Physics Archives

1. What is the length of a simple pendulum, which has a frequency of 0.5

What is the length of a simple pendulum, which has a frequency of 0.5 Hz (take g = 10 m/s²)?

[amp_mcq option1=”1 m” option2=”2 m” option3=”1.5 m” option4=”3 m” correct=”option1″]

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UPSC CISF-AC-EXE – 2024

The correct answer is A) 1 m.

The frequency (f) of a simple pendulum is related to its length (L) and the acceleration due to gravity (g) by the formula: f = 1 / (2π√(L/g)). Rearranging this formula to solve for L gives L = g / (4π²f²). Given f = 0.5 Hz and g = 10 m/s², and using the common approximation π² ≈ 10 when g ≈ 9.8 or 10 m/s², we get L = 10 / (4 * 10 * (0.5)²) = 10 / (40 * 0.25) = 10 / 10 = 1 meter.

The period (T) is the reciprocal of the frequency (T = 1/f = 1/0.5 = 2 seconds). The period formula is T = 2π√(L/g). Squaring both sides gives T² = 4π²(L/g), so L = gT² / (4π²). Using T=2s, g=10m/s², and π²≈10: L = 10 * (2)² / (4 * 10) = 10 * 4 / 40 = 40 / 40 = 1 meter. This confirms the result.

2. Which one among the following metals is not a ferromagnetic material?

Which one among the following metals is not a ferromagnetic material?

[amp_mcq option1=”Cobalt” option2=”Silver” option3=”Iron” option4=”Nickel” correct=”option2″]

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UPSC CISF-AC-EXE – 2024

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The correct answer is B) Silver.

Ferromagnetic materials are substances that are strongly attracted by a magnetic field and can be permanently magnetized. Common examples include Iron (Fe), Cobalt (Co), and Nickel (Ni). Silver (Ag) is a diamagnetic material, meaning it is weakly repelled by a magnetic field.

Materials can be classified based on their magnetic properties as diamagnetic, paramagnetic, or ferromagnetic. Diamagnetic materials have paired electrons and oppose external magnetic fields. Paramagnetic materials have unpaired electrons and are weakly attracted to magnetic fields. Ferromagnetic materials exhibit a strong attraction to magnetic fields due to the alignment of magnetic domains.

3. The temperature of a body decreases from 240 K to 210 K. The correspon

The temperature of a body decreases from 240 K to 210 K. The corresponding change (without sign) in the temperature in degree Fahrenheit is close to :

[amp_mcq option1=”54° F” option2=”30° F” option3=”17° F” option4=”44° F” correct=”option1″]

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UPSC CISF-AC-EXE – 2023

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The temperature change from 240 K to 210 K is 30 K. The corresponding change in degree Fahrenheit is 54°F, which is option A.

A change in temperature on the Kelvin scale is equal to the same change in temperature on the Celsius scale (ΔT_K = ΔT_C). The relationship between a change in Celsius and a change in Fahrenheit is ΔT_F = (9/5) * ΔT_C.

Given the initial temperature T1 = 240 K and final temperature T2 = 210 K, the change in Kelvin is ΔT_K = T2 – T1 = 210 K – 240 K = -30 K. The question asks for the change *without sign*, so the magnitude of the change is 30 K. Since ΔT_K = ΔT_C, the change in Celsius is ΔT_C = 30°C. The corresponding change in Fahrenheit is ΔT_F = (9/5) * ΔT_C = (9/5) * 30 = 9 * 6 = 54°F.

4. According to the Big Bang theory of the origin of the Universe, the ag

According to the Big Bang theory of the origin of the Universe, the age of the Universe is estimated at about :

[amp_mcq option1=”0-3 million years” option2=”4 – 6 billion years” option3=”7 – 10 billion years” option4=”13 – 18 billion years” correct=”option4″]

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UPSC CISF-AC-EXE – 2023

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According to the Big Bang theory, the age of the Universe is estimated at about 13 – 18 billion years, which corresponds to option D.

The standard cosmological model, which is based on the Big Bang theory, estimates the age of the Universe by observing the expansion rate (using the Hubble constant) and the composition of the Universe (from Cosmic Microwave Background data, e.g., from Planck or WMAP satellites). Current estimates based on these observations place the age of the Universe at approximately 13.8 billion years.

Early estimates varied, but increasingly precise measurements over the past couple of decades have converged on a value close to 13.8 billion years. Options A, B, and C represent ages that are significantly younger than the currently accepted scientific estimate for the age of the Universe.

5. Which one of the following represents the correct order for the wavele

Which one of the following represents the correct order for the wavelengths of different waves ?

[amp_mcq option1=”Used for FM radio > Used for AM radio > Microwaves > Infrared > Visible light” option2=”Used for AM radio > Used for FM radio > Microwaves > Infrared > Visible light” option3=”Microwaves > Used for AM radio > Used for FM radio > Infrared > Visible light” option4=”Used for AM radio > Used for FM radio > Infrared > Visible light > Microwaves” correct=”option2″]

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UPSC CISF-AC-EXE – 2023

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The correct order for the wavelengths of different waves is Used for AM radio > Used for FM radio > Microwaves > Infrared > Visible light. This corresponds to option B.

Electromagnetic waves are ordered by their wavelength (or equivalently, frequency). Longer wavelengths correspond to lower frequencies, and shorter wavelengths correspond to higher frequencies. The order of these waves from longest wavelength to shortest is: Radio waves (including AM and FM) > Microwaves > Infrared radiation > Visible light > Ultraviolet radiation > X-rays > Gamma rays. Within the radio wave spectrum, AM radio waves typically have longer wavelengths and lower frequencies than FM radio waves.

The approximate wavelength ranges are:
– AM Radio: Hundreds to thousands of meters
– FM Radio: A few meters
– Microwaves: Centimeters to decimeters (e.g., 1 cm to 1 meter)
– Infrared: Micrometers to millimeters (e.g., 700 nm to 1 mm)
– Visible Light: Nanometers (e.g., 400 nm – 700 nm)
Based on these ranges, the correct order from longest to shortest wavelength is AM Radio > FM Radio > Microwaves > Infrared > Visible Light.

6. Which one among the following statements regarding sound is correct ?

Which one among the following statements regarding sound is correct ?

[amp_mcq option1=”High pitch sound has high frequency” option2=”High pitch sound has high amplitude” option3=”High pitch sound has high speed” option4=”High pitch sound has low frequency and high amplitude” correct=”option1″]

This question was previously asked in

UPSC CISF-AC-EXE – 2023

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The correct answer is A.

Pitch of a sound is related to its frequency. A higher frequency corresponds to a higher pitch, and a lower frequency corresponds to a lower pitch. Amplitude of a sound wave is related to its loudness or intensity. Speed of sound depends on the medium through which it travels and generally does not change with pitch or amplitude.

This is a fundamental concept in the physics of sound waves. Frequency is measured in Hertz (Hz), and amplitude is related to the displacement or pressure variation in the medium.

7. A person of mass 50 kg is standing in a lift. If the lift moves in upw

A person of mass 50 kg is standing in a lift. If the lift moves in upward direction with an acceleration of 1 m/s², then the weight of the person will be closest to :

[amp_mcq option1=”490 N” option2=”540 N” option3=”440 N” option4=”50 N” correct=”option2″]

This question was previously asked in

UPSC CISF-AC-EXE – 2023

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The correct answer is B.

When a person is in a lift accelerating upwards, the apparent weight of the person is greater than their true weight. The forces acting on the person are the gravitational force (mg) downwards and the normal force (N) exerted by the lift floor upwards. According to Newton’s second law, the net force (N – mg) is equal to mass times acceleration (ma). Thus, the normal force N = mg + ma = m(g + a). This normal force is the apparent weight.

Given mass m = 50 kg and upward acceleration a = 1 m/s². Using the standard value of g ≈ 9.8 m/s²: Apparent weight N = 50 kg * (9.8 m/s² + 1 m/s²) = 50 kg * 10.8 m/s² = 540 N. If we use g ≈ 10 m/s², N = 50 kg * (10 m/s² + 1 m/s²) = 50 kg * 11 m/s² = 550 N. The option closest to 540 N (or 550 N) is 540 N.

8. The Nobel Prize in Physics in 2022 was awarded for :

The Nobel Prize in Physics in 2022 was awarded for :

[amp_mcq option1=”Evolution of Universe” option2=”Quantum entanglement” option3=”Black hole” option4=”Laser Physics” correct=”option2″]

This question was previously asked in

UPSC CISF-AC-EXE – 2023

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The correct answer is B.

The Nobel Prize in Physics in 2022 was awarded jointly to Alain Aspect, John F. Clauser, and Anton Zeilinger for “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” This work is directly related to quantum entanglement, a phenomenon where two or more particles are linked in such a way that they cannot be described independently of each other, even when separated by large distances.

While the evolution of the Universe, black holes, and laser physics are significant areas of physics, the 2022 Nobel Prize specifically recognized foundational work in quantum mechanics related to entanglement and its potential applications in quantum computing and communication.

9. Which one of the following is the energy source of stars ?

Which one of the following is the energy source of stars ?

[amp_mcq option1=”Nuclear fission” option2=”Nuclear fusion” option3=”Chemical reaction” option4=”Photochemical reaction” correct=”option2″]

This question was previously asked in

UPSC CISF-AC-EXE – 2023

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The correct answer is B.

Stars generate energy primarily through nuclear fusion reactions in their core. Specifically, in stars like our Sun, hydrogen nuclei fuse to form helium nuclei, releasing a vast amount of energy according to Einstein’s mass-energy equivalence principle (E=mc²).

Nuclear fission is the process of splitting heavy atomic nuclei, used in nuclear power plants on Earth, but not the main energy source of stars. Chemical reactions involve the rearrangement of electrons and release far less energy than nuclear reactions, insufficient to power a star for its lifetime. Photochemical reactions are triggered by light, not the primary energy generation mechanism in the core of a star.

10. What is the state of matter in a glowing fluorescent tube ?

What is the state of matter in a glowing fluorescent tube ?

[amp_mcq option1=”Gas” option2=”Liquid” option3=”Plasma” option4=”Bose-Einstein Condensate” correct=”option3″]

This question was previously asked in

UPSC CISF-AC-EXE – 2022

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The correct answer is Plasma.

A fluorescent tube typically contains an inert gas (like argon) and a small amount of mercury vapour at low pressure. When an electric voltage is applied across the electrodes, it accelerates electrons within the tube. These electrons collide with the gas atoms and mercury atoms, causing them to become ionized (losing electrons) and excited. This creates a state of matter consisting of a collection of positively charged ions and negatively charged electrons, along with neutral atoms, that is overall electrically neutral. This state is known as plasma. The excited mercury atoms and ions emit ultraviolet (UV) light. The inside of the tube is coated with a phosphorescent material (phosphor) that absorbs the UV light and re-emits it as visible light, causing the tube to glow.

Plasma is often referred to as the fourth state of matter, distinct from solid, liquid, and gas. It is essentially an ionized gas. Plasma is the most abundant state of matter in the visible universe, found in stars, nebulae, lightning, auroras, and certain human-made applications like fluorescent lamps, neon signs, and plasma televisions.