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Physics — UPSC Prelims Science & Environment previous year questions — page 2

177 solved previous year questions on Physics, papers from 1990 to 2023, drawn from UPSC CSE Prelims, UPPCS, UPSC CDS. Each carries the printed answer key and the explanation from the source compilation.

1. #41UPPCS 2020
   Which of the following is used in fire alarm?
   • APhotocell
   • BWater
   • CSolar cell
   • DNone of the above
   Answer: (A) Photocell
   Explanation

   Option a is the correct photoelectric smoke detector works by using light to Inside the alarm, there is a special room that can In this room, a small light called an LED shines a in a straight line across the room. The alarm can there's smoke around. When smoke goes into the room, the LED light change direction and go to a of the same room where there's a special light this sensor gets light on it, the alarm starts

2. #42UPPCS 2019
   What happens when some charge is placed on a soap bubble?
   • AIts radius increases
   • BIts radius decreases
   • CThe bubble collapses
   • DNone of the above
   Answer: (A) Its radius increases
   Explanation

   Option a is the correct some charge is placed on a soap bubble, its This occurs because the charge on the creates electrostatic repulsion forces that push molecules away from each other, causing the expand and increase in size. The radius of the bubble to increase until the repulsive force between is balanced by the surface tension of the bubble. Surface Bubble with positive charge

3. #43Previous year
   A liquid remains hot or cold for a long time in thermos flask because there is no loss or gain of heat by- [. (Pre) 2019)
   • AConduction
   • BConvection and radiation
   • CBoth (a) and (b)
   • DNone of the above
   Answer: (C) Both (a) and (b)
   Explanation

   Option c is the correct liquid remains hot or cold for a long time in a because there is no loss or gain of heat by or radiation. A thermos flask has a chamber with a vacuum between the walls. The heat transfer by conduction and convection, of these processes require the presence of a inner and outer walls of the glass chamber are to reflect heat radiation, which is the only of heat transfer. Thus, a thermos flask minimizes of heat between the liquid and its it at a constant temperature for a long time. E

4. #44Previous year
   A sample of any of the following substances disappears after sometime when exposed to air like camphor, naphthalene or dry ice. This phenomenon is called: [. (Pre) 2019)
   • ASublimation
   • BEvaporation
   • CDiffusion
   • DRadiation
   Answer: (A) Sublimation
   Explanation

   Option a is the correct is the process of a solid changing directly gas without passing through a liquid state. It is a meaning that it does not change the of the substance. Important Tips Here are some examples of sublimation in everyday life: Camphor balls are used to repel moths because they slowly sublimate and release a moth-repelling vapor. Naphthalene is used in mothballs because it also sublimate and repels moths. Dry ice is used in shipping because it sublimate and keeps food and other products cold. Evaporation is the process of a liquid changing into a gas. Examples of evaporation include water evaporating from the surface of a lake or pond and sweat evaporating from your skin. Diffusion is the movement of particles from an area of high concentration to an area of low concentration. Examples of diffusion include the smell of perfume diffusing through the air and the movement o

5. #45UPPCS 2017
   A cut diamond shines brilliantly due to
   • ASome inherent property
   • BTotal internal reflection
   • CIts molecular structure
   • DAbsorption of light
   Answer: (B) Total internal reflection
   Explanation

   Option b is the correct cut diamond shines brilliantly due to total Total internal reflection occurs when from one medium to another medium at an than the critical angle. The critical angle is of incidence at which the refracted ray is parallel surface. When total internal reflection occurs, all light is reflected back into the original medium. A has many facets, which are flat surfaces that When light enters a diamond, it is reflected from to another. As long as the angle of incidence is the critical angle, total internal reflection will the light will be kept inside the diamond. The inside of is very reflective, so the light bounces around before it finally escapes. This multiple reflection causes diamonds to sparkle and shine so brilliantly Air Water

6. #46UPPCS 2017
   A body weight is highest in which among the following?
   • AAir
   • BWater
   • CHydrogen
   • DNitrogen
   Answer: (C) Hydrogen
   Explanation

