Heat and Thermodynamics (Physics) Part-I

Total Questions: 50

1. In an ecosystem, flow of energy declines as it passes from lower to higher trophic level. This is explained by the following: [M.P.P.C.S. (Pre) 2023]

Correct Answer: (b) Second law of thermodynamics
Solution:

In an ecosystem, flow of energy declines as it passes from lower to higher trophic level. This is explained by the second law of thermodynamics. The second law of thermodynamics states that as energy is transferred or transformed, more and more of it is wasted.

That means, every activity and energy transformation is accompanied by dissipation of energy. The energy decreases from one trophic level to the next trophic level because energy is lost as metabolic heat during transfer of energy.

When food energy passes from producers to herbivores to carnivores, only 10% of energy is transferred from one trophic level to another trophic level.

2. The temperature point at which solid, liquid and gaseous states may stay together is known as: [67ᵗʰ B.P.S.C. (Pre) 2022]

Correct Answer: (d) triple point
Solution:

The temperature point at which solid, liquid and gaseous states may stay together is known as triple point. In thermodynamics, a substance's triple point is the unique combination of temperature and pressure at which the solid phase, liquid phase, and gaseous phase can all coexist in thermodynamic equilibrium.

By international agreement, the triple point of water has been assigned a value of 273.16 K (0.01°C, 32.018 °F) at partial vapour pressure of 611.657 pascals (6.11657 mbar; 0.0060366 atm). The triple point of water was earlier used to define the Kelvin, the base unit of thermodynamic temperature in the SI system.

3. Transfer of heat by convection can take place in [M.P.P.C.S. (Pre) 2017]

Correct Answer: (c) Gases and liquids
Solution:

Convection is a mode of heat transfer by actual motion of matter. It is possible only in fluids and gases. Convection can be natural and forced.

4. A liquid remains hot or cold for a long time in thermos flask because there is no loss or gain of heat by [U.P.P.C.S. (Pre) 2019]

Correct Answer: (c) Both (a) and (b)
Solution:

Thermos flasks have a vacuum gap between the inner chamber and the outer wall, so there is no loss or gain of heat by conduction, convection and radiation. Thus, a liquid remains hot or cold for a long time in thermos flask.

5. Steam at 100°C causes more severe burns than the water at the same temperature, because: [Chhattisgarh P.C.S. (Pre) 2008]

Correct Answer: (b) Steam provides more heat
Solution:

Steam at 100°C causes more severe burns than the water at the same temperature because steam provides more heat. The boiling point of water is 100 degree celsius. At this temperature, the water will begin to change from the liquid state to the gaseous state.

For this change to take place, additional energy is required. In fact, every gram of liquid water requires 540 calories of heat energy to convert it to steam. This is called the latent heat of vaporization or latent heat of steam.

So while they both would exist at the same temperature, the steam would have a lot more heat energy due to the addition of 540 calories per gram of heat energy that has been absorbed. This is why steam burns are worse than water burns.

6. The hand burns more by steam comparatively to boiling water, because - [U.P.P.C.S. (Pre) 1993]

Correct Answer: (a) The steam has latent heat
Solution:

Steam at 100°C causes more severe burns than the water at the same temperature because steam provides more heat. The boiling point of water is 100 degree celsius. At this temperature, the water will begin to change from the liquid state to the gaseous state.

For this change to take place, additional energy is required. In fact, every gram of liquid water requires 540 calories of heat energy to convert it to steam. This is called the latent heat of vaporization or latent heat of steam.

So while they both would exist at the same temperature, the steam would have a lot more heat energy due to the addition of 540 calories per gram of heat energy that has been absorbed. This is why steam burns are worse than water burns.

7. The heat required in calories to convert one gram of ice at 0° C to steam at 100° C is approximately. [Uttarakhand Lower Sub. (Pre) 2010]

Correct Answer: (c) 720
Solution:

Latent heat of Melting = 80 calorie/gram

The heat required for ice at 0°C to convert into water = 80 calorie

Required heat to enhance the temperature of water from 0°C to 100°C = 100 calorie

Latent heat of vaporization = 540 calorie/gram

Hence the energy required to convert water into vapour at 100°C = 540 calorie.

Total heat required = 80 + 100 + 540
= 720 calorie.

8. At what temperature are the reading of a Centigrade and Fahrenheit thermometer the same? [U.P.P.C.S. (Mains) 2015, U.P.U.D.A/L.D.A. (Pre) 2003, I.A.S. (Pre) 1993]

Correct Answer: (a) - 40
Solution:

-40° is the temperature at which the readings are the same in both degree Centigrade and Fahrenheit scales. Relation between degree Centigrade and degree Fahrenheit is:

C / 5 = (F - 32) / 9

Placing F = -40 to verify:

C = [(-40 - 32) × 5] / 9
= (-72 × 5) / 9
= -40

9. At what degree are the Centigrade and Fahrenheit temperatures the same? [M.P.P.C.S. (Pre) 1994-95]

Correct Answer: (d) -40°
Solution:

-40° is the temperature at which the readings are the same in both degree Centigrade and Fahrenheit scales. Relation between degree Centigrade and degree Fahrenheit is:

C / 5 = (F - 32) / 9

Placing F = -40 to verify:

C = [(-40 - 32) × 5] / 9
= (-72 × 5) / 9
= -40

10. At which temperature, the Celsius and Fahrenheit scales coincides? [66ᵗʰ B.P.S.C. (Pre) (Re. Exam) 2020]

Correct Answer: (c) -40°
Solution:

-40° is the temperature at which the readings are the same in both degree Centigrade and Fahrenheit scales. Relation between degree Centigrade and degree Fahrenheit is:

C / 5 = (F - 32) / 9

Placing F = -40 to verify:

C = [(-40 - 32) × 5] / 9
= (-72 × 5) / 9
= -40