Heat Transfer — Set 3
Physics · ऊष्मा स्थानांतरण · Questions 21–30 of 50
In a room, a ceiling fan helps cooling mainly by increasing?
Correct Answer: C. convection and evaporation
A fan increases air movement, which increases convection and evaporation from skin. Faster moving air removes the warm air layer near the body. This makes you feel cooler without changing room temperature much.
Thermal resistance of an insulating layer generally increases when its thickness?
Correct Answer: C. increases
Thermal resistance increases with thickness because heat must travel a longer path. For a slab, R is proportional to L/(kA). Higher resistance means lower heat flow for the same temperature difference.
Which statement about conduction is correct?
Correct Answer: A. It can happen only in vacuum
Conduction occurs mainly in solids through particle collisions and electron movement. It does not require bulk motion like convection. In vacuum, conduction cannot occur because there are no particles to collide.
A room is kept warm by a heater placed near the floor mainly because warm air?
Correct Answer: D. does not change density
The correct answer is 'does not change density'. Warm air becomes less dense and rises, creating convection currents. This circulates heat through the room. That is why heaters are often placed lower.
Which of these reduces heat transfer by convection from a hot cup of tea?
Correct Answer: D. Stirring the tea continuously
The correct answer is 'Stirring the tea continuously'. Covering the cup reduces convection by limiting air movement above the tea. It also reduces evaporation, which carries away heat. This helps the tea stay warm longer.
A 2 m long rod has cross-sectional area 1×10⁻⁴ m², thermal conductivity 200 W m⁻¹ K⁻¹, and ends at 100°C and 0°C. What is the heat flow rate?
Correct Answer: B. 1 W
Use Q/t = kAΔT/L for steady conduction. Here Q/t = 200×(1×10⁻⁴)×100/2 = 1 W, so option B is correct for 1 W? Actually it equals 1 W, and the correct option is 1 W placed at B. Conduction rate increases with k and area but decreases with length.
Which process transfers heat in solids without movement of matter as a whole?
Correct Answer: D. Conduction
Conduction transfers heat through collisions and electron motion without bulk movement. Solids mainly transfer heat this way. Convection needs fluid motion and cannot be the main mode inside a solid.
Which color of a surface generally reduces heat gain from sunlight the most?
Correct Answer: A. White
White surfaces reflect most incident radiation and absorb less heat. Dark surfaces absorb more radiation and heat up faster. This is why white clothes feel cooler in strong sunlight.
Which term describes heat transfer that does not require contact between bodies?
Correct Answer: A. Radiation
Radiation transfers energy through electromagnetic waves without contact. It can travel through vacuum as well as through media. Infrared is the main type of thermal radiation for ordinary temperatures.
In a vacuum bottle, silvered surfaces are used mainly to reduce?
Correct Answer: A. radiation
Silvered surfaces reflect thermal radiation and reduce radiative heat transfer. Vacuum already reduces conduction and convection strongly. Reflective coatings are used to minimize remaining radiation loss.