✏️ Chapter 11: Transfer of Thermal Energy – Constructed Response Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This section covers constructed response questions from the official PECTAA 2026 curriculum: Leslie's cube experiment (radiation differences), surface area effect on emission, convection principles in solids vs fluids, soaring birds using thermals, and thermal equilibrium via radiation. Each answer is presented in the exact exam-ready format with show/hide functionality.
📚 Related Resources – Chapter 11: Transfer of Thermal Energy
Constructed response questions help build deeper conceptual understanding for board exams.
✏️ Constructed Response Questions & Answers (PECTAA 2026)
11.1 During the Leslie's Cube experiment, why does the dull black surface radiate more heat than the shiny silver one, even though they are at the same temperature?
The dull black surface radiates more heat because surface characteristics affect emission. Dull, dark surfaces are good emitters of infrared radiation, while shiny silver surfaces are poor emitters and mostly reflect radiation. The emissivity of a blackened surface is close to 1, whereas a polished metallic surface has very low emissivity, thus radiating significantly less thermal energy even at identical temperatures.
The dull black surface radiates more heat because surface characteristics affect emission. Dull, dark surfaces are good emitters of infrared radiation, while shiny silver surfaces are poor emitters and mostly reflect radiation. The emissivity of a blackened surface is close to 1, whereas a polished metallic surface has very low emissivity, thus radiating significantly less thermal energy even at identical temperatures.
11.2 How does the size (surface area) of a radiator affect the amount of heat it emits through radiation? Explain using a real-world example.
A larger surface area increases the rate of heat emission or absorption because more radiation can occur at once. For example, radiators have fins to increase surface area and release more heat. Similarly, cooling fins on motorcycles or heat sinks in electronics maximize surface area to enhance radiative and convective heat transfer, making the system more efficient.
A larger surface area increases the rate of heat emission or absorption because more radiation can occur at once. For example, radiators have fins to increase surface area and release more heat. Similarly, cooling fins on motorcycles or heat sinks in electronics maximize surface area to enhance radiative and convective heat transfer, making the system more efficient.
11.3 Why is convection not possible in solids, and how do convection currents in liquids and gases compensate for their poor thermal conductivity?
Convection does not occur in solids because their particles cannot move freely; they are fixed in a lattice structure. In liquids and gases, heated particles become less dense and rise, creating convection currents that transfer heat efficiently. This bulk fluid motion compensates for the generally lower thermal conductivity of fluids compared to metals, enabling faster heat distribution throughout the volume.
Convection does not occur in solids because their particles cannot move freely; they are fixed in a lattice structure. In liquids and gases, heated particles become less dense and rise, creating convection currents that transfer heat efficiently. This bulk fluid motion compensates for the generally lower thermal conductivity of fluids compared to metals, enabling faster heat distribution throughout the volume.
11.4 How do birds like eagles and gliders manage to stay aloft without flapping their wings for hours? Explain the scientific principle involved.
They use the principle of convection by riding thermals, which are rising currents of warm air. By stretching their wings and circling within these thermals, gliders and birds can gain altitude and stay aloft for extended periods as the warm air rises. The sun heats the ground unevenly, creating pockets of warmer, less dense air that ascend; birds and gliders exploit these rising columns to conserve energy.
They use the principle of convection by riding thermals, which are rising currents of warm air. By stretching their wings and circling within these thermals, gliders and birds can gain altitude and stay aloft for extended periods as the warm air rises. The sun heats the ground unevenly, creating pockets of warmer, less dense air that ascend; birds and gliders exploit these rising columns to conserve energy.
11.5 Why does a hot metal object placed in a cooler room eventually cool down? Explain using the concepts of radiation and thermal equilibrium.
Because the object's temperature is higher than its surroundings, it radiates more heat than it absorbs. This process of thermal energy transfer continues until the object's temperature matches the room's temperature, reaching a state of thermal equilibrium. All objects emit infrared radiation; the hotter object emits at a higher rate, leading to a net loss of energy until both temperatures equalize.
Because the object's temperature is higher than its surroundings, it radiates more heat than it absorbs. This process of thermal energy transfer continues until the object's temperature matches the room's temperature, reaching a state of thermal equilibrium. All objects emit infrared radiation; the hotter object emits at a higher rate, leading to a net loss of energy until both temperatures equalize.
📐 Key Concepts – Transfer of Thermal Energy (Constructed Response Context)
Stefan-Boltzmann Law (Radiation Power): \( P = e \sigma A T^4 \)
Thermal Equilibrium: Heat transfer continues until \( T_{object} = T_{surroundings} \)
Convection Current Principle: Density decreases with temperature → \( \rho_{hot} < \rho_{cold} \) → buoyant rise
💡 Exam Tip:
For board exams, always explain the reason behind the phenomenon in constructed response questions — don't just state the fact. Mention relevant concepts like emissivity, surface area dependence, particle motion in convection, density gradients, and the radiation balance leading to equilibrium. These constructed response questions follow the official PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Unit 10✨ Thermal Physics
Unit 11Transfer of Thermal Energy
Unit 12Waves
Unit 13Sound
Unit 14Light
Unit 15Electrostatics
Unit 16Electricity
Unit 17Electromagnetism
Unit 18Electromagnetic Induction & EM Waves
Unit 19Electronics
Unit 20Atomic and Nuclear Physics
Unit 21Space and Environment
📖 Complete syllabus coverage for Class 10 Physics (PECTAA 2026) – Units 10 to 21
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus