the molecular weight of the body Specific heat capacity of a body,s=mΔθQ​Here,Q = Heat suppliedm = Mass of bodyΔθ = Change in temperatureMolar heat capacity of a body,C=nΔθQ​Here,Q = Heat suppliedn = Number of molesΔθ = Change in temperature∴ The ratio of the specific heat capacity and molar heat capacity is given byCs​=nΔθQ​mΔθQ​​=mn​=nMn​=M1​Here,M = Molar mass related to number of moles m = MassAs the value of M is different for different bodies of different composition, the ratio cannot be a universal constant.Also, the ratio is independent of the mass of the body.The ratio of the specific heat and molar heat capacity depends on the molecular weight of the body.Clearly, the unit of molecular weight is kg/mole. So, the ratio that depends only on the molecular weight cannot be dimensionless.

Question 20

If heat is supplied to a solid, its temperature:

Accepted Answer

(b) May increase(c) May remain constant When heat is supplied to a solid, it is used up either to increase the temperature of the body or to change its state from one form to another by breaking the bonds between the molecules (without raising the temperature).When heat is supplied to the solid, the internal energy of the solid increases, so the temperature does not decrease.

Chapter 3 : Calorimetry