Physics

221. Sita, 1·5 m high, stands before a plane mirror fixed on a wall to view

Sita, 1·5 m high, stands before a plane mirror fixed on a wall to view her full image. What should be the minimum height of the plane mirror so that Sita can view her image fully ?

[amp_mcq option1=”0·50 m” option2=”0·35 m” option3=”0·75 m” option4=”0·25 m” correct=”option3″]

This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
The minimum height of the plane mirror required for a person to view their full image is half of the person’s height.
For a person of height H standing in front of a plane mirror, the minimum height of the mirror needed to see the entire image is H/2. This is because of the law of reflection; light rays from the top of the head and the bottom of the feet reflect off the mirror into the observer’s eyes. The reflection point for the feet is halfway between the feet and the eyes’ height, and similarly for the top of the head.
In this case, Sita’s height is 1.5 m. The minimum mirror height is 1.5 m / 2 = 0.75 m. The position of the mirror is also important; the bottom edge of the mirror should be at a height halfway between the person’s eyes and their feet.

222. The heating element in an electric iron is usually made of

The heating element in an electric iron is usually made of

[amp_mcq option1=”Constantan” option2=”Tungsten” option3=”Nichrome” option4=”Copper” correct=”option3″]

This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
The heating element in an electric iron is usually made of Nichrome.
Nichrome is an alloy primarily composed of nickel and chromium. It is chosen for heating elements because it has a high electrical resistance, allowing it to efficiently convert electrical energy into heat. It also has a high melting point and resistance to oxidation at high temperatures, ensuring durability and safety during operation.
Other materials like Constantan also have high resistance but are less commonly used for high-temperature heating elements compared to Nichrome. Tungsten has a very high melting point and is used for filaments in incandescent light bulbs, but not typically for resistive heating elements like those in irons or toasters. Copper has low resistance and is used for electrical wiring, not for generating heat.

223. The potential difference between the two end terminals of an electric

The potential difference between the two end terminals of an electric heater is 220 V and the current through it is 0·5 A. What would be the current through the heater if the potential difference across the terminals of the heater is reduced to 120 V ?

[amp_mcq option1=”1·0 A” option2=”0·5 A” option3=”0·27 A” option4=”0·7 A” correct=”option3″]

This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
Assuming the resistance of the electric heater remains constant, the current through it would be approximately 0.27 A when the potential difference is reduced to 120 V.
– We can use Ohm’s Law, which states that the voltage (V) across a resistor is directly proportional to the current (I) flowing through it, provided the temperature and other physical conditions remain unchanged (V = I * R).
– First, calculate the resistance (R) of the heater using the initial conditions: V1 = 220 V, I1 = 0.5 A.
– R = V1 / I1 = 220 V / 0.5 A = 440 ohms.
– Now, use this resistance to find the new current (I2) when the potential difference (V2) is 120 V.
– V2 = I2 * R
– 120 V = I2 * 440 ohms
– I2 = 120 V / 440 ohms = 12 / 44 A = 3 / 11 A.
– Calculating the decimal value: 3 / 11 ≈ 0.2727 A.
In reality, the resistance of a heater element made of materials like nichrome increases with temperature. However, standard Ohm’s Law problems involving fixed resistors or devices like heaters typically assume constant resistance unless otherwise specified. The calculated current represents the value based on this common assumption.

224. The flash of lightning is seen before the thunderstorm is heard. It ve

The flash of lightning is seen before the thunderstorm is heard. It verifies that

[amp_mcq option1=”sound travels much faster than light” option2=”light travels much faster than sound” option3=”light and sound both travel with same speed” option4=”intensity of flash of lightning is very high during thunderstorm” correct=”option2″]

This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
The fact that the flash of lightning is seen before the thunderstorm is heard verifies that light travels much faster than sound.
– Lightning and thunder occur simultaneously as a result of an electrical discharge in the atmosphere.
– Light (the flash) and sound (the thunder) are both produced by the same event.
– We see the light almost instantly because the speed of light in air is extremely high (approximately 3 x 10^8 m/s).
– We hear the thunder later because the speed of sound in air is much lower (approximately 343 m/s at 20°C).
– The time delay between seeing the flash and hearing the thunder is directly proportional to the distance of the thunderstorm.
For every 3 seconds delay between the flash and the thunder, the thunderstorm is approximately 1 kilometer (or 5 seconds per mile) away. This difference in speeds makes it possible to estimate the distance to a thunderstorm.

225. The streaming of light beams coming from the Sun through trees is said

The streaming of light beams coming from the Sun through trees is said to have suggested that light travels in straight line. The particles on the path of light beams are visible to us because

[amp_mcq option1=”dust particles in the air reflect light into our eyes” option2=”dust particles in the air scatter light into our eyes” option3=”dust particles in the air refract light into our eyes” option4=”dust particles in the air polarize light into our eyes” correct=”option2″]

This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
The particles on the path of light beams coming from the Sun are visible to us because dust particles in the air scatter light into our eyes.
– In a perfectly clean atmosphere, a beam of light passing through it would be invisible from the side. We see light when it enters our eyes.
– When light encounters particles in the air, such as dust, smoke, or water droplets, it interacts with them.
– Scattering occurs when light waves are deflected in various directions by these particles.
– Some of the scattered light is directed towards our eyes, making the path of the light beam visible.
– This phenomenon is related to the Tyndall effect, which is the scattering of light by particles in a colloid or suspension.
Reflection typically occurs from surfaces, refraction is the bending of light as it passes from one medium to another, and polarization is the restriction of the direction of oscillation of light waves. While dust particles can reflect and refract light, scattering is the primary mechanism by which the path of a light beam through a dusty or smoky medium becomes visible.

226. A microphone converts

A microphone converts

[amp_mcq option1=”electrical signals to sound waves” option2=”sound waves to electrical signals” option3=”microwaves to sound waves” option4=”sound waves to microwaves” correct=”option2″]

This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
A microphone converts sound waves into electrical signals.
– A microphone is a transducer, a device that converts one form of energy into another.
– Its function is to capture sound (which consists of variations in air pressure) and transform it into a corresponding electrical voltage or current signal.
– Different types of microphones use different principles (e.g., electromagnetic induction in dynamic microphones, capacitance change in condenser microphones) to achieve this conversion.
– The electrical signal produced by a microphone can then be amplified, recorded, or transmitted.
Conversely, a loudspeaker or headphone is a transducer that performs the opposite function: it converts an electrical signal into sound waves.

227. A bullet of mass 10 g is horizontally fired with velocity 300 m s$^{-1

A bullet of mass 10 g is horizontally fired with velocity 300 m s$^{-1}$ from a pistol of mass 1 kg. What is the recoil velocity of the pistol?

[amp_mcq option1=”0·3 m s$^{-1}$” option2=”3 m s$^{-1}$” option3=”−3 m s$^{-1}$” option4=”−0·3 m s$^{-1}$” correct=”option3″]

This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
The recoil velocity of the pistol is -3 m/s.
– This problem can be solved using the principle of conservation of linear momentum.
– The total momentum of the system (pistol + bullet) before firing is zero, as both are at rest.
– According to the conservation of momentum, the total momentum of the system after firing must also be zero.
– Let $m_b$ and $v_b$ be the mass and velocity of the bullet, and $m_p$ and $v_p$ be the mass and recoil velocity of the pistol.
– Momentum before firing = 0
– Momentum after firing = $m_b v_b + m_p v_p$
– By conservation of momentum: $m_b v_b + m_p v_p = 0$
– Given: $m_b = 10 \text{ g} = 0.01 \text{ kg}$, $v_b = 300 \text{ m/s}$, $m_p = 1 \text{ kg}$.
– Substituting the values: $(0.01 \text{ kg})(300 \text{ m/s}) + (1 \text{ kg}) v_p = 0$
– $3 \text{ kg} \cdot \text{m/s} + v_p \text{ kg} = 0$
– $v_p = -3 \text{ m/s}$
The negative sign for the recoil velocity indicates that the pistol moves in the opposite direction to the bullet. The magnitude of the recoil velocity is 3 m/s. This demonstrates Newton’s third law of motion (action-reaction) applied to momentum conservation.

228. Which one of the following metals is most commonly used for making fil

Which one of the following metals is most commonly used for making filament of incandescent electric bulbs?

[amp_mcq option1=”Aluminium” option2=”Silver” option3=”Copper” option4=”Tungsten” correct=”option4″]

This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
Tungsten is the metal most commonly used for making the filament of incandescent electric bulbs.
– The filament in an incandescent bulb needs to glow white-hot (reach a very high temperature) to produce light.
– This requires a material with a very high melting point that can withstand such extreme temperatures without melting or vaporizing quickly.
– Tungsten (chemical symbol W) has the highest melting point of all known metals (3422°C or 6192°F).
– It also has high tensile strength, even at high temperatures, and relatively low vapor pressure, reducing evaporation of the filament.
Early incandescent bulbs used carbon filaments, but tungsten proved to be significantly more efficient and durable. The bulb is often filled with inert gases like argon or nitrogen to reduce tungsten evaporation and prolong the filament’s life.

229. Which one of the following statements about the speed of sound waves i

Which one of the following statements about the speed of sound waves is not correct?

[amp_mcq option1=”The speed of sound waves in steel is higher than that in water.” option2=”The speed of sound waves in air decreases with increase in temperature.” option3=”The speed of sound waves in air increases with increase in temperature.” option4=”The speed of sound waves in water is higher than that in air.” correct=”option2″]

This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
The statement that the speed of sound waves in air decreases with increase in temperature is incorrect.
– The speed of sound is generally highest in solids, lower in liquids, and lowest in gases. This is because molecules are closer together and interact more strongly in denser media, allowing vibrations to propagate faster.
– The speed of sound in steel is higher than in water, and the speed of sound in water is higher than in air.
– The speed of sound in a gas (like air) increases with increasing temperature. As temperature rises, the average kinetic energy of the gas molecules increases, and they collide more frequently and vigorously, allowing the sound wave to travel faster.
The speed of sound in dry air at 0°C is approximately 331.3 m/s. For every degree Celsius increase in temperature, the speed increases by about 0.6 m/s. Therefore, speed increases with temperature.

230. SONAR is a device that is used to measure the distance of underwater o

SONAR is a device that is used to measure the distance of underwater objects by a ship. Which of the following types of waves does it use for this purpose?

[amp_mcq option1=”Infrasonic waves” option2=”Sound waves in audible range for human beings” option3=”Ultrasonic waves” option4=”All of the above” correct=”option3″]

This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
SONAR (Sound Navigation and Ranging) systems typically use ultrasonic waves to measure distances and detect underwater objects.
– SONAR works by emitting sound pulses and measuring the time it takes for the echo to return after bouncing off an object.
– The speed of sound in water is known, so the distance can be calculated from the travel time.
– Ultrasonic waves (frequencies above the human hearing range, typically >20 kHz) are preferred in SONAR for several reasons, including better directionality, less attenuation over typical ranges compared to higher frequencies, and superior resolution compared to lower (infrasonic or audible) frequencies for object detection.
SONAR can be active (emitting pulses) or passive (listening to sounds). Active SONAR, used for distance measurement, relies heavily on the properties of ultrasonic pulses. While lower frequencies might be used for long-range detection (where attenuation is lower), ultrasonic frequencies are standard for detailed navigation and object location applications described in the question.
Page 1Page 2Page 3Page 4Page 5