Physics MCQs

Question #641

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The refractive index of a medium is 1.5. The speed of light in this medium is:

A. 3×10⁸ m/s

B. 4.5×10⁸ m/s

C. 2×10⁸ m/s

D. 1.5×10⁸ m/s

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Correct Answer: Option C

Explanation:
Refractive index n = c/v, where c = 3×10⁸ m/s (vacuum speed). Thus v = c/n = 3×10⁸ / 1.5 = 2×10⁸ m/s. This direct calculation tests optics fundamentals. Memory aid: 'Higher n ⇒ slower light; v = c/n'. Competitive exams frequently test this with common refractive indices. Always use c = 3×10⁸ m/s unless specified; verify units (m/s). This problem assesses basic formula application skills essential for wave optics sections.

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Question #642

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The kinetic energy of a body is increased by 300%. The percentage increase in its momentum is:

A. 200%

B. 300%

C. 100%

D. 400%

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Correct Answer: Option C

Explanation:
KE ∝ p² (since KE = p²/(2m)). If KE increases by 300%, new KE = 4× original KE. Thus p_new² / p_original² = 4 ⇒ p_new / p_original = 2 ⇒ momentum doubles, i.e., 100% increase. Memory tip: 'KE ∝ p²; so %Δp = ½ %ΔKE for small changes, but for large changes use ratio'. This proportional reasoning problem tests energy-momentum relationship, frequently appearing in competitive exams. Always derive from fundamental relations when percentages are large; linear approximations fail here.

This question belongs to: Science Physics

Question #643

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The time taken for a radioactive sample to decay to 1/16th of its initial activity is:

A. 8 half-lives

B. 4 half-lives

C. 2 half-lives

D. 3 half-lives

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Correct Answer: Option B

Explanation:
After n half-lives, activity reduces to (1/2)ⁿ of initial. Set (1/2)ⁿ = 1/16 ⇒ (1/2)ⁿ = (1/2)⁴ ⇒ n = 4 half-lives. Thus time = 4 × half-life. Memory aid: '1/2ⁿ remaining after n half-lives; 1/16 = 1/2⁴ ⇒ 4 half-lives'. This radioactivity calculation is frequently tested in competitive exams. Always express fraction as power of 1/2 to find half-life count. This problem assesses understanding of exponential decay without heavy computation.

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Question #644

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In a potentiometer, the sensitivity can be increased by:

A. Using a thicker wire

B. Increasing the current in the primary circuit

C. Decreasing the length of the wire

D. Increasing the length of the wire

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Correct Answer: Option D

Explanation:
Potentiometer sensitivity (ability to detect small EMF changes) increases with longer wire because potential gradient (V/cm) decreases, allowing finer measurement of balancing length. Sensitivity ∝ length of wire. Decreasing length (B) reduces sensitivity; increasing current (C) increases potential gradient, reducing sensitivity; thicker wire (D) reduces resistance but doesn't directly affect sensitivity if current adjusted. Memory tip: 'Longer potentiometer wire ⇒ smaller potential gradient ⇒ higher sensitivity'. This experimental physics concept is frequently tested in competitive exams. Always link sensitivity to potential gradient: smaller gradient allows detection of smaller voltage differences.

This question belongs to: Science Physics

Question #645

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The phenomenon of superconductivity is characterized by:

A. Zero electrical resistance below critical temperature

B. Zero magnetic permeability

C. High thermal conductivity

D. Infinite electrical resistance

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Correct Answer: Option A

Explanation:
Superconductivity occurs in certain materials below a critical temperature T_c, where electrical resistance drops abruptly to zero, and magnetic fields are expelled (Meissner effect). Option B describes insulators; C and D are unrelated. Memory aid: 'Superconductor: R=0 below T_c; perfect diamagnetism'. This modern physics concept is frequently tested in competitive exams. Always distinguish superconductivity (zero resistance) from perfect conductivity (theoretical limit); real superconductors exhibit Meissner effect, a defining property.

This question belongs to: Science Physics

Question #646

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The dimensional formula of torque is the same as that of:

A. Force

B. Work

C. Power

D. Momentum

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Correct Answer: Option B

Explanation:
Torque τ = r × F, so dimensions [L][MLT⁻²] = [ML²T⁻²]. Work W = F·d, dimensions [MLT⁻²][L] = [ML²T⁻²], identical. Momentum is [MLT⁻¹], force [MLT⁻²], power [ML²T⁻³]. Though torque and work have same dimensions, they are physically distinct: torque is vector (rotational effect), work is scalar (energy transfer). Memory tip: 'Torque and work share [ML²T⁻²]; but torque = rF sinθ, work = Fd cosθ'. This dimensional analysis question tests ability to compare quantities, frequently appearing in competitive exams. Always verify dimensions but remember physical context differs.

