Heat Transfer — Set 4
Physics · ऊष्मा स्थानांतरण · Questions 31–40 of 50
Which equation represents Fourier’s law for one-dimensional steady heat conduction?
Correct Answer: B. Q/t = kA(ΔT/L)
Fourier’s law for a slab is Q/t = kAΔT/L. It shows heat flow rate is proportional to area and temperature difference. It is widely used for steady conduction in solids.
Which expression is commonly used for Newton’s law of cooling (simple form)?
Correct Answer: B. dT/dt ∝ -(T - Ts)
Newton’s law of cooling states dT/dt is proportional to the negative of (T - Ts). This means cooling is faster when the temperature difference is larger. It is an approximation valid under certain conditions.
Which mode of heat transfer is dominant in liquids and gases under gravity when heated from below?
Correct Answer: D. Convection
Convection becomes dominant because warmer fluid rises and cooler fluid sinks. This creates a circulation that transports heat efficiently. Heating a fluid from above reduces convection currents.
Stefan–Boltzmann law for an ideal black body gives radiated power as?
Correct Answer: B. P ∝ T²
Stefan–Boltzmann law states radiated power is proportional to T⁴ for a black body. A small rise in temperature can greatly increase radiation. Real bodies use emissivity to modify the ideal value.
What is the primary reason woolen clothes keep the body warm?
Correct Answer: A. Wool increases radiation from the body
Wool traps still air, which is a poor conductor and reduces convection. This lowers the heat loss from the body. Many insulating fabrics work by trapping air in small pockets.
Why are handles of cooking pans often made of wood or plastic?
Correct Answer: D. They increase heat radiation
Wood and plastic are poor conductors, so they reduce heat transfer to the hand. This keeps the handle cooler for safer holding. Metals are avoided for handles because they conduct heat quickly.
A black surface compared to a white surface at the same temperature generally has?
Correct Answer: C. higher emissivity
A black or dull surface generally has higher emissivity than a white or shiny surface. Higher emissivity means better emission of thermal radiation. Good absorbers are usually good emitters at the same wavelength.
Which statement about a perfect reflector is correct?
Correct Answer: D. It emits maximum radiation
A perfect reflector reflects all incident radiation. It absorbs almost none, so its absorptivity is near zero. Highly polished metals approximate this behavior for some wavelengths.
If a surface has emissivity 0.8, its radiation compared to a black body at the same temperature is?
Correct Answer: D. 0% of black body
Radiation from a surface equals emissivity times the black body radiation at the same temperature. With emissivity 0.8, it emits 0.8 times the black body value. Emissivity lies between 0 and 1 for ordinary surfaces.
Which situation best represents heat transfer mainly by radiation?
Correct Answer: D. Cooling by a fan only
The correct answer is 'Cooling by a fan only'. You can feel heat from a fire at a distance mainly due to radiation. Thermal radiation travels as electromagnetic waves through air. Conduction needs contact and convection needs moving fluid.