   Option c is the correct body's weight is highest in hydrogen among the Weight is the force exerted on an object due and it depends on the mass of the object and acceleration. Hydrogen has a very low to other substances, such as air, water, Therefore, when a body is in an environment it experiences less buoyancy and thus a weight due to the weaker buoyant force acting on it

7. #47Previous year
   One important characteristic of sound is 'Pitch', which depends upon: [. (Mains) 2017)
   • AIntensity
   • BFrequency
   • CQuality
   • DAmplitude
   Answer: (B) Frequency
   Explanation

   Option bis the correct pitch of a sound is determined by its is the number of sound waves that pass a in a given period of time. It is measured in which is the number of cydes per second. A corresponds to a higher pitch. For example, voice typically has a higher pitch than a man's it has a higher frequency

8. #48UPPCS 2017
   Television viewers using dish antenna to receive satellite signals do not receive signals during rain because: 1. of small size of antenna. 2. rain droplets absorb the energy of radio waves. 3. rain droplets disperse the energy of radio waves from their original direction. Which of the above statements are correct?
   • Aonly 1
   • Bonly 1 and 2
   • Conly 2 and 3
   • D1, 2 and 3
   Answer: (D) 1, 2 and 3
   Explanation

   Option d is the correct l is correct- Satellite signals are typically so a large dish antenna is needed to collect enough signal to produce a clear image. A small dish antenna be able to collect as much of the signal, so it will be to signal attenuation from 2 is correct- Rain droplets are made of is a dielectric material. Dielectric materials the energy of radio waves. The more rain are in the atmosphere, the more energy will be the satellite 3 is correct- Rain droplets can scatter of radio waves. This is because the rain different refractive indices than the surrounding the radio waves hit the rain droplets, they are different directions. This can cause the signal to and difficult to receive

9. #49UPPCS 2016
   During respiration energy is produced in the form of
   • AADP
   • BATP
   • CNADP
   • DCO 2
   Answer: (B) ATP
   Explanation

   Option bis the correct answer. Common semiconductor materials include Si, Ge, the process of respiration, glucose is broken into carbon dioxide and water, and the energy A pure semiconductor is neither p-type nor n-type is released is stored in the form of ATP (Adenosine remains electrically ATP is a molecule that contains a high- Extrinsic semiconductors result from bond that can be easily broken to release energy for suitable impurities to enhance conductivity and cellular processes. ATP is also called the energy classified of the cell. Intrinsic: Pure semiconductors like Germanium (Ge)

10. #50UPPCS 2016
    In wind power, which form of energy is converted into electrical energy?
    • AKinetic energy
    • BPotential energy
    • CSolar energy
    • DRadiant energy
    Answer: (A) Kinetic energy
    Explanation

    and Silicon Option a is the correct answer. Extrinsic: Semiconductors mixed with power is the conversion of wind energy into electricity further categorized mechanical energy using wind turbines. In wind power (1) n-type: Mixed with pentavalent impurities like the energy source that is converted into and Sb, where negatively charged electrons act as energy is kinetic energy. This kinetic energy is by the moving air molecules, and it's harnessed by (2) p-type: Mixed with trivalent impurities like Al and turbines. When wind blows, it causes the turbine's where positively charged holes serve as charge to rotate, which transfers mechanical energy to The generator then utilizes electromagnetic 284. Solution: to convert this mechanical energy into electrical Exp) Option d is the correct Nylon is not a natural polymer. Nylon is a synthetic polymer

11. #51State PCS 2014
    The electric current does not flow between two prnpedy connected charged bodies if they are having
    • ASame charge
    • BSame capacity
    • CSame resistivity
    • DSame potential
    Answer: (D) Same potential
    Explanation

    Option d is the correct current flows from a region of higher a region of lower potential. If two charged bodies same potential, there is no potential difference and no current will flow

