Question  set 143

Thermal Conductivity of Materials

1.     Thermal conductivity is defined as the heat flow per unit time
a) When the temperature gradient is unity
b) Across the wall with no temperature
c) Through a unit thickness of the wall
d) Across unit area where the temperature gradient is unity
Answer: d
Explanation: Thermal conductivity of a material is because of migration of free electrons and lattice vibrational waves.

2.     Mark the matter with least value of thermal conductivity
a) Air
b) Water
c) Ash
d) Window glass
Answer: a
Explanation: For air, it is .024 W/ m degree i.e. lowest.

3.     Which one of the following forms of water have the highest value of thermal conductivity?
a) Boiling water
b) Steam
c) Solid ice
d) Melting ice
Answer: c
Explanation: For ice, it is 2.25 W/m degree i.e. maximum.

4.     The average thermal conductivities of water and air conform to the ratio
a) 50:1
b) 25:1
c) 5:1
d) 15:1
Answer: b
Explanation: For water, it is 0.55-0.7 W/m degree and for air it is .024 W/m degree.

5.     Identify the very good insulator
a) Saw dust
b) Cork
c) Asbestos sheet
d) Glass wool
Answer: d
Explanation: Glass wool has a lowest thermal conductivity of 0.03 W/m degree amongst given option.

6.     Most metals are good conductor of heat because of
a) Transport of energy
b) Free electrons and frequent collision of atoms
c) Lattice defects
d) Capacity to absorb energy
Answer: b
Explanation: For good conductors, there must be electrons that are free to move.

7.     Heat conduction in gases is due to
a) Elastic impact of molecules
b) Movement of electrons
c) EM Waves
d) Mixing of gases
Answer: a
Explanation: If there is elastic collision then after sometime molecules regain its natural position.

8.     The heat energy propagation due to conduction heat transfer will be minimum for
a) Lead
b) Water
c) Air
d) Copper
Answer: c
Explanation: It is because air has lowest value of thermal conductivity amongst given options.

9.     Cork is a good insulator because
a) It is flexible
b) It can be powdered
c) Low density
d) It is porous
Answer: d
Explanation: Cork has thermal conductivity in the range of 0.05-0.10 which is very low so it can be porous.

10. Choose the false statement
a) For pure metal thermal conductivity is more
b) Thermal conductivity decreases with increase in the density of the substance
c) Thermal conductivity of dry material is lower than that of damp material
d) Heat treatment causes variation in thermal conductivity
Answer: b
Explanation: Thermal conductivity increase with increase in the density of a substance.