A charged particle with charge q and mass m moves in a magnetic field. The cyclotron frequency is:

The correct answer is A: f = qB/(2πm). Cyclotron frequency: f = qB/(2πm) = ω/(2π), where ω = qB/m is the angular frequency

In a single slit diffraction pattern, the first minimum occurs at an angle of 30°. If the slit width is doubled, at what angle will the first minimum occur?

The correct answer is A: 15°. For single slit diffraction, first minimum: a·sin(θ) = λ. If slit width is doubled, 2a·sin(θ') = λ, so sin(θ') = sin(θ)/2. Since sin(30°) = 0.5, sin(θ') = 0.25, therefore θ' ≈ 15°.

Consider a uniformly charged disc of radius R with total charge Q. What is the electric field at the center of the disc?

The correct answer is B: E = σ/(2ε₀), where σ = Q/(πR²). For a uniformly charged disc, the field at the center involves integrating contributions from rings. Result: E = σ/(2ε₀) = Q/(2πε₀R²).

Two resistors R₁ and R₂ are connected in series with a battery. If R₁ = 2R₂ and the voltage across R₁ is 8V, the voltage across R₂ is:

The correct answer is A: 4V. In series, current is same. V₁/V₂ = R₁/R₂ = 2R₂/R₂ = 2. Therefore V₂ = V₁/2 = 8/2 = 4V

A uniform rod of length L is suspended vertically from one end. Its center of mass falls from height L/2 to height 0 when released. The velocity of COM when it reaches bottom is:

The correct answer is A: √(gL/2). For rotating rod, energy: mg(L/2) = (1/2)Iω² + (1/2)mv². With I = mL²/3 about pivot, v_cm = √(gL/2)

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Modern Physics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

34 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 31–34 of 34

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.31 Easy Modern Physics

A radioactive nucleus has half-life of 10 days. After 30 days, the fraction of original nuclei remaining is:

A 1/2

B 1/4

C 1/8

D 1/16

Correct Answer: C. 1/8

EXPLANATION

Number of half-lives = 30/10 = 3. Remaining fraction = (1/2)³ = 1/8

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Q.32 Easy Modern Physics

The binding energy per nucleon is maximum for:

A Iron-56

B Uranium-238

C Deuterium

D Helium-4

Correct Answer: A. Iron-56

EXPLANATION

Iron-56 has the maximum binding energy per nucleon (~8.8 MeV/nucleon), making it the most stable nucleus. This is a key concept in nuclear physics.

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Q.33 Easy Modern Physics

The de Broglie wavelength of a particle is given by:

A λ = h/p

B λ = p/h

C λ = hp

D λ = h²/p

Correct Answer: A. λ = h/p

EXPLANATION

de Broglie proposed that λ = h/p, where h is Planck's constant and p is momentum. This is fundamental to wave-particle duality.

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Q.34 Easy Modern Physics

The rest mass energy of an electron is approximately:

A 0.511 MeV

B 1.022 MeV

C 5.11 MeV

D 9.11 MeV

Correct Answer: A. 0.511 MeV

EXPLANATION

Using E=mc², the rest mass energy of electron (m=9.11×10⁻³¹ kg) is 0.511 MeV, a fundamental constant in modern physics.

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