Heat and Thermodynamics (Physics) Part-I

Total Questions: 50

31. What is the function of the thermostat of a refrigerator? [U.P.P.C.S. (Pre) 1996]

Correct Answer: (c) To maintain constant temperature
Solution:

Thermostat is a device which automatically regulates temperature or provides a signal used by another device to regulate temperature.

32. The safest temperature for keeping food fresh in a refrigerator is [U.P.P.C.S. (Pre) 2007]

Correct Answer: (a) 4°C
Solution:

Refrigerator temperatures do not destroy pathogenic or spoilage microorganisms. The lower temperature slows the growth of microorganisms already in the food. Some bacteria grow most rapidly in the range of temperature between 40° and 140°F, the "Danger Zone".

Thus it is necessary to make sure that the refrigerator is maintaining the recommended temperature of 35 to 40 degree F or below. According to international standards, the recommended temperature of the refrigerator is 36°F to 38°F (1.7°C to 3.3°C). Thus option (a) is correct.

33. When soggy biscuits are kept inside the fridge for sometime they become crisp because: [U.P.P.C.S. (Pre) 2009]

Correct Answer: (b) Humidity inside the fridge is low and extra moisture is absorbed
Solution:

Most of the modern frost-free refrigerators use dehumidification process in which the moisture or water vapour or the humidity is removed from the air by keeping its dry bulb (DB) temperature constant.

As water vapour or moisture condenses, it drips down into a drain, which removes water to the bottom of the refrigerator. This is the main reason for soggy biscuits turns into crisp in the refrigerator.

34. Assertion (A): A piece of ice added to the drink, cools it. [U.P. Lower Sub. (Pre) 2009]

Reason (R) : Ice takes latent heat from the drink for melting resulting in the cooling of the drink.

Select the correct answer using the codes given below:

Correct Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Solution:

Heat transmits from hot body to cold body. Ice takes latent heat from the drink to melt and makes drink cooler.

35. Assertion (A): Large cold storage plants use ammonia as refrigerant while domestic refrigerators use chlorofluorocarbons. [I.A.S. (Pre) 2000]

Reason (R): Ammonia can be liquefied at ambient temperatures at low pressures.

Code:

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

Assertion (A) is correct but reason (R) is false because ammonia can be liquefied at ambient temperature at high pressure, not low pressure.

36. Consider the following statements - [U.P.P.C.S (Pre) 2010]

Assertion (A): If Ice collects in the freezer, the cooling in the refrigerator is affected adversely.

Reason (R): Ice is a poor conductor.

Select the correct answer using the codes given below:

Code:

Correct Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Solution:

Ice collects in the freezer is the result of vapour and moisture which is expelled by the food and ice tray. It adversely affects the cooling of the refrigerator, as ice is a poor conductor.

37. When a glass rod is placed in steam, its length increases but its width: [U.P.P.C.S. (Pre) 1994]

Correct Answer: (c) Increases
Solution:

When a glass rod is placed in steam, its length and width both will increase.

38. Consider the figures of a metal ball and a metal ring given below : [I.A.S. (Pre) 1993]

The metal ball can just pass through the hole of a metal ring formed out of a strip. When the ball is heated it gets stuck. But when the metal is heated-

Correct Answer: (a) The ball can still pass through the ring as diameter expands on heating.
Solution:

Most solids expand when heated and contract when cooled. The rate at which solids expand when heated depends on the substance. Metals tend to have higher rates of expansion than non-metal solids.

When a metal ring is heated, there is an increase in outer diameter along with inner diameter. Thus, the ball can pass easily through it.

39. Strips of two metals A and B are firmly jointed together as shown in the figure.

Correct Answer: (b)
Solution:

When two strips of different metals, say one of brass and the other of iron are welded or riveted together, the greater expansion of one metal results in the bending shown in option (b).

Such a compound thin bar is called a bimetallic strip. When the strip is heated one side of the double strip bent into a curve.

40. Assertion (A): A piece of copper and a piece of glass are heated to the same temperature. When touched, thereafter, the copper piece appears hotter than the glass piece. [I.A.S. (Pre) 2001]

Reason (R): The density of copper is more than that of glass.

Code :

Correct Answer: (b) Both A and R are true but R is not a correct explanation of A
Solution:

Copper is a very good conductor of heat. So it allows the maximum amount of heat to pass through it from one point to other. But glass is a poor conductor of heat and hence it allows a very little amount of heat to pass through it.

The density of copper is 8920-8960 kg/m³ and the density of glass is equal to 2400-2800 kg/m³. Thus the density of copper is more than that of glass, but it is not the correct reason of the given assertion. Hence the correct answer is option (b).