Assertion (A): The land surfaces get heated and cooled quickly compared to oceans.
Reason (R): Land surfaces are practically opaque to solar radiation and only few inches of the ground is affected.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Land has lower specific heat than water and absorbs solar radiation only at the surface, leading to rapid temperature changes. Water has high specific heat and radiation penetrates deeply. Both A and R are true, and R explains A.
Assertion (A): An ice skater can slide over ice smoothly if the skate blades are sharp.
Reason (R): Melting point of ice decreases with increase in pressure.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Sharp blades increase pressure, lowering ice's melting point (regelation). A thin water layer forms, reducing friction, allowing smooth sliding. Thus, R correctly explains A.
Assertion (A): On the sea shore, cool breeze flows in the evening.
Reason (R): Convection currents are set up from sea to the land since land cools slower than water.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Land heats and cools faster than water. In the evening, land is cooler than the sea. This causes a breeze from land to sea (land breeze), not sea to land. Thus, A is true but R is false.
Assertion (A): Two thin blankets put together are less warmer than a single blanket of double the thickness.
Reason (R): Thickness increases because of vacuum layer enclosed between the two blankets.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Two thin blankets are warmer than one thick one due to trapped air layers acting as insulation. No vacuum forms. Both assertion and reason are incorrect.
Assertion (A): Density of humid air is less then density of dry air at the same temperature and pressure.
Reason (R): Mass of humid air is more than mass of dry air.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Water vapor \( H_2 O \) has a lower molar mass (approx. \(18 g/mol\) than nitrogen \( 28 g/mol\) and oxygen (\32 g/mol). Thus, replacing dry air molecules with \( H_2 O\) makes humid air less dense. Reason (R) is incorrect.
Assertion (A): Temperature near the sea-coast are moderate.
Reason (R): Water has a high thermal conductivity.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Coastal temperatures are moderate because water has a high specific heat capacity, absorbing and releasing large amounts of heat without significant temperature change.
Water's thermal conductivity is not high. Thus, R is false.
Assertion (A): Bodies radiate heat at all temperature.
Reason (R): Rate of radiation of heat is proportional to the fourth power of absolute temperature.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
All bodies above 0 K radiate thermal energy. The Stefan-Boltzmann law states that the rate of radiation P is proportional to the fourth power of absolute temperature \(T^4\), i.e., \(P \propto T^4\). This law confirms that radiation occurs at any temperature above absolute zero and quantifies it. Thus, R explains A.
Assertion (A): For an ideal black body, both absorption coefficient and reflection coefficient are one.
Reason (R): Perfect absorbers are perfect reflectors.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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A black body is defined as a perfect absorber (absorption coefficient = 1) and a perfect emitter, but it reflects no radiation (reflection coefficient = \(0\)).
Perfect absorbers are the opposite of perfect reflectors. Both Assertion (A) and Reason (R) are false.
Assertion (A): Heat radiations and light have identical properties.
Reason (R): A cold body does not radiate heat to the hotter surroundings.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
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Heat radiation (infrared) and visible light are both forms of electromagnetic waves, sharing properties like speed, reflection, and refraction. All bodies above 0 K radiate heat. A colder body radiates heat but receives more from hotter surroundings, leading to net heat gain. Thus, R is false.
Assertion (A): Two spheres of same material have radius \(r_1\) and \(r_2\) respectively and temperature \(4000\text{ K}\) and \(2000\text{ K}\) respectively. The energy radiated per second by first sphere is more than second sphere.
Reason (R): In thermal conduction, energy is transferred by transference of particles of conducting body.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
The power radiated by a sphere is `\(P = e \sigma (4\pi r^2) T^4\)`. The ratio of powers is `\(frac{P_1}{P_2} = \frac{r_1^2 (4000\text{ K})^4}{r_2^2 (2000\text{ K})^4} = 16 \frac{r_1^2}{r_2^2}\)`.
For `\(P_1 > P_2\)`, we need `\(16 r_1^2 > r_2^2\)` or `\(r_1 > r_2/4\)`. This is not universally true (e.g., if `\(r_1 = r_2/5\)`). Thus, Assertion (A) is false. Reason (R) describes convection, not conduction. Conduction involves energy transfer through molecular vibrations and collisions, not by the bulk transference of particles. Therefore, Reason (R) is also false.