Conduction, Convection, and Radiation - UNSOLVED PRACTICE SET

Q7. Distinguish between conduction, convection, and radiation. Give one example of each.

Q8. Define thermal conductivity. Write its SI unit and dimensional formula.

Q9. Why are cooking utensils made of metals while their handles are made of plastic or wood?

Q10. In your school, students notice that a tiled floor feels colder than a carpeted floor at the same room temperature. Explain using thermal conductivity.

Q11. Explain why a flame rises upward. What type of heat transfer is primarily responsible?

Q12. Why do we wear light-coloured clothes in summer and dark-coloured clothes in winter?

Q13. Explain the three modes of heat transfer in detail:

(i) Conduction — derive the expression for rate of heat flow, define thermal conductivity, discuss factors affecting it

(ii) Convection — explain natural and forced convection, give examples

(iii) Radiation — explain electromagnetic nature, Stefan's law, and Wien's displacement law (qualitative)

Compare and contrast the three modes with examples from daily life.

Q14. A composite wall consists of two layers: brick (thickness 20 cm, k = 0.84 W/m·K) and plaster (thickness 5 cm, k = 0.5 W/m·K). The inner surface is at 25°C and the outer surface is at 5°C. The wall area is 20 m².

(a) Calculate the thermal resistance of each layer.

(b) Calculate the equivalent thermal resistance.

(c) Calculate the rate of heat flow through the wall.

(d) Calculate the temperature at the brick-plaster interface.

(e) Compare this with a single brick wall of the same total thickness.

Q15. Analyse the following situations:

(i) Sea breeze and land breeze

(ii) Double-glazed windows

(iii) Thermos flask design

For each case, discuss:

(a) The modes of heat transfer involved

(b) How the design minimises heat transfer

(c) The physical principles applied

Q16. A copper rod (length 50 cm, cross-section 2 cm²) has one end in boiling water and the other in melting ice. (k_copper = 400 W/m·K)

(a) Calculate the rate of heat flow through the rod.

(b) Calculate the mass of ice melted per minute. (Lf = 3.36 × 10⁵ J/kg)

(c) If the rod is replaced by an iron rod of same dimensions (k_iron = 80 W/m·K), calculate the new rate of heat flow.

(d) Calculate the temperature gradient in each rod.

(e) Discuss why copper is preferred over iron for heat exchangers.

Q17. In a school experiment, students compare the thermal conductivity of copper and aluminium rods:

Both rods: length = 30 cm, diameter = 1 cm

One end in steam at 100°C, other end in ice at 0°C

Mass of ice melted by copper rod in 10 minutes = 15 g

Mass of ice melted by aluminium rod in 10 minutes = 10 g

(a) Calculate the rate of heat flow for each rod.

(b) Calculate the ratio of thermal conductivities.

(c) If the accepted value for copper is 400 W/m·K, calculate the thermal conductivity of aluminium.

(d) List two sources of error in this experiment.

(e) Why must the rods be coated with wax or have thermometers along their length?

Q18. A room has walls of total area 50 m², thickness 25 cm (k = 0.5 W/m·K), and windows of area 5 m² with glass thickness 4 mm (k = 1.0 W/m·K). The inside temperature is 20°C and outside is 5°C.

(a) Calculate the rate of heat loss through the walls.

(b) Calculate the rate of heat loss through the windows.

(c) Calculate the total rate of heat loss.

(d) If the room is heated by a 2 kW heater, is it sufficient to maintain the temperature?

(e) Suggest two ways to reduce heat loss through windows.