Physics: Heat and Thermodynamics: UPSC Previous Year Questions (Science and Technology)

Explanation

Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is defined as the amount of heat that passes through a unit area of a material over a unit of time, when there is a temperature difference across the material. Assertion (A) is true: When copper and glass are heated to the same temperature, copper feels hotter to the touch because it has a much higher thermal conductivity than glass. Copper transfers heat to your skin more quickly than glass which makes it feel hotter. Reason (R) is true: While it is true that copper has a higher density than glass, the reason copper feels hotter is due to its thermal conductivity, not its density. R is Not a Correct Explanation of A: The reason (R) correctly states that copper is denser than glass, but density alone does not determine how hot an object feels to the touch. The primary factor is thermal conductivity, which determines how quickly heat is transferred from the object to your skin. Directions: The following item consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below.

Explanation

The boiling point of a substance is the temperature at which its vapour pressure equals the external atmospheric pressure, causing it to change from a liquid to a gas. Assertion (A) is true: The boiling point of water decreases with increasing altitude because atmospheric pressure decreases at higher altitudes. At lower pressure the water molecules require less energy to escape into the vapor phase, so water boils at a lower temperature. For example, at sea level, water boils at 100°C, but at higher altitudes, it can boil at temperatures as low as 90°C or even lower. Reason (R) is false: Atmospheric pressure decreases with increasing altitude. As you go higher in the atmosphere, the air becomes less dense, and the pressure exerted by the atmosphere decreases. This is why mountaintops have lower atmospheric pressure compared to sea level.

Explanation

When water is heated from to, it exhibits an unusual behavior due to the anomalous expansion of water. Between 0°C and 4°C, water contracts, meaning its volume decreases as the temperature increases. At 4°C, water reaches its minimum volume (maximum density). Above 4°C the water starts expanding which means its volume increases as the temperature increases further. This behavior is quite unique to water and is crucial for the survival of aquatic life in cold climates.

Explanation

Option (b) is correct: Clouds act as a blanket by trapping and reflecting back the longwave infrared radiation (heat) emitted by the Earth’s surface. This phenomenon is known as the greenhouse effect. On cloudy nights, this trapped heat keeps the surface warmer compared to clear nights when heat escapes more easily into space.

Option (a), (c) and (d) are incorrect:

Clouds do not block "cold waves" from descending on earth. The temperature of the Earth’s surface is primarily influenced by the balance of incoming solar radiation and outgoing terrestrial radiation, not by "cold waves" from the sky. Clouds do not produce heat. They are formed by the condensation of water vapor and do not have any internal mechanism to generate heat. Clouds do absorb some heat from the atmosphere but their primary role in warming the Earth’s surface at night is to reflect back the heat radiated by the Earth and not to absorb and send heat from the atmosphere.