A heat engine absorbs 1000 J of heat and rejects 600 J to the cold reservoir in one cycle. What is its efficiency?

The correct answer is A: 40%. Efficiency η = (Qh - Qc)/Qh = (1000 - 600)/1000 = 400/1000 = 0.40 = 40%

A radioactive nucleus ₆₀₂₇Co undergoes beta-plus decay. The daughter nucleus is:

The correct answer is A: ⁶⁰₂₆Ni. In beta-plus decay, Z decreases by 1, A remains constant. ⁶⁰₂₇Co → ⁶⁰₂₆Ni + e⁺ + νₑ.

Light of wavelength 500 nm in vacuum enters a medium of refractive index 1.5. The wavelength in the medium is:

The correct answer is A: 333.3 nm. Wavelength in medium = λ_vacuum / n = 500/1.5 = 333.3 nm. Frequency remains constant in different media.

A string of length 2 m vibrates in its third harmonic. The number of nodes (excluding ends) is:

The correct answer is B: 2. In the nth harmonic, number of nodