Heat Transfer — Set 1

Correct Answer: B. Radiation

• **Radiation** = Radiation transfers heat as electromagnetic waves, requiring no material medium — it is the only mode that can travel through vacuum. • **Speed of light (3×10⁸ m/s)** — Radiation travels at the speed of light; sunlight reaching Earth across 150 million km of vacuum is a classic example. • 💡 Wrong-option analysis: Convection: requires a fluid (liquid or gas) to carry heat by bulk motion; All three equally: conduction and convection cannot occur in vacuum; Conduction: requires direct particle contact or electron transfer through matter.

Correct Answer: A. Convection

• **Convection** = Convection transfers heat by bulk (mass) movement of a fluid — warm fluid rises while cooler fluid sinks, creating circulation currents. • **Natural vs. forced convection** — Natural convection is driven by density differences; forced convection uses a pump or fan to move the fluid. • 💡 Wrong-option analysis: Radiation: transfers heat by electromagnetic waves without bulk fluid motion; Evaporation: involves phase change, not just bulk fluid motion; Conduction: transfers heat through molecular collisions without bulk movement.

Correct Answer: C. Copper

• **Copper** = Copper has high thermal conductivity (~400 W m⁻¹ K⁻¹) because its many free electrons efficiently carry energy through the lattice. • **k(copper) ≈ 400 W m⁻¹ K⁻¹** — Among common metals, copper and silver are the best thermal conductors; silver is slightly better but copper is more widely used. • 💡 Wrong-option analysis: Glass: a poor conductor (k ≈ 1 W m⁻¹ K⁻¹) with no free electrons; Wood: an insulator (k ≈ 0.1–0.2 W m⁻¹ K⁻¹); Rubber: an excellent insulator with very low thermal conductivity.

Correct Answer: A. W m⁻¹ K⁻¹

• **W m⁻¹ K⁻¹** = Thermal conductivity k is defined by Fourier's law Q/t = kAΔT/L; SI units are watts per metre per kelvin (W m⁻¹ K⁻¹). • **Fourier's law** — A higher k means more heat flows for the same temperature gradient; metals have high k, insulators have low k. • 💡 Wrong-option analysis: J m⁻¹: joules per metre has wrong dimensions; J/s: that is watts — the unit of power, not of conductivity; W m K: incorrect dimensional combination.

Correct Answer: A. higher temperature to lower temperature

• **Higher temperature to lower temperature** = Conduction (and all heat transfer) obeys the second law of thermodynamics — heat flows spontaneously from hot to cold. • **Temperature gradient (ΔT/L)** — The rate of conduction is proportional to the temperature gradient; steeper gradient means faster heat flow. • 💡 Wrong-option analysis: Higher mass to lower mass: mass does not drive heat flow; Lower temperature to higher temperature: this would violate the second law; Higher pressure to lower pressure: pressure gradient drives fluid flow, not heat conduction.

Correct Answer: A. Sea breeze formation

• **Sea breeze formation** = Sea breeze forms by convection: land heats faster than sea, warm air over land rises, and cooler air from the sea moves in to replace it — a classic convection phenomenon. • **Convection cycle** — During the day land heats up; warm lighter air rises; cool denser sea air flows in as sea breeze. • 💡 Wrong-option analysis: Heat from the Sun reaching Earth: this is radiation (electromagnetic waves through vacuum), not convection; Heat traveling along a metal rod: this is conduction; Heating of a spoon in hot tea only at its handle: this is also conduction through the metal spoon.

Correct Answer: C. emitter of radiation

• **Emitter of radiation** = A shiny or polished surface has low emissivity — it is a poor emitter of thermal radiation and a good reflector. • **Emissivity ε** — A perfect mirror has ε ≈ 0 (poor emitter); a perfect black body has ε = 1 (best emitter); shiny metals have very low ε. • 💡 Wrong-option analysis: Conductor of heat: metals (shiny or not) are good conductors of heat; Absorber of sound: emissivity of thermal radiation is unrelated to sound absorption; Insulator of electricity: metals are good electrical conductors regardless of surface finish.

Correct Answer: B. Dull black surface

• **Dull black surface** = A dull black surface has emissivity close to 1, absorbing nearly all incident thermal radiation — it is the best absorber. • **Kirchhoff's law** — Good absorbers are also good emitters; a dull black surface both absorbs and emits radiation most effectively at any given temperature. • 💡 Wrong-option analysis: White painted surface: reflects most visible light and infrared, so absorbs poorly; Transparent glass surface: transmits rather than absorbs radiation in certain wavelength ranges; Highly polished silver: has very high reflectivity and very low absorptivity.

Correct Answer: B. Convection requires fluid motion

• **Convection requires fluid motion** = Convection needs the bulk movement of a fluid (liquid or gas); it cannot occur in solids because the particles are fixed in place. • **Density-driven flow** — Heated fluid becomes less dense and rises; cooler, denser fluid sinks — this creates convection currents in liquids and gases. • 💡 Wrong-option analysis: Convection transfers heat only by waves: waves describe radiation, not convection; Convection is strongest in solids: solids cannot convect because their molecules cannot flow; Convection occurs without density change: without density differences, there is no buoyancy force to drive convection.

Correct Answer: A. To increase radiation absorption

• **To increase radiation absorption** = Spreading wings increases the surface area exposed to solar radiation, allowing more heat to be absorbed by radiation. • **Area ∝ absorbed power** — By Stefan-Boltzmann and absorption law, the power absorbed is proportional to surface area; larger area = more heat gain. • 💡 Wrong-option analysis: To stop evaporation completely: spreading wings does not prevent evaporation; To reduce convection only: this is not the primary reason birds spread wings in sunlight; To increase conduction to air: air is a poor conductor; spreading wings does not significantly aid conduction.