Test: Basic Concepts of Heat Transfer - 3 - Mechanical Engineering MCQ

Mica, Bakelite and fibre glass are insulator material and carbon in the form of diamond is the good conductor of heat but bad conductor of electricity

Wood is considered as orthotropic material since property of grain along the wood is different and across the grain is different.

Due to high temperature difference between your finger and the tray, some heat will be transferred to the tray, depending on its capacity for thermal conductivity.
Since the surface of your skin is naturally moisturized (it’s not a function of the air humidity), the loss of heat will cause a thin layer of ice to form on the skin which in turn sticks to the tray. No matter how dry is the air and how cold is the freezer, your finger will not stick to a wooden tray whose thermal conductivity is low. So the most important factor is the thermal conductivity of the tray.

Since thermal diffusivity, α = k/ρc
(ρC)_liquid > (ρC)_gas
α_gas > α_liquid

Thermal diffusivity, α = K/ρC
Hence its unit = m²/hr or m²/sec
Note that unit of thermal diffusivity and kinematic viscosity is same.

Note that the concept of overall heat transfer is useful in combined mode of heat transfer like conduction and convection

The ratio of thermal conductivity to electrical conductivity is always constant for all the conductors at the given temperature. This is know as Wideman and Frange law.

The constant is known as Lorentz number (Lo) whose value is equal to 2.045 x 10^-8 W/μK².