Chapter 1: Heat Capacity and Modes of Heat Transfer

The SI unit of specific heat capacity is:

Which method of heat transfer does not require a material medium?

The transfer of heat due to the bulk movement of molecules in fluids is called:

Which colour surface is the best absorber of infrared radiation?

The greenhouse effect is the process by which:

The temperature of the Earth's core is about:

The layer of the Earth between the core and the crust is the:

Sea breeze blows from:

The handle of a cooking pan is made of plastic because plastic is:

Thermal equilibrium is reached when:

Correct Answer: C. J kg⁻¹ K⁻¹ Explanation: From , the unit is joule per kilogram per kelvin. MCQ 2. Correct Answer: C. Radiation Explanation: Radiation transfers heat via electromagnetic waves and can travel through a vacuum. MCQ 3. Correct Answer: B. Convection Explanation: Convection is heat transfer by the bulk movement of fluid molecules. MCQ 4. Correct Answer: C. Black Explanation: Black surfaces are the best absorbers (and emitters) of radiation. MCQ 5. Correct Answer: A. Heat is trapped near Earth's surface by greenhouse gases Explanation: The greenhouse effect is the trapping of heat by greenhouse gases in the atmosphere. MCQ 6. Correct Answer: C. 6000 °C Explanation: The textbook states the core temperature is about 6000 °C. MCQ 7. Correct Answer: C. Mantle Explanation: The mantle lies between the core and the crust. MCQ 8. Correct Answer: B. Sea to land during the day Explanation: During the day, hot air rises over land and cool air from the sea moves in — a sea breeze. MCQ 9. Correct Answer: C. A poor conductor of heat Explanation: Plastic is an insulator, so heat does not easily pass through to the handle. MCQ 10. Correct Answer: A. Absorption rate equals emission rate Explanation: At thermal equilibrium, an object absorbs and radiates heat at equal rates. 387 J kg C −1 ∘ −1 0.9 J g C −1 ∘ −1 4.18 J g C −1 ∘ −1 c = QmΔT Section B — Short Answers Q1. Define specific heat capacity and state its SI unit. Answer: Specific heat capacity is the amount of heat energy required to raise the temperature of 1 kg of a substance by 1 K (or 1 °C). It is denoted by and is a property of the material itself. Its SI unit is joule per kilogram per kelvin (J kg⁻¹ K⁻¹). The formula is , where is the heat absorbed or lost, is the mass, and is the temperature change. A substance with a high specific heat (like water) heats slowly and retains heat, while one with a low specific heat (like metals) heats and cools quickly. Q2. Why are metals good conductors of heat? Answer: Metals are good conductors of heat because they have a large number of free electrons that are loosely bound to their atoms. When one end of a metal is heated, these free electrons gain energy and move rapidly through the metal, transferring energy quickly to the colder end. In addition, the atoms at the hot end vibrate with larger amplitude and collide with neighboring atoms, passing energy along. The combination of rapid free electron motion and atomic vibrations makes metals excellent conductors compared to non-metals like wood or glass, which lack free electrons. Q3. What is meant by thermal equilibrium? Answer: Thermal equilibrium is the state in which an object absorbs and radiates heat at equal rates, so its temperature remains constant. When a hot object is placed in cooler surroundings, it initially emits radiation faster than it absorbs, so its temperature drops. After some time, the rate of emission equals the rate of absorption, and the object's temperature becomes constant, matching its surroundings. At this point, the object is in thermal equilibrium. For example, a hot cup of tea cools until it reaches the temperature of its surroundings and then stays at that temperature. Q4. State two uses of the high specific heat of water. Answer: Water has a very high specific heat capacity (4200 J kg⁻¹ K⁻¹), which gives it two important uses. First, it is used as a coolant in car radiators and industrial systems because it can absorb a large amount of heat with only a small rise in temperature. Second, large bodies of water (oceans, lakes) help moderate the climate of coastal areas: water absorbs a lot of heat in summer and releases it in winter, keeping coastal temperatures more stable. This is why coastal areas have cooler summers and milder winters than inland areas. Q5. What is the greenhouse effect? Answer: The greenhouse effect is the natural process by which greenhouse gases in the atmosphere (such as water vapour, carbon dioxide, and methane) trap heat near Earth's surface. Earth absorbs high-energy radiation from the Sun and emits lower- energy infrared radiation back into the atmosphere. Greenhouse gases absorb this infrared radiation and re-emit it back towards Earth, warming the planet. This warming is c c = QmΔT Q m ΔT necessary for life — without it, Earth's temperature would drop to about -180 °C at night. However, human activities have increased greenhouse gas concentrations, enhancing the effect and causing global warming. Section C — Long Answers Q1. Explain the three modes of heat transfer (conduction, convection, and radiation) with one example of each. Answer: Introduction: Heat transfer occurs when thermal energy moves from a hotter object to a colder object due to a temperature difference. There are three modes of heat transfer: conduction, convection, and radiation. Conduction: Conduction is the transfer of heat through a solid material without the material itself moving. When one end of a solid is heated, the atoms at that end gain kinetic energy and vibrate with larger amplitude. These vibrating atoms collide with neighboring atoms and transfer energy. In metals, free electrons also carry energy rapidly. Metals are good conductors because of these free electrons. Non-metals like wood and glass are poor conductors. Example: When a metal spoon is placed in hot tea, the handle becomes hot by conduction. Convection: Convection is the transfer of heat by the bulk movement of a fluid (liquid or gas). When a fluid is heated, it expands and becomes less dense, so it rises. Cooler, denser fluid sinks to take its place, creating a convection current. Convection occurs only in liquids and gases, not in solids. Example: Water boiling in a pot — heated water at the bottom rises, and cooler water sinks. Radiation: Radiation is the transfer of heat through electromagnetic waves (mainly infrared). It does not require a material medium and can travel through a vacuum. All objects emit radiation, and hotter objects emit more. The nature of the surface (colour, texture) affects absorption and emission: black surfaces are the best absorbers and emitters, while shiny silver surfaces are the best reflectors. Example: Heat from the Sun reaches Earth through the vacuum of space by radiation. Conclusion: Conduction transfers heat through direct contact in solids, convection through fluid movement, and radiation through electromagnetic waves without a medium. Understanding these modes explains many everyday phenomena, from cooking to climate. Q2. Describe the structure of the Earth (core, mantle, crust) and explain how the Earth's core maintains its high temperature. Answer: Introduction: The Earth's interior is divided into three main layers: the core, the mantle, and the crust. The core is the hottest part, at about 6000 °C. Structure of the Earth: 1. Core: The core is the central part of the Earth, mainly composed of iron and nickel. Its temperature is about 6000 °C. The core is divided into an inner core (solid, maintained by extreme pressure) and an outer core (molten, at about 4000 °C). 2. Mantle: The mantle lies between the core and the crust and is the thickest layer. It is composed of silicate minerals containing iron and magnesium. It is solid but shows plasticity — it can flow slowly under high pressure. 3. Crust: The crust is the outer layer, composed of silicate materials. It is divided into continental crust and oceanic crust. The lithosphere is the layer between the crust and upper mantle, and the asthenosphere lies just beneath it and plays a role in plate tectonics. How the core maintains its high temperature: