Heat Transfer — Set 3

Correct Answer: C. convection and evaporation

• **Convection and evaporation** = A ceiling fan increases air movement, which enhances convective heat removal from the skin and accelerates evaporation of sweat, both causing cooling. • **Evaporative cooling** — Evaporation of sweat absorbs latent heat from the skin; a fan speeds up this evaporation, significantly enhancing the cooling effect. • 💡 Wrong-option analysis: Thermal conductivity of air: a fan does not change air's thermal conductivity; Radiation from walls: a fan does not alter radiation from walls; Conduction through floor: a fan has no effect on floor conduction.

Correct Answer: C. increases

• **Increases** = Thermal resistance R = L/(kA) — resistance increases with thickness L; a thicker insulating layer makes it harder for heat to flow through. • **R ∝ L** — Doubling the thickness doubles the thermal resistance and halves the heat flow rate for the same temperature difference. • 💡 Wrong-option analysis: Decreases: decreasing thickness reduces resistance and increases heat flow; Becomes zero: zero thickness means no material, so no insulating effect; Does not matter: thickness directly determines the thermal resistance.

Correct Answer: C. It occurs mainly in solids

• **It occurs mainly in solids** = Conduction transfers heat through atomic/molecular collisions and free-electron movement without bulk flow — this is most effective in solids where particles are closely packed. • **Free electrons in metals** — Metals are excellent conductors because free electrons carry energy rapidly through the lattice; non-metals conduct by lattice vibrations only. • 💡 Wrong-option analysis: It can happen only in vacuum: conduction requires matter (particle contact) — it cannot occur in vacuum; It requires bulk fluid motion: that describes convection; It always involves visible light: that describes radiation.

Correct Answer: B. rises up due to lower density

• **Rises up due to lower density** = When air is heated by a floor heater, it expands, its density decreases, and it rises; cooler denser air sinks to replace it — creating convection currents that heat the room. • **ρ ∝ 1/T (at constant pressure)** — Warm air is less dense and buoyant; placing the heater at the floor allows warm air to rise and circulate throughout the entire room. • 💡 Wrong-option analysis: Moves only sideways: warm air rises, not just moves sideways; Sinks down: cold air sinks, warm air rises; Does not change density: heating air causes thermal expansion, which reduces its density.

Correct Answer: A. Covering the cup with a lid

• **Covering the cup with a lid** = A lid traps the warm air layer above the tea and blocks air circulation, reducing convection heat loss from the surface significantly. • **Convection suppression** — Without a lid, warm air above the tea rises freely (convection) and cooler air replaces it; a lid stops this circulation. • 💡 Wrong-option analysis: Blowing air over the cup: blowing increases convection, accelerating heat loss; Using a fan near the cup: same effect as blowing — increases convection heat loss; Stirring the tea continuously: stirring increases convection inside the cup, accelerating cooling.

Correct Answer: B. 1 W

• **1 W** = Using Fourier's law Q/t = kAΔT/L: Q/t = 200 × (1×10⁻⁴) × 100 / 2 = 1 W. • **Q/t = kAΔT/L = 200×10⁻⁴×100/2 = 1 W** — All values substituted correctly give 1 W; the answer is option B. • 💡 Wrong-option analysis: 100 W: would require A = 0.01 m², 100 times larger; 2 W: would require twice the area or twice the conductivity; 10 W: would require 10 times the area.

Correct Answer: D. Conduction

• **Conduction** = Conduction transfers heat in solids through molecular vibrations and free-electron movement — no bulk movement of matter is required. • **Particle collision mechanism** — Energy passes from high-energy (hot) particles to neighboring lower-energy particles via collisions, creating a heat flow along the temperature gradient. • 💡 Wrong-option analysis: Boiling: a phase change process, not a mode of heat transfer; Convection: requires bulk fluid motion, so it doesn't occur in rigid solids; Radiation: transfers heat by electromagnetic waves, not by particle contact.

Correct Answer: A. White

• **White** = White surfaces reflect most incident solar radiation and absorb very little — they minimise heat gain from sunlight. • **High reflectivity** — A white or silver surface can reflect up to 90% of incident radiation, while a black surface absorbs up to 95% — this is why white clothes feel cooler in sunlight. • 💡 Wrong-option analysis: Black: absorbs most radiation, maximising heat gain; Dull gray: absorbs more than white; Dark blue: absorbs significantly more radiation than white.

Correct Answer: A. Radiation

• **Radiation** = Radiation transfers heat by electromagnetic waves that can travel through vacuum — no contact between the source and receiver is needed. • **Infrared radiation** — Most thermal radiation at everyday temperatures is in the infrared range; it travels at the speed of light through space. • 💡 Wrong-option analysis: Diffusion: a process of mass transport, not specifically heat transfer without contact; Convection: requires a fluid medium, and the fluid must be in contact with the heat source; Conduction: requires direct physical contact between bodies.

Correct Answer: A. radiation

• **Radiation** = Silvered inner walls of a thermos reflect thermal radiation back, preventing radiative heat loss — the vacuum already stops conduction and convection. • **Silvering reflects infrared** — A silvered surface has very low emissivity, so it both emits and absorbs very little thermal radiation. • 💡 Wrong-option analysis: Conduction: the vacuum (not silvering) stops conduction; Melting: silvering has nothing to do with melting; Convection: the vacuum stops convection, not the silvered walls.