12. #52UPPCS 2013
    Red light is used in traffic signals because
    • AIt has the longest wavelength
    • BIt is beautiful
    • CIt is visible to people even with bad eyesight
    • DNone of the above reasons
    Answer: (A) It has the longest wavelength
    Explanation

    Option a is the correct answer Red light is used in traffic signals because it has the among visible colors in the The longer wavelength of red light allows scatter less and penetrate atmospheric as fog or mist, more effectively compared to like blue or green. This makes red light to drivers even in adverse weather conditions visibility situations, enhancing road safety. choice of red light is primarily based on its ensuring that traffic signals are effective and see

13. #53UPPCS 2013
    The red colour of the Sun at sunset and sumrise is due to
    • AScattering of light
    • BRefraction of light
    • CTotal internal reflection of light
    • DDispersion of light
    Answer: (A) Scattering of light
    Explanation

    Option a is the correct red color of the Sun during sunset and sunrise due to the scattering of light in the When the Sun is lower on the horizon, has to pass through a larger portion of the This causes shorter wavelengths, like blue to scatter more, leaving the longer wavelengths, red and orange, dominant in the visible spectrum. As we perceive the Sun as red or orange during - WHY THE SKY IS BLUE Blue Sky ond Red Molecules in atmosphere scatters lights rays Shorter wavelengths (blue, violet) are scattered more easlly More atmosphere

14. #54UPPCS 2012
    Why do pendulum docks usually lose time in summed
    • ADue to longer days in summer
    • BDue to friction caused by coils
    • CThe length of the pendulum increases, increasing the time period of unit oscillation
    • DThe weight of the pendulum changes in summer
    Answer: (C) The length of the pendulum increases, increasing the time period of unit oscillation
    Explanation

    Option c is the correct clocks usually lose time in summer because of the pendulum increases, increasing the of unit oscillation. In summer, due to the increase the metal rod of the pendulum expands, and increases slightly. This causes the pendulum to time to complete one oscillation. As a result, the time in winter, the opposite happens. The metal rod its length decreases slightly. This causes the take less time to complete one oscillation. As a result, gains time in winter

15. #55Previous year
    Which of the following lens is used to minimize Myopianu.. (Mains) 2011
    • AConvex lens
    • BConcave lens
    • CCylindrical lens
    • DNone of these
    Answer: (B) Concave lens
    Explanation

    Option b is the correct concave lens is used to correct myopia, also known Myopia is a refractive error where appear blurred, and it occurs when light focuses of the retina rather than on it. A concave lens light rays, allowing them to focus correctly on thereby correcting the myopic condition and distant vision. (Sharp image of ob,iucr at infmily) R Myopic Eye P {Blurred image of object al infinity and sharp image of object at F (far point)] x - ( b )' / myopic eyo (Sharp image of object at infinity) -x

16. #56UPPCS 2011
    Retina of the eye is comparable to which of the following parts of a traditional camera?
    • AFilm
    • BLens
    • CShutter
    • DCover
    Answer: (A) Film
    Explanation

    Option a is the correct retina of the eye is comparable to the film in camera. Just as the film in a camera captures and incoming light lo create an image, the retina in the and processes light s.ignals, ultinrntely to the hnin for im,igc formation. Both the retina play sirnilar roles in capturing inforrnatiou.it images or photographs

17. #57UPPCS 2011
    Helium is preferred to Hydrogen in air balloons because it
    • AIs cheaper
    • BIs less dense
    • CHas greater lifting power
    • DDoes not form an explosive mixture with air
    Answer: (D) Does not form an explosive mixture with air
    Explanation

    Option d is the correct is preferred over hydrogen in air because it does not form an explosive air. Unlike hydrogen, which is highly flammable lead to catastrophic accidents if ignited, helium and non-flammable. This makes helium a safer lifting gas in balloons, as it eliminates the risk of explosions that hydrogen poses. While hydrogen is and slightly more buoyant than helium, the makes helium the preferred choice for air balloons

