Heat and Thermodynamics (Physics) Part-II

Total Questions: 41

11. On a specific day and time, the temperature was 48°C in Churu and 24°C in Shimla. Two metallic cups identical in all respects contained water at 95°C in Churu and 71°C in Shimla. Which one of these two cups reached room temperature first? [R.A.S./R.T.S. (Pre) 2008]

Correct Answer: (c) Both cups reached room temperature at the same time
Solution:

The rate of heat loss of an object is called cooling rate
Rate of heat loss (ΔQ) ∝ Temperature difference (ΔT)

∴ 95° – 48° = 47° and 71° – 24° = 47°

Because of the temperature difference (ΔT) is similar in both. Thus, both cups reach the room temperature at the same time.

12. A hot body cools from 90°C to 75°C in 25 seconds. The same body in further cooling from 75°C to 60°C will take how much time? [U.P.P.C.S. (Mains) 2017]

Correct Answer: (c) More than 25 seconds
Solution:

Newton's law of cooling states that the rate of loss of heat by a body is directly proportional to the temperature difference between the body and its surroundings.

As the temperature difference between the body and its surroundings will be lesser in the second case, so the time taken in cooling from 75°C to 60°C will be more than 25 seconds.

13. Consider the following statements regarding 'heat wave' : [69ᵗʰ B.P.S.C. (Pre) 2023]

1. Heat wave is considered if the maximum temperature of a station reaches at least 30°C or more for plains.

2. Heat wave is considered if the maximum temperature of a station reaches at least 40°C or more for hilly regions.

Which of the above statements is/are correct?

Correct Answer: (d) Neither 1 nor 2
Solution:

Heat wave is considered when temperature of a station reaches at least 40°C or more in plains and at least 30°C or more in hilly regions. Hence, both statements are incorrect.

14. In the season of summer, we experience a humid heat when weather : [U.P.P.C.S. (Pre) 2003]

Correct Answer: (d) Humid weather
Solution:

In the season of summer, when the weather is humid, we experience humid heat.

15. Xeric condition refers to - [U.P.R.O/A.R.O. (Pre) 2014]

Correct Answer: (b) low humidity
Solution:

Xeric condition refers to low humidity. Xeric condition is characterized by an inadequate supply of water. These conditions exist in arid habitats, extremely cold habitats, and in salt marshes.

16. The atmosphere exerts enormous pressure on us. But, we do not feel it because [U.P.P.C.S (Pre) 2010]

Correct Answer: (a) Our blood exerts a pressure slightly more than that of the atmosphere.
Solution:

The atmosphere exerts enormous pressure on us but we do not feel it, because our blood exerts pressure slightly more than that of the atmosphere. We feel this pressure in water because the pressure under water is greater than our normal blood pressure.

17. Which of the following statement is not true : [I.A.S. (Pre) 2002]

Correct Answer: (b) Addition of solute to a solution increases its water potential.
Solution:

Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure or matrix effects such as capillary action (which is caused by surface tension).

The addition of solutes in water, lowers its water potential by which it becomes negatively charged. Adding solutes makes the water molecules less concentrated and this reduces the chemical potential of water in the system.

18. Why does water boil at a temperature below 100°C at higher altitudes ? [U.P.P.C.S. (Pre) 1994]

Correct Answer: (a) The atmospheric pressure decreases and hence boiling point is lowered.
Solution:

At higher altitudes of a mountain, the atmospheric pressure decreases due to lack of air. Therefore, the boiling water pressure decreases and water boils below 100°C due to lowering of boiling point. The boiling point of a liquid varies depending upon the surrounding environmental pressure.

A liquid in a partial vacuum has a lower boiling point than that of liquid at atmospheric pressure. A liquid at high pressure has a higher boiling point than when that liquid is at atmospheric pressure.

19. Assertion (A): The boiling point of water decreases as the altitude increases. [I.A.S. (Pre) 2001, U.P.R.O/A.R.O. (Pre) 2023]

Reason (R): The atmospheric pressure increases with altitude.

Code :

Correct Answer: (c) (A) is true, but (R) is false.
Solution:

The atmospheric pressure decreases as one climbs higher in the atmosphere and increases closer to the Earth's surface. The boiling point of water decreases as the altitude increases. Thus, (A) is true but (R) is false.

20. Food gets cooked faster in a pressure cooker because - [U.P.P.C.S. (Pre) 2007]

Correct Answer: (b) Water starts boiling at a higher temperature due to high pressure
Solution:

Food can be cooked faster in a pressure cooker since the boiling point of water increases with increase in pressure. As the water boils, the steam produced cannot evaporate so the pressure inside the cooker increases.

The boiling point of water now becomes about 120°C. The load pin fixed on the heavy aluminum lid prevents any escape of steam. The rubber ring around the lid deals the liquid inside. Thus food gets cooked faster in a pressure cooker.

Pressure cookers are more common in the mountains where elevation is high. At higher elevations, atmospheric pressure is lower and therefore water boils at lower temperature. In order to make up for that difference, people use pressure cookers.