Heat Transfer — Set 5
Physics · ऊष्मा स्थानांतरण · Questions 41–50 of 50
Which factor does NOT directly affect radiative heat transfer between two bodies?
Correct Answer: C. Thickness of the body only
Radiative heat transfer depends on temperature, area, and emissivity of surfaces. Thickness alone does not directly set the radiation emitted from the surface. Surface properties matter more than bulk thickness for radiation.
Why is the bottom of a cooking pan often blackened?
Correct Answer: D. To increase absorption of heat radiation
Blackened surfaces absorb heat radiation better than shiny surfaces. This helps the pan gain heat more effectively from a flame or hot surroundings. Black surfaces are generally good absorbers and good emitters.
A thermal insulator has thermal conductivity k close to?
Correct Answer: B. infinite value
The correct answer is 'infinite value'. A good insulator has very low thermal conductivity. Low k means heat flows slowly through the material. Examples include foam, wool, and still air trapped in pores.
In free convection, the driving factor for fluid motion is mainly?
Correct Answer: C. magnetic field
The correct answer is 'magnetic field'. Free convection is driven by density differences created by temperature changes. Warmer fluid becomes lighter and rises while cooler fluid sinks. Forced convection happens when a fan or pump drives motion.
If heat flow Q through a wall is 600 J in 10 s, what is the heat transfer rate?
Correct Answer: D. 60 W
Heat transfer rate is power P = Q/t. Here P = 600/10 = 60 W, so Option D is correct. Power in heat transfer is measured in watts.
A metal plate of area 0.5 m² emits radiation with emissivity 0.6 at temperature T. Compared to a black body of same area and temperature, its emitted power is?
Correct Answer: A. 1.0 times
Emitted power equals emissivity times black body power for the same T and A. With emissivity 0.6, it emits 0.6 times the black body value, and that correct option is placed at B. Emissivity cannot exceed 1 for ordinary surfaces.
Which statement about convection is correct?
Correct Answer: C. It involves heat transfer with bulk movement of fluid
Convection involves bulk motion of a fluid carrying heat from one place to another. It occurs in liquids and gases, not in vacuum. Natural convection is caused by buoyancy due to density differences.
A slab has k = 0.5 W m⁻¹ K⁻¹, thickness 0.1 m, area 2 m², and temperature difference 20 K. What is the heat flow rate?
Correct Answer: A. 5 W
Use Q/t = kAΔT/L for steady conduction. Here Q/t = 0.5×2×20/0.1 = 200 W, and that correct value is placed at D. Increasing thickness reduces conduction rate for the same k, A, and ΔT.
Which material is commonly used as insulation in refrigerators due to trapped gas cells?
Correct Answer: C. Glass rod
The correct answer is 'Glass rod'. Foam insulation works because it traps gas in many small cells, reducing conduction and convection. The trapped gas has low thermal conductivity. This helps keep the inside cold with less heat gain from outside.
Why does a blackened kettle cool faster than a shiny kettle in the same room?
Correct Answer: C. Black kettles stop evaporation completely
A blackened surface has higher emissivity, so it emits thermal radiation more strongly. This increases heat loss by radiation compared to a shiny surface. Shiny surfaces are poor emitters and good reflectors.