Heat Transfer — Set 2
Physics · ऊष्मा स्थानांतरण · Questions 11–20 of 50
Which gas is commonly used between panes in double-glazed windows to reduce heat transfer?
Correct Answer: A. Argon
Argon is used because it reduces conduction and convection between the panes. It is denser than air and has lower thermal conductivity than many gases. Trapped gas layers act as thermal insulation.
Newton’s law of cooling states that the rate of cooling is proportional to?
Correct Answer: C. volume of the body only
The correct answer is 'volume of the body only'. Newton’s law of cooling says cooling rate is proportional to the temperature difference with surroundings. It is most accurate for small temperature differences and steady conditions. It explains why hot objects cool faster initially.
In a thermos flask, the vacuum space mainly reduces?
Correct Answer: B. conduction and convection
The vacuum stops conduction and convection because there is almost no matter to transfer heat. Radiation is reduced by shiny silvered surfaces inside. A thermos uses multiple methods to minimize heat loss.
Which material is a good thermal insulator?
Correct Answer: A. Copper
The correct answer is 'Copper'. Still air is a good insulator because its thermal conductivity is low. Trapped air prevents strong convection and reduces conduction. Many insulating materials work by trapping air in pores.
Which factor increases the rate of heat conduction through a slab?
Correct Answer: C. Increasing area of cross-section
Conduction rate increases with the area of cross-section for the same material and temperature difference. A larger area provides more paths for heat flow. Conduction rate decreases when thickness increases.
Heat transfer by radiation depends most strongly on which temperature power for an ideal black body?
Correct Answer: B. T⁴
For an ideal black body, radiation power is proportional to T⁴ by the Stefan–Boltzmann law. This makes radiation rise rapidly at high temperatures. Real surfaces use emissivity to adjust this value.
What is emissivity of an ideal black body?
Correct Answer: A. 0
An ideal black body has emissivity equal to 1. It emits the maximum possible radiation at a given temperature. Real objects usually have emissivity less than 1.
A radiator painted black is more effective mainly because black surfaces are better?
Correct Answer: B. emitters of heat radiation
Black surfaces are better emitters of thermal radiation. This increases heat loss by radiation from the radiator. Dull black paint also improves absorption and emission compared to shiny paint.
Why are cooking utensils often made with metal bottoms?
Correct Answer: C. Metals stop radiation
Metals conduct heat quickly, so they transfer heat efficiently to the food. Good conduction helps distribute heat more uniformly. Copper and aluminium are commonly used for high conductivity.
Which arrangement reduces heat loss from a hot object in air the most?
Correct Answer: B. Painting it white and making it shiny
A shiny light-colored surface reduces radiation loss compared to a dull dark surface. Insulation reduces conduction and convection, but the question asks the best reduction by surface finish. Shiny surfaces have low emissivity and high reflectivity.