Chapter 10: Heat and Modes of Transfer
Short Response Questions
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1. Why do we prefer wearing dark-colored clothes in winter and light-colored clothes in summer?
Dark colors absorb more sunlight due to their lower reflectivity, which helps retain heat in winter and keeps the body warm. In contrast, light-colored clothes reflect most of the sunlight and absorb less heat, making them suitable for summer. This behavior relates to the absorption and reflection of radiant energy.
Example: Black absorbs most of the sunlight, while white reflects it.
2. How can water reach the upper floors in houses without using a pump when geysers or boilers are placed on the ground floor?
This happens due to the principle of thermal convection. Hot water, being less dense, rises naturally while cold water sinks. In a closed pipe system, when the geyser heats water, it moves upward to the upper floors due to its lower density. This creates a natural circulation loop without requiring a pump.
Example: Hot water rising through a vertical pipe connected to an overhead tap.
Formula:
ρ(hot) < ρ(cold) ⇒ hot water rises
3. From where will you feel more heat from a wood fire: 1 meter above it or 1 meter from the side? Why?
You will feel more heat 1 meter above the fire due to convection. Hot air rises because it becomes less dense, so more heat is transferred upward. Sideways heat transfer is mainly through radiation, which is less intense compared to the direct upward convection of hot air.
Example: You feel intense heat directly above a bonfire compared to standing beside it.
Concept:
- Convection: Vertical movement of heat
- Radiation: Transfers heat in all directions, but less efficiently
4. Why do the central parts of cities feel hotter than the outskirts during hot summer days?
This is due to the urban heat island (UHI) effect. Cities have concrete, asphalt, and buildings that absorb and retain more heat. There’s less vegetation and water, which would otherwise provide cooling through evaporation and transpiration. This leads to higher temperatures in city centers compared to greener outskirts.
Example: Cities like Lahore and Karachi record 2–5°C higher temperatures than surrounding rural areas.
5. Why is the metallic handle of a door colder to touch than the wooden part of the same door?
Metals have high thermal conductivity, so they quickly draw heat from your hand, making them feel colder. Wood is a poor conductor and doesn't absorb heat as efficiently, so it feels warmer. The actual temperature may be the same, but the rate of heat transfer differs.
Formula:
Q = kA(ΔT/d), where Q is heat flow, k is thermal conductivity (higher in metals), A is area, ΔT is temperature difference, and d is thickness.
6. How do trees help in reducing the impact of climate change and what could happen if they are cut down?
Trees absorb carbon dioxide during photosynthesis, reducing greenhouse gases in the atmosphere. They also provide shade, increase humidity through transpiration, and cool surroundings. If trees are cut, CO₂ increases, leading to global warming, extreme weather, and loss of biodiversity.
Formula:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Example: Amazon Rainforest is often called the 'lungs of Earth' due to its carbon absorption role.
7. How does gravity play a role in maintaining the Earth's core temperature?
Gravity contributes by compressing the Earth's core, causing high pressure and temperature. As the Earth formed, gravitational potential energy was converted to thermal energy, which still contributes to the heat at the core. Additionally, gravity retains radioactive elements in the core, which release heat over time.
Formula:
U = -GMm/r, where G is the gravitational constant, M and m are masses, and r is the radius.
8. Why do certain gases trap more heat in the atmosphere than others?
Gases like CO₂, CH₄, and H₂O vapor are greenhouse gases. They absorb and emit infrared radiation effectively due to their molecular structure and vibrations. Diatomic gases like N₂ and O₂ don't absorb infrared efficiently, so they don't trap heat as much.
Example: Methane (CH₄) is 25 times more potent than CO₂ in trapping heat.
9. What are the possible environmental consequences of extracting geothermal energy?
Though geothermal energy is renewable, its extraction can cause:
- Surface instability and minor earthquakes
- Depletion of underground reservoirs
- Release of toxic gases like H₂S
- High water usage and contamination risk
Example: Geothermal plants in Iceland are efficient but need careful monitoring.
10. How does the specific heat capacity of a material affect its use in cookware?
Specific heat capacity determines how much heat a material can store. A material with low specific heat (e.g., metals like copper or aluminum) heats up quickly and is ideal for cookware. Materials with high specific heat (e.g., water, ceramic) heat slowly but retain heat longer.
Formula:
Q = mcΔT
Where Q is heat energy, m is mass, c is specific heat capacity, and ΔT is temperature change.
Example: Copper pans heat fast, while cast iron retains heat longer.
11. Why does 1 kg of steam release more energy than 1 kg of hot water when cooled from 100°C to 20°C?
Steam at 100°C must first condense into water before cooling further. During condensation, it releases latent heat of vaporization, which is very high (≈ 2260 kJ/kg). Hot water doesn’t have this latent heat, so it only releases sensible heat.
Formula:
Q_steam = mL + mcΔT
Q_water = mcΔT
Where L = latent heat, c = specific heat, m = mass.
Conclusion: Steam releases more energy due to the additional latent heat during phase change.
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