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Q.11 Hard Physics

Which phenomenon explains why stars twinkle but planets do not?

A Refraction of light in Earth's atmosphere

B Point sources vs extended sources in atmospheric turbulence

C Dispersion of light

D Diffraction by dust particles

Correct Answer: B. Point sources vs extended sources in atmospheric turbulence

Explanation:

Stars appear as point sources. Turbulent layers in the atmosphere refract starlight randomly, causing intensity variations (twinkling). Planets appear as extended sources, so refraction averaging effect minimizes visible twinkling.

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Q.12 Hard Physics

Which of the following pairs correctly represents the SI units of momentum and impulse respectively?

A kg·m/s and N·s

B N·s and kg·m/s

C Both kg·m/s and N·s (they are equivalent)

D kg·m/s² and N·s

Correct Answer: C. Both kg·m/s and N·s (they are equivalent)

Explanation:

Momentum = mass × velocity = kg·m/s. Impulse = force × time = N·s = (kg·m/s²)·s = kg·m/s. Both have equivalent units.

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Q.13 Hard Physics

A transformer has a primary coil with 500 turns and secondary coil with 100 turns. If the primary voltage is 220V, what is the secondary voltage?

A 22 V

B 44 V

C 110 V

D 220 V

Correct Answer: B. 44 V

Explanation:

Using transformer equation: V_s/V_p = N_s/N_p. Therefore V_s = V_p × (N_s/N_p) = 220 × (100/500) = 220 × 0.2 = 44 V.

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Q.14 Hard Physics

The first law of thermodynamics relates to which principle?

A Conservation of energy

B Conservation of momentum

C Conservation of angular momentum

D Conservation of charge

Correct Answer: A. Conservation of energy

Explanation:

The first law of thermodynamics (ΔU = Q - W) states that energy cannot be created or destroyed, only transferred. It is based on conservation of energy.

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Q.15 Hard Physics

A projectile is launched at an angle of 45° with initial velocity 20 m/s. What is the maximum height reached? (g = 10 m/s²)

A 5 m

B 10 m

C 15 m

D 20 m

Correct Answer: B. 10 m

Explanation:

Vertical component of velocity: u_y = 20 sin(45°) = 20/√2 = 10√2 m/s. Maximum height h = u_y²/(2g) = (10√2)²/(2×10) = 200/20 = 10 m.

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Q.16 Hard Physics

A charged particle moves in a magnetic field. Under what condition does it move in a straight line?

A Velocity is parallel to magnetic field

B Velocity is perpendicular to magnetic field

C Magnetic field is very strong

D Particle has positive charge

Correct Answer: A. Velocity is parallel to magnetic field

Explanation:

Lorentz force F = q(v × B). When v is parallel to B, the cross product is zero, so no force acts

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Q.17 Hard Physics

Which phenomenon causes the sky to appear blue during daytime?

A Reflection of light

B Refraction of light

C Rayleigh scattering

D Total internal reflection

Correct Answer: C. Rayleigh scattering

Explanation:

Blue light has shorter wavelength and is scattered more by atmospheric molecules (Rayleigh scattering)

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Q.18 Hard Physics

A wire of resistance R is stretched to double its length. What is the new resistance?

A R

B 2R

C 4R

D R/2

Correct Answer: C. 4R

Explanation:

Resistance R = ρL/A. When length doubles, L becomes 2L and area becomes A/2 (volume constant). New R = ρ(2L)/(A/2) = 4ρL/A = 4R

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Q.19 Hard Physics

Which quantum number determines the shape of an orbital?

A Principal quantum number (n)

B Azimuthal quantum number (l)

C Magnetic quantum number (m)

D Spin quantum number (s)

Correct Answer: B. Azimuthal quantum number (l)

Explanation:

The azimuthal quantum number (l) determines the shape of the orbital (s, p, d, f)

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Q.20 Hard Botany

Which structure in plant cells is analogous to the centrosome in animal cells?

A Cell wall

B Plasmodesmata

C Chloroplast

D Nucleus

Correct Answer: B. Plasmodesmata

Explanation:

While plant cells lack true centrosomes with centrioles, plasmodesmata serve similar intercellular communication functions. However, most plant cells lack a distinct MTOC (microtubule-organizing center).

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