18. #58UPPCS 2010
    Which one of the following is not correctly matched?
    • AKnot - Measure of speed of ship
    • BNautical mile - Unit of distance used in navigation
    • CAngstrom - Unit of wavelength of light
    • DLight year - Unit of measuring time
    Answer: (D) Light year - Unit of measuring time
    Explanation

    Option d is the correct light year is not a unit of measuring time, but a unit distance. It is the distance that light travels year in a vacuum, which is about 9.46 trillion 5.88 trillion miles. Important Tips A knot is a measure of speed of a ship, equal to one nautical mile per hour. A nautical mile is a unit of distance used in navigation, equal to 1.852 kilometers or 1.151 miles. An angstrom is a unit of wavelength of light, equal to 10-10 meter or 0.1 nanometers

19. #59UPPCS 2009
    The mirror, which is used in searchlights, is
    • AConcave mirror
    • BConvex mirror
    • CSimple mirror
    • DNone of these
    Answer: (A) Concave mirror
    Explanation

    Option a is the correct mirror used in searchlights is a concave mirrors are curved inward, and they have of converging light rays. This makes them reflecting and focusing light from a source to powerful, directional beam. In searchlights, a high intensity light source, such as a bulb or lamp, is the focal point of the concave mirror. The mirror and concentrates the light into a parallel can be projected over long distances for including signaling and illumination. Important Tips Examples of where concave mirrors are used: Makeup mirrors: Concave mirrors are commonly used in makeup mirrors to provide an enlarged and upright virtual image of the face, making it easier to apply cosmetics with precision. Reflecting telescopes: Many telescopes, especially Newtonian telescopes, use concave mirrors as the primary mirror to collect and focus incoming light, allowing astronomers to observe dista

20. #60UPPCS 2009
    Consider the following statements:l. Coal, petroleum and natural gas are fossil fuels. 2. Gasohol is a mixture of benzene and alcohol. 3. Geothermal energy is a non-conventional energy source. 4. Gobar gas contains mainly methane. Of these statements
    • AOnly 1 and 2 are correct
    • BOnly 2 and 3 are correct
    • COnly 1, 2 and 3 are correct
    • D1, 3 and 4 are correct
    Answer: (D) 1, 3 and 4 are correct
    Explanation

    Option d is the correct I is correct: Coal, petroleum and natural gas fuels, which are formed from the remains of ancient plants and animals under high pressure and millions of 2 is incorrect: Gasohol is not a mixture and alcohol, but a mixture of gasoline and is a type of alcohol that can be produced plant materials, such as corn, sugarcane, or is a carcinogenic compound that is found in and 3 is correct: Geothermal energy is a nonconventional energy source, which means it is not or easily accessible. Geothermal energy is the comes from the Earth's interior, which can be used electricity or provide heating and 4 is correct: Gobar gas, also known as mainly methane. It is produced through decomposition of organic wastes from animals such as cow dung, crop residues, or sewage. a flammable gas that can be used as a fuel or a fertilizer

21. #61UPPCS 2009
    If the distance between the Earth and the Sun were twice what it is now, the gravitational force exerted on the Earth by the Sun would be
    • ATwice as large as it is now
    • BFour times as large as it is now
    • COne fourth of what it is now
    • DHalf of what it is now
    Answer: (C) One fourth of what it is now
    Explanation

    Option c is the correct gravitational force between two objects is given by Fg Fg is the gravitational force, G is the constant, ml and are the masses of and r is the distance between the distance between the Earth and the Sun were the gravitational force would be reduced by a factor since it is inversely proportional to the square of Therefore, the new gravitational force would fourth of the original force

22. #62UPPCS 2009
    Given below a.re two statements: Assertion (A): Lighting a candle in space does not produce a flame. Reason A flame exists due to the pull of gravity. In the context of above, which one of the following is correct?
    • ABoth (A) and (R) are true, and (R) is the correct explanation of (A)
    • BBoth (A) and (R) are true, but (R) is not the correct explanation of (A)
    • C(A) is true, but (R) is false
    • D(A) is false, but (R) is true
    Answer: (A) Both (A) and (R) are true, and (R) is the correct explanation of (A)
    Explanation

    Option a is the correct (A) is correct: When we light a candle, it classic teardrop shape. However, that's only because When the oxygen is consumed by the fire, the rises and the cooler uncombusted air sinks to the without gravity in space, there's no separation and heavier air, so the fire burns in all causing a slow-burning globe of combustion n (R) correctly explains Assertion (A): On plays a crucial role in creating and sustaining a causes hot air and combustion products to rise, creating of fresh air to feed the flame. This convection fundamental to the formation and sustenance of a condition is not satisfied in space with no gravity

23. #63UPPCS 2009
    While lifting a bucket of water from a well, we feel that the bucket
    • ABecomes heavier above the surface of water
    • BBecomes lighter above the surface of water
    • CLoses stability on coming out of the water
    • DHas gained mass on coming out of water
    Answer: (A) Becomes heavier above the surface of water
    Explanation

    Option a is the correct lifting a bucket of water from a well, we feel bucket becomes heavier above the surface of the sensation is due to the buoyant force acting on while it's submerged. Underwater, the buoyant offsets the weight of the water inside the it feel lighter. However, when the bucket the water, it loses this buoyant force, and its weight longer counteracted, causing it to feel heavier. Buoyant F'orce, General force "'weight of -? V volume ol submerged object. fl since V _m_ the buoyant force force Pobject be expressed as mg Important Tips Buoyant force: The buoyant force is the upward force exerted on an object wholly or partly immersed in a fluid. This upward force is also called Upthrust. Due to the buoyant force, a body submerged partially or fully in a fluid appears to lose its weight, i.e. appears to be lighter. Factors Affecting Buoyant Force: Density of the Fluid: The

24. #64State PCS 2009
    In comparison to an electric bulb, a fluorescent tube is preferred because
    • AIt has greater external outer surface to radiate/emit the light
    • BThere is no effect of voltage fluctuation on it
    • CIn the tube, electric energy almost converted into light energy
    • DNone of these. m
    Answer: (D) None of these. m
    Explanation

    Option d is the correct tubes are preferred to electric bulbs are more energy-efficient and have a longer tubes use about less energy bulbs to produce the same amount oflight, typically last 8,000-10,000 hours, while typically last 1,000-2,000 hours

25. #65UPPCS 2008
    The colour of the star is an indication of its
    • ADistance from the earth
    • BTemperature
    • CLuminosity
    • DDistance from the sun
    Answer: (B) Temperature
    Explanation

    Option b is the correct color of a star is primarily an indication of This is described by Wien's law, which hotter objects emit shorter-wavelength radiation, objects emit longer-wavelength radiation. a continuous spectrum of light, but the that light depends on their temperature. Cooler reddish because they emit more in the red part spectrum, while hotter stars appear bluish because more in the blue part. Thus, by observing a star's can infer its temperature. X10 K {blue star) K (white star like our sun) K (red star) 100 500 1000 1500 2000 Wavelength (nm} ¼483 nm 3 "A T 2.898 x peak The wavelength of the peak of the blackbody radiation curve gives a measure of temperature. 966 nm (IR} Wavelength (nm) Important Tips Wien's Displacement Law: Wien's Displacement Law is a fundamental. in physics that relates the temperature of an object to the peak wavelength of its black-body radiation. It

26. #66UPPCS 2007
    To hear a dear echo, the minimum distance between the reflecting surface and the observer should be
    • A165 feet
    • B165 metre
    • C16.5 feet
    • D16.5 metre
    Answer: (D) 16.5 metre
    Explanation

    Option dis the correct minimum distance between the reflecting the observer to hear a clear echo is 16.5 meters. because sound travels at a speed of about 343 meters To hear a clear echo, the sound must have to travel to the reflecting surface and back before sound wave is produced. If the reflecting surface is the two sound waves will overlap and the echo be dear. Important Tips Comparison between Echo and Reverberations: Characteristic Echo Reverberation Definition A single reflection The persistence of a sound wave of sound after the off a surface sound source has been stopped Cause Reflection of a Multiple reflections sound wave off a of a sound wave off single surface multiple surfaces Delay The delay between The delay between the original sound the original and the echo is sound and the determined by the reverberation is distance to the too short to be reflecting surface perceived

27. #67UPPCS 2007
    Food gets cooked faster in a pressure cooker because
    • AWater starts boiling at a lower temperature due to high pressure
    • BWater starts boiling at a higher temperature due to high pressure
    • CWater boils only at but the heat content is higher at high pressure
    • DConvection currents are set inside the cooker
    Answer: (B) Water starts boiling at a higher temperature due to high pressure
    Explanation

    Option b is the correct answer. Food gets cooked faster in a pressure cooker because water starts boiling at a higher temperature due to high pressure. A pressure cooker works on the principle of the Ideal Gas Law, which states that the pressure and temperature of a gas are directly proportional in a closed system. This means that when the pressure inside the cooker increases, the temperature also increases. This allows the water to boil at a higher temperature than the normal boiling point of at sea level. The higher temperature cooks the food faster and more evenly

28. #68UPPCS 2003
    Consider the following statements: Assertion (A): When a ship enters a sea from a river, it rises a little. Reason The density of sea water is greater than that of river water. In the context of the above statements, which of the following is true?
    • ABoth A and Rare true, and R is the correct explanation of A
    • BBoth A and R are true, but R is not the correct explanation of A
    • CA is true, but R is false
    • DA is false, but R is true
    Answer: (A) Both A and Rare true, and R is the correct explanation of A
    Explanation

    Option a is the correct A and R are true, and R is the correct A: When a ship enters a sea from a river, it rises a seavvater is denser than river water. This affects the buoyant force acting on the ship. In seawater, the buoyant force is greater than in the river water, causing the ship to experience an As a result, the ship rises slightly in the sea, the new equilibrium between its weight and the

29. #69UPPCS 2003
    Direct viewing of the Sun during total solar eclipse causes irreversible damage to the eyes. The retinal bum is caused by which one of the following components of the sun rays?
    • AHeat
    • BRainbow light
    • CUltraviolet light
    • DInfrared light
    Answer: (C) Ultraviolet light
    Explanation

    Option c is the correct viewing the Sun during a total solar eclipse irreversible damage to the eyes due to the of the Sun's rays, which is ultraviolet Ultraviolet light, specifically the UV-B and can damage the cells in the retina of the damage is often referred to as solar retinopathy and c NOLOGY can lead to permanent vision loss. That's why it's crucial proper eye protection, such as eclipse glasses or when viewing a solar eclipse

30. #70UPPCS 2002
    Consider the following statements:'I' 1. The heart vibrates at infrasonic. 2. The speed of sound is more in gas than in liquid and solid. 3. Mach number is used to describe the speed of sound. 4. Ultrasonic sound has frequency more than 20,000 Hz. Of these statements
    • A1, 2 and 3 are correct
    • B2, 3 and 4 are correct
    • C1, 3 and 4 are correct
    • D1, 2 and 4 are correct
    Answer: (C) 1, 3 and 4 are correct
    Explanation

    Option c is the correct I is correct- Infrasonic waves are sound frequencies below the range of human hearing. vibrates at a frequency of about l Hz, which is range of human 2 is incorrect- The speed of sound is solids, followed by liquids, and then gases. This solids have the highest density and the 3 is correct- The Mach number is a that is used to describe the speed of an to the speed of 4 is correct- Ultrasonic waves are sound frequencies above the range of human hearing. of human hearing is about 20 Hz to 20,000 Hz

31. #71UPPCS 1994
    It is difficult to walk on the ice than on the road because
    • AIce is harder than the road
    • BRoad is harder than ice
    • CIce does not offer any reaction when we push it with our foot
    • DIce has lesser friction than the road
    Answer: (D) Ice has lesser friction than the road
    Explanation

    Option d is the correct is the force that opposes the relative motion of in contact. Friction depends on the nature of the surfaces, as well as the normal force together. The coefficient of friction is a measure of friction a pair of surfaces can generate. The coefficient of friction, the more friction there is and it is to slide one surface over has a very low coefficient of friction compared to materials, especially when it is wet or covered with thin layer of water. This means that ice offers very to sliding, and it is easy to slip or skid on ice. on the other hand, are usually rougher and coefficients of friction, which means they grip and traction for walking or driving. is more difficult to walk on ice than on the road has less friction than the road

32. #72UPPCS 1994
    Oil spreads on water surface because
    • AOil is denser than water
    • BOil is less dense than water
    • CSurface tension of oil is more than water
    • DSurface tension of oil is less than water
    Answer: (D) Surface tension of oil is less than water
    Explanation

    Option d is the correct spreads on water because the surface tension of oil is that of water. Surface tension is the force that allows to minimize its surface area. Oil molecules are to each other than they are to water molecules. because oil molecules are non polar, meaning that they uneven distribution of charge, while water molecules As a result, the surface tension of oil is less than water. When oil is placed on water, the oil pulled towards the surface of the water by the tension. This causes the oil to spread out into film. Important Tips Other factors why oil spread on the water surface are: The amount of oil that spreads depends on the difference in surface tension between the oil and the water. Oils with a lower surface tension will spread out more than oils with a higher surface tension. The surface tension of oil also depends on its temperature. As the temperature of oil incr

33. #73UPPCS 1993
    In order to avoid sunlight, which of the following colour combinations for an umbrella is most appropriate?
    • ATop black bottom white
    • BTop white bottom black
    • COnly black
    • DOnly white
    Answer: (B) Top white bottom black
    Explanation

    Option b is the correct most appropriate color combination for an avoid sunlight is top white and bottom black. is based on the principle of albedo, which refers reflectivity of an object's surface. White surfaces have higher albedo, meaning they reflect more sunlight and conversely, black surfaces absorb more sunlight This means that a white top will reflect sunlight us, while a black bottom will absorb any does pass through the umbrella. This will help us cool and avoid sunburn

34. #74UPPCS 1993
    A fuse wire is characterized by
    • ALow resistance and high melting point
    • BLow resistance and low melting point
    • CHigh resistance and low melting point
    • DHigh resistance and high melting
    Answer: (C) High resistance and low melting point
    Explanation

    Option c is the correct fuse wire is a piece of wire that is designed to melt if flowing through it exceeds a certain safe level. done to protect the electrical circuit from fuse wire needs to have two properties: High resistance: This is so that the fuse wire heats up quickly when a high current flows through it. Low melting point: This is so that the fuse wire melts quickly when it heats up, breaking the circuit and preventing further damage

35. #75UPPCS 1990
    If a person is sitting in a lift, then when he feel that his weight gets increased?
    • AWhen the elevator is going upward expeditiously
    • BWhen the elevator goes down expeditiously
    • CGoing upward with constant velocity
    • DGoing downward with constant velocity
    Answer: (A) When the elevator is going upward expeditiously
    Explanation

    Option a is the correct a person sits in a lift, he will feel that his weight when the elevator goes upward reason is that when the elevator accelerates upward, weight of the person sitting in the lift apparent weight is the normal force that the lift the person to balance the downward force of gravity. force is greater than the actual weight of the the lift accelerates upward, so the person feels heavier

36. #76UPSC CDS 2023
    Consider the following statements about the microphone and the speaker of a mobile phone: 1. The microphone converts sound to a mechanical signal. 2. The microphone converts sound to an electrical signal. 3. The speaker converts a mechanical signal to sound. 4. The speaker converts an electrical signal to sound.Which of the statements given above are correct?
    • A1 and 3
    • B1 and 4
    • C2 and 3
    • D2 and 4
    Answer: (D) 2 and 4
    Explanation

    Option d is the correct l is incorrect- The primary function of in a mobile phone is to convert sound energy) into an electrical signal. signal can then be processed and transmitted phone's 2 is correct- Microphones use transducers, condenser or dynamic elements, to convert sound electrical 3 is incorrect- Speakers convert electrical sound waves. The electrical signal passes speaker's voice coil, which interacts with a causing the coil to vibrate and produce sound 4 is correct- The speaker's main purpose is electrical signals into audible sound. The passing through the voice coil of the speaker that generate sound waves, which can be the user

37. #77UPSC CDS 2023
    The area under the velodty-time graph for particle moving in a straight line with uniform acceleration gives in air in air lk
    • Aits average velocity
    • Bits net displacement
    • Cthe distance travelled by it
    • Dits average speed 369, Sound and light waves are
    Answer: (B) its net displacement
    Explanation

    Option h is the correct area under the velocity-time graph for a particle a straight line with uniform acceleration particle's net displacement. When the acceleration the velocity-tirne graph forms a straight the area under the graph corresponds to the change or displacement of the particle during the interval. By calculating this area, we can determine displacement of the particle

38. #78UPSC CDS 2022
    Plants prepare glucose in the process of
    • Arespiration
    • Bphotosynthesis
    • Cdegradation
    • Dmineral absorption rmnl.m
    Answer: (B) photosynthesis
    Explanation

    Option b is the correct is the process by which plants carbon dioxide, and water into glucose (a and oxygen. Important Tips Photosynthesis takes place in the chloroplasts of plant cells, specifically in the chlorophyll-containing structures called thylakoids. During photosynthesis, plants capture energy from sunlight using the pigment chlorophyll. This energy is used to convert carbon dioxide and water into glucose, a form of stored energy that plants can use for growth, development, and other metabolic processes. The overall reaction of photosynthesis can be represented as follows: 6 CO2 (carbon dioxide) 6 H20 (water) sunlight energy 6 02 (oxygen) Glucose is a carbohydrate that serves as the primary source of energy for plants. It is used to produce ATP (adenosine triphosphate), which is the energy currency of cells. Glucose can also be stored in various forms such as starch or converte

39. #79UPSC CDS 2022
    At the time of short-circuit, the current in the circuit
    • Areduces substantially
    • Bdoes not change
    • Cincreases heavily
    • Dkeeps on fluctuating
    Answer: (C) increases heavily
    Explanation

    Option c is the correct a short circuit, a low-resistance path is created circuit when the live wire and neutral wire come with each other, bypassing a significant the circuit's normal resistance. This results in a in the overall resistance of the circuit. Ohm's Law (V IR), when the resistance decreases, voltage remains constant, the current in the Therefore, during a short circuit, the current circuit increases heavily. Important Tips A domestic circuit consists of two wires: live wire and neutral wire. The voltage difference between the live wire and neutral wire is maintained by electricity companies, with the live wire at a potential of 220 V and the neutral wire at a potential of 0 V. To prevent damage from short-circuiting and overloading, electric fuses are used. Electric fuses are designed to melt in the presence of excess current, breaking the circuit and stopping the flow of ov

40. #80UPSC CDS 2022
    What happens to the gravitational force between two objects if the mass of one object is doubled and the distance between them is also doubled?
    • AThe force would remain the same
    • BThe force would be doubled
    • CThe force would be halved
    • DThe force would increase by a factor of 4
    Answer: (C) The force would be halved
    Explanation

    Option c is the correct know, F (G ml /, - is the force of is the gravitational constant 6.673 x l 0-11 N and are the masses of the two objects, is the distance between their (i) implies force is directly proportional to of the object and inversely proportional to between their to the question, new mass 1112' 2 and new force values in Equation (i), we G ml 2 1112 / ½ (G ml / ) gravitational force would be halved of gravitational force

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