Physics (Part-I) (UPSC)

Total Questions: 50

41. The following items 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. [2001]

Assertion (A): The boiling point of water decreases as the altitude increases.

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

(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

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

Solution:The boiling point of an element or a substance is the temperature at which the vapour pressure of the liquid equals the environmental pressure surrounding the liquid. As altitude increases, atmospheric pressure decreases, so boiling point decreases, so the liquid would boil at a lower temperature.

42. When water is heated from 0°C to 10°C. Its volume: [2001]

(a) increases
(b) decreases
(c) does not change
(d) first decreases and then increases

Correct Answer: (d) first decreases and then increases

Solution:Initially at start of heating from 0°C to 4°C there will be a contraction as a result of which volume decreases. On further heating beyond 4°C to 10°C the molecules gain kinetic energy and start moving more randomly. Thus, intermolecular distance increases as a result of which its volume increases.

43. A hollow sphere of radius R. a hollow cube of side R and a thin circular plate of radius R, made up of the same material, are all heated to 20°C above room temperature. When left to cool in the room, which of them will reach the room temperature first? [2002]

(a) Circular plate
(b) Cube
(c) Sphere
(d) All of them will reach the room temperature at the same time

Correct Answer: (a) Circular plate

Solution:According to Newton's Law of Cooling the rate at which an object cools down is influenced by its surface area, volume, and the nature of heat transfer. A thin circular plate has the largest surface area relative to its volume, allowing rapid heat dissipation. A cube has a moderate surface area to volume ratio. A sphere, with the smallest surface area relative to its volume, retains heat longer than the other two shapes. Therefore, the thin circular plate will reach room temperature first due to its high surface area to volume ratio, enabling quicker heat dissipation.

44. Consider the following statements: [2003]

1. Steam at 100°C and boiling water at 100°C contain same amount of heat.

2. Latent heat of fusion of ice is equal to the latent heat of vaporization of water.

3. In an air-conditioner, heat is extracted from the room air at the evaporator coils and is rejected out at the condenser coils.

Which of these statements is/are correct?

(a) 1 and 2
(b) 2 and 3
(c) Only 2
(d) Only 3

Correct Answer: (d) Only 3

Solution:Only statement 3 is correct. An air conditioner (often referred to as AC) is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. Steam (at 100 degree Celsius) has more energy than water (at 100 degree Celsius) because it takes energy to break the bonds that keep it liquid. Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. Latent heat of fusion of water is 334 kJ/kg whereas Latent heat of vaporization of water is 2260 kJ/kg.

45. Cloudy nights are warmer compared to clear cloudless nights, because clouds: [2001]

(a) prevent clod waves from the sky from descending on earth
(b) reflect back the heat given off by earth
(c) produce heat and radiate it towards earth
(d) absorb heat from the atmosphere and send it towards earth

Correct Answer: (b) reflect back the heat given off by earth

Solution:Cloudy nights are warmer than clear nights because, the cloud cover provides a shield which act as an insulator and store up the heat radiated by the earth and do not permit heat to escape.

46. The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason? [2011-1]

(a) Ice is a bad conductor of heat.
(b) Since the surface of the lake is at the same temperature as the air, no heat is lost.
(c) The density of water is maximum at 4°C.
(d) None of the statements (a), (b) and (c) given is correct.

Correct Answer: (c) The density of water is maximum at 4°C.

Solution:The anomalous expension of water as it cools from 4°C to 0°C means that the density of water is greatest at 4°C. So the ice that forms in cold weather forms at the top of the water (where the less dense coldest water gathers) and then acts as an insulating barrier preventing the water underneath from getting much colder (because heat cannot escape).

47. In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following? [2021]

1. Area of the hole in the lid

2. Temperature of the flame

3. Weight of the lid

Select the correct answer using the code given below.

(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1,2 and 3

Correct Answer: (c) 1 and 3 only

Solution:By increasing the surface pressure on the liquid inside the closed cooker we can raise the cooking temperature.

This pressure can be adjusted by changing the weight placed on the top of the cooker lid. #3 is right. The larger hole will lead to an increase in the steam going out hence a greater reduction in temperature. So, statement ! is correct.

48. When the same note is played on a sitar and a flute, the sound produced can be distinguished from each other because of the difference in: [1995]

(a) pitch, loudness and quality
(b) pitch and loudness
(c) quality only
(d) loudness only

Correct Answer: (c) quality only

Solution:Sounds may be generally characterized by pitch, loudness, and quality. Sound "quality" or "timbre" describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness. Quality is then a general term for the distinguishable characteristics of a tone.

49. The following items consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this items using the code given below. [1995]

Assertion (A): A diamond sparkles more than a glass imitation cut to the same shape.

Reason (R): The refractive index of diamond is less than that of glass.

In context of the above two statements, which one of the following is correct?

(a) Both A and R are true and R is the correct explanation of A
(b) Both 4 and R are true and R is not a correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

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

Solution:This is a phenomenon of total internal reflection of light, dependent on the critical angle' of the incidence of light in a material medium at its bounding surface with air.

The higher the refractive index of a transparent material the smaller is the critical angle and hence the larger is the range of angles of incidence for more light to be totally reflected.

A diamond has a large refractive index and very small critical angle as against glass, which has a lower refractive index and large critical angle.

A skilled diamond cutter exploits the large range of angles of incidence in the diamond to cut multiple faces at suitable angles.

Light entering diamond from different faces, suffers multiple total internal reflection and comes out of the diamond as intense beams from selected directions. Hence, a diamond shines brilliantly.

50. Optical fibre works on the principle of: [1995]

(a) total internal reflection
(b) refraction
(c) scattering
(d) interference

Correct Answer: (a) total internal reflection

Solution:An optical fibre is a thin, flexible, transparent fibre that acts as a waveguide or "light pipe" to transmit light between the two ends of the fibre. An optical fibre transmits light along its axis, by the process of total internal reflection.

When light traveling in a dense medium hits a boundary at an angle larger than the "critical angle" for the boundary, the light will be completely reflected. This effect is used in optical fibres to confine light in the core.

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