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Question #647

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The escape velocity from a planet depends on:

A. Mass of the escaping body

B. Both mass and radius of the planet

C. Temperature of the planet

D. Radius of the planet only

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Correct Answer: Option B

Explanation:
Escape velocity v_e = √(2GM/R), where M is planet mass, R its radius. Thus v_e depends on both M and R. It is independent of the escaping body's mass (as gravitational and inertial mass cancel). Memory tip: 'v_e ∝ √(M/R); larger M or smaller R ⇒ higher escape velocity'. This gravitation formula application is frequently tested in competitive exams. Always recall that escape velocity is a property of the planet, not the projectile. This problem assesses understanding of gravitational potential energy concepts.

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Question #648

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The phenomenon of interference of light demonstrates that light has:

A. Wave nature

B. Particle nature

C. Neither

D. Both particle and wave nature

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Correct Answer: Option A

Explanation:
Interference (constructive/destructive superposition) is a characteristic wave phenomenon, requiring coherent sources and path difference. Young's double-slit experiment demonstrated light interference, confirming its wave nature. Particle nature is shown by photoelectric effect. Memory aid: 'Interference/diffraction ⇒ wave nature; photoelectric effect ⇒ particle nature'. This conceptual question tests wave-particle duality fundamentals, frequently examined in competitive exams. Always link specific phenomena to the aspect of light they demonstrate; competitive exams often combine both aspects in advanced questions.

This question belongs to: Science Physics

Question #649

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The root mean square value of alternating current is defined as the equivalent DC current that produces the same:

A. Power dissipation

B. Charge flow

C. Magnetic field

D. Voltage

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Correct Answer: Option A

Explanation:
RMS value of AC is defined such that I_rms²R = average power dissipated, same as DC current I_dc would produce (I_dc²R). Thus I_rms = I₀/√2 for sinusoidal AC. This definition ensures power calculations use RMS values. Memory tip: 'RMS: heating effect equivalent to DC; I_rms = 0.707 I₀ for sine wave'. This AC fundamentals concept is frequently tested in competitive exams. Always use RMS values for power calculations in AC circuits; peak values overestimate power.

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Question #650

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The time period of revolution of a satellite close to Earth's surface is approximately:

A. 90 minutes

B. 12 hours

C. 24 hours

D. 84 minutes

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Correct Answer: Option D

Explanation:
For low Earth orbit (r ≈ R_earth), orbital period T = 2π√(r³/GM). Using g = GM/R², T = 2π√(R/g). With R = 6.4×10⁶ m, g = 9.8 m/s², T ≈ 2×3.14×√(6.4e6/9.8) ≈ 6.28×√(653061) ≈ 6.28×808 ≈ 5076 s ≈ 84.6 minutes. Memory aid: 'Low orbit period ≈ 84 min; geostationary = 24 hours'. This standard result is frequently tested in competitive exams. Always recall that period increases with orbital radius; competitive exams often compare low orbit vs geostationary periods.

This question belongs to: Science Physics

Question #651

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The specific heat capacity of water is high. This property is useful for:

A. Neither

B. Cooling systems in engines

C. Thermal regulation in living organisms

D. Both (a) and (b)

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Correct Answer: Option D

Explanation:
Water's high specific heat (4186 J/kg·K) means it absorbs/releases large heat with small temperature change. This stabilizes temperatures: in engines, water coolant absorbs heat without large ΔT; in organisms, it buffers against temperature fluctuations. Memory tip: 'High specific heat = thermal buffer; water moderates climate and body temperature'. This application question tests understanding of material properties, frequently appearing in competitive exams. Always link physical properties to real-world uses; competitive exams emphasize practical implications of physics concepts.

This question belongs to: Science Physics

Question #652

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The phenomenon of photoelectric emission occurs only when incident light has:

A. Long wavelength

B. Any frequency if intensity is high

C. Frequency above threshold

D. High intensity

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Correct Answer: Option C

Explanation:
Photoelectric effect requires photon energy hν ≥ work function φ of metal. Thus frequency ν must exceed threshold ν₀ = φ/h, regardless of intensity. Intensity affects number of electrons, not emission possibility. Memory aid: 'Photoelectric: frequency threshold (quantum effect); intensity affects current, not emission'. This quantum physics concept is frequently tested in competitive exams. Always distinguish classical wave prediction (intensity-dependent) from quantum reality (frequency-dependent); this was key evidence for photons.

This question belongs to: Science Physics

Question #653

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The instrument used to measure the wavelength of light is:

A. Telescope

B. Spectrometer

C. Microscope

D. Interferometer

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Correct Answer: Option B

Explanation:
Spectrometer measures wavelengths of light using diffraction gratings or prisms to disperse light and angular measurements. Interferometer (D) can also measure wavelength but spectrometer is the standard instrument for spectral analysis. Microscope and telescope magnify images, not measure wavelength. Memory tip: 'Spectrometer = wavelength measurement via dispersion; used in spectroscopy'. This instrument-application question tests practical physics knowledge, frequently examined in competitive exams. Always associate instruments with their primary function; competitive exams often test context-specific uses.

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Question #654

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The work done in charging a capacitor to charge Q is stored as:

A. Heat energy

B. Magnetic energy

C. Chemical energy

D. Electrostatic potential energy

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Correct Answer: Option D

Explanation:
Work done to charge a capacitor is stored as electrostatic potential energy in the electric field between plates: U = ½QV = ½CV² = Q²/(2C). This energy can be recovered when capacitor discharges. Heat (A) is dissipated in resistance during charging, but ideal capacitor stores energy electrostatically. Memory aid: 'Capacitor energy = ½CV², stored in electric field'. This electrostatics concept is frequently tested in competitive exams. Always distinguish ideal capacitor (no resistance) from real circuits where some energy is lost as heat during charging.

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Question #655

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The phenomenon of beats is used in:

A. Neither

B. Measuring speed of sound

C. Both (a) and (b)

D. Tuning musical instruments

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Correct Answer: Option C

Explanation:
Beats (periodic amplitude variation from interference of close frequencies) are used to tune instruments: adjust until beats disappear (frequencies match). Also, in Kundt's tube or other methods, beat frequency helps measure sound speed by comparing known and unknown frequencies. Memory tip: 'Beats: tuning (zero beat = matched frequency); speed measurement (beat frequency = |f₁-f₂|)'. This application question tests wave phenomena uses, frequently appearing in competitive exams. Always link physical phenomena to practical applications; competitive exams emphasize real-world relevance of physics concepts.

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Question #656

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The dimensional formula of gravitational constant G is:

A. [M⁻¹L²T⁻²]

B. [M⁰L⁰T⁰]

C. [ML³T⁻²]

D. [M⁻¹L³T⁻²]

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Correct Answer: Option D

Explanation:
From Newton's law: F = Gm₁m₂/r² ⇒ G = Fr²/(m₁m₂). Force F has [MLT⁻²], r² has [L²], m₁m₂ has [M²]. Thus G = [MLT⁻²][L²]/[M²] = [M⁻¹L³T⁻²]. Memory aid: 'G dimensions: [M⁻¹L³T⁻²]; small value (6.67×10⁻¹¹) reflects weak gravity'. This dimensional analysis question tests ability to derive constants, frequently appearing in competitive exams. Always verify by substituting into fundamental equations; dimensional consistency is a powerful tool for error checking in physics problems.

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Question #657

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A body moves with uniform acceleration. The graph between velocity and time is:

A. Straight line with positive slope

B. Parabola

C. Straight line parallel to time axis

D. Hyperbola

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Correct Answer: Option A

Explanation:
Uniform acceleration means constant rate of change of velocity: a = dv/dt = constant. Thus v = u + at, a linear equation in t. Graph is straight line with slope = acceleration. Parallel to time axis (A) implies zero acceleration (constant velocity). Parabola (C) is for position-time under constant acceleration. Memory tip: 'v-t graph slope = acceleration; area = displacement'. This graph interpretation question tests kinematics fundamentals, frequently examined in competitive exams. Always link graph features to physical quantities: slope, area, intercepts have specific meanings in motion graphs.

This question belongs to: Science Physics

Question #658

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The phenomenon of total internal reflection is used in:

A. Optical fibers

B. Both (a) and (b)

C. Diamond cutting

D. Neither

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Correct Answer: Option B

Explanation:
Total internal reflection (TIR) is exploited in optical fibers for light transmission with minimal loss (core-cladding interface). In diamonds, high refractive index (n≈2.4) gives small critical angle (~24°), so most light undergoes TIR, enhancing brilliance through multiple reflections. Memory aid: 'TIR applications: optical fibers (communication), diamonds (sparkle), prisms (binoculars)'. This application question tests optics uses, frequently appearing in competitive exams. Always link physical principles to technological applications; competitive exams emphasize practical relevance of theoretical concepts.

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Question #659

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The efficiency of a Carnot engine depends on:

A. Size of the engine

B. Temperatures of source and sink only

C. Both (a) and (b)

D. Working substance

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Correct Answer: Option B

Explanation:
Carnot efficiency η = 1 - T₂/T₁, depending solely on absolute temperatures of heat source (T₁) and sink (T₂). It is independent of working substance, engine size, or other details. This universality makes Carnot cycle the theoretical maximum efficiency benchmark. Memory tip: 'Carnot efficiency: only T_hot and T_cold matter; real engines have lower efficiency'. This thermodynamics concept is frequently tested in competitive exams. Always recall that Carnot efficiency is an ideal limit; real engines have additional losses reducing efficiency below this value.

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Question #660

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The time period of oscillation of a spring-mass system in a satellite orbiting Earth is:

A. Infinite

B. Depends on orbital radius

C. Zero

D. Same as on Earth

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Correct Answer: Option D

Explanation:
Spring-mass period T = 2π√(m/k), depending only on mass and spring constant, not on gravity. In orbit, apparent weightlessness doesn't affect spring force (which depends on displacement, not gravity). Thus period remains unchanged. Pendulum period would change (depends on g), but spring-mass does not. Memory aid: 'Spring period: independent of g; pendulum period: depends on g'. This conceptual question tests oscillations fundamentals, frequently examined in competitive exams. Always distinguish gravity-dependent (pendulum) from gravity-independent (spring-mass) oscillators.

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