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.

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.
Water can boil, raise temperature or decrease air pressure, in two ways. At sea level, it is the pressure of air that causes water to boil at 100oC. Water can boil at a much lower temperature in vacuum, where there’s no air. That is, if not for the skin that keeps the blood pressurized, body temperature would be sufficient to cause the blood to boil with water. At low air pressure the water boils significantly below 100oC at temperatures.
The boiling point of water is the temperature at which the liquid vapor pressure is equal to the pressure surrounding the body, and the body transforms into a vapour. The boiling point is the temperature for a particular liquid to boil. For example, the boiling point for water, at a pressure of 1 atm, is 100 degrees Celsius. A liquid’s boiling point depends upon the liquid’s temperature, atmospheric pressure, and vapor pressure.

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

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.
Water can boil, raise temperature or decrease air pressure, in two ways. At sea level, it is the pressure of air that causes water to boil at 100oC. Water can boil at a much lower temperature in vacuum, where there’s no air. That is, if not for the skin that keeps the blood pressurized, body temperature would be sufficient to cause the blood to boil with water. At low air pressure the water boils significantly below 100oC at temperatures.
The boiling point of water is the temperature at which the liquid vapor pressure is equal to the pressure surrounding the body, and the body transforms into a vapour. The boiling point is the temperature for a particular liquid to boil. For example, the boiling point for water, at a pressure of 1 atm, is 100 degrees Celsius. A liquid’s boiling point depends upon the liquid’s temperature, atmospheric pressure, and vapor pressure.

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]

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?

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]

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.
• On cloudy days, clouds reflect the sun's light into space, keeping much of the energy away from the surface and leading to cooler temperatures.
• The airborne water molecules are called water vapor.
• When the molecules are bouncing around in the atmosphere, they don't normally stick together.
• Clouds on Earth form when warm air rises and its pressure is reduced.
• The air expands and cools, and clouds form as the temperature drops below the dew point.
• When it is cloudy, the clouds absorb (hold) some of the heat so it can’t escape. Clouds also reflect some of the heat back towards the earth.
• Sometimes it gets even warmer on cloudy days because of advection.
• Advection is the movement of heat, cold, and moisture when air moves (when there is wind).

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]

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).
• Most substances become denser as they cool down. Water does this too, but only down to 4°C.
• As water cools from higher temperature (e.g., 10°C) towards 4°C, its density increases. Colder, denser water sinks, displacing warmer, less dense water upwards. This process is called convection.
• This convection continues until the entire body of water reaches approximately 4°C. At this point, the densest water is at the bottom of the lake.
• However, as the water cools from 4°C down to 0°C (its freezing point), its density starts to decrease. Water at 3°C is less dense than water at 4°C, water at 2°C is less dense, and so on. Ice at 0°C is significantly less dense than water at 0°C or 4°C, which is why ice floats.
• Because water cooler than 4°C is less dense than water at 4°C, it remains near the surface once it cools below 4°C. It does not sink.
• This means that once the lake's temperature profile reaches 4°C throughout, further cooling only affects the surface layers. The surface cools from 4°C to 0°C and freezes, while the denser 4°C water remains undisturbed at the bottom.
• Convection currents stop effectively transferring heat downwards once the surface layer cools below 4°C.

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.

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]

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.
• Pitch & Loudness (Same): Both instruments produce the same fundamental frequency (pitch) and intensity (loudness) for the chosen note.
• Quality/Timbre (Different): The unique mix of overtones and the way the sound wave is shaped (waveform) differs for each instrument, creating a distinct sound quality that we recognize as the sitar or the flute.

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?

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]

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.