Thermal Physics: Important part-A 2 marks Short Questions and Answers

ANNA UNIVERSITY 'PART‒A' QUESTIONS & ANSWERS

1) What are the basic entities responsible for thermal conduction of a solid ?

(i) Area of cross section (A)

(ii) Temperature difference between the hot and cold layers of the solid (θ₁‒ θ₂)

(iii) Time of conduction (t)

(iv) Thickness of the solid (x)

2) Define Coefficient of thermal conductivity.

The Coefficient of thermal conductivity is defined as the amount of heat conducted per second normally across the unit area of cross section, maintained at unit temperature gradient.

(i.e.) K = Q_x / A(θ₁‒ θ₂)t     Wm^‒1K^‒1

3) What is a bimetallic strip? Give its applications.

Definition:

Bimetallic strips are made up of two thin metal strips as shown in Fig. with different co‒efficient of thermal expansion.

Applications

1. Bimettalic strips are commonly used in water heaters as temperature controller.

2. They are used as a switch to maintain the temperature of the liquid.

4. Distinguish between conduction and convection.

Conduction

1. It is the process in which the heat is transferred from hot end to cold end without the actual movement of the particles.

2. This process take place in solids

Convection

1. It is the process in which the heat is transmitted from hot end to cold end by the actual movement of the particles.

2. This process take place in liquids

ADDITIONAL PART A QUESTIONS & ANSWERS

1. What are modes by which transfer of heat energy takes place.

Normally there are three modes by which transfer of heat take from one place to another viz., conduction, convection and radiation.

Conduction: It is the process in which heat is transferred from hotter end to colder end without the actual motion of the particles.

Convection: It is the process in which heat is transmitted from hotter end to colder end by the actual movement of heated particles.

Radiation: It is the process in which heat is transmitted from one place to another without the necessity of the intervening medium.

2) Define radiation and give an example.

It is the process in which heat is transmitted from one place to another without the necessity of the intervening medium.

Example: The says from the Sun reacting the Earth.

3) What is meant by thermal expansion in solids?

When a metal is heated, due to increase in temperature it will expand and when it is cooled it contracts. However, an internal force will always act so as to keep the metal to regain its original length/position.

Thermal expansion: The expansion of a metal, when subjected to heat is called thermal expansion.

4) Define Co‒efficient of Thermal expansion.

The co‒efficient of thermal expansion (α): The co‒efficient of thermal expansion can be defined as the ratio between the change in length to the original length per unit rise of temperature.

The co‒efficient of thermal expansion α = dl/lθ

If θ = 1, then α = dl/lθ

5) Define Co‒efficient of volume expansion.

The co‒efficient of volume expansion 'β' is defined as ratio between the fractional change (ΔV) in volume to the original volume (V) per unit rise of temperature (ΔT)

i.e., ΔV / V = β ΔT

When ΔT = 1

β = ΔV / V

6) What is meant by Thermal expansion in liquids?

We know when as ordinary alcohol‒in‒glass thermometer (or) mercury‒ in‒glass thermometer is kept in the temperature bath, the alcohol (or) mercury rises, due to thermal expansion.

In this case it should be noted that the temperature rises not because of the expansion of liquid, it is only due to the volume expansion of the liquid, it is only due to the volume expansion of the liquid.

7) What do you understand by the term. "Bimetallic Strip"? Give its use.

Definition:

As the name itself implies that Bimetallic strips are made up of two thin metal strips with different co‒efficient of thermal expansion.

Bimettalic strips are commonly used in water heaters as temperature controller.

8) What is meant by temperature gradient ?

The rate of fall of temperature with respect to the distance is called as temperature gradient. In general it is denoted as ‒dθ/dx. The negative sign indicates the fall of temperature with the increase in distance.

9) Define thermal diffusivity.

It is defined as the ratio of thermal conductivity to the thermal capacity per unit volume of the material

Thermal diffusivity (h) = Thermal conductivity / Thermal capacity.

Since thermal capacity is the product of specific heat capacity(s) and density of the material (ρ), we can write

 h = k / ρs         m²s^‒1

10) Derive the unit for thermal conductivity.

The thermal conductivity of material is K = Qx / A(θ₁‒ θ₂)t

= [ Joules × metre ] / [ (metre)² × kelvin × second ] = watts / ( metre × kelvin)

The unit of thermal conductivity is Wm^‒1K^‒1.

11) Write any four uses of expansion joints.

(i) Buildings: Expansion joints prevents the excess stress in buildings and minimize the cracks.

(ii) Walls: In walls, especially in long walls, expansion joints will allow expansion & contraction and maintain building structure.

(iii) Floors: It is used to accomodate movement in large floor slabs.

(iv) Foundations: Though the effect of temperature less in foundations, seperate joints shall be used to address the load differences and hence it helps the building foundations to withstand natural calamites like earth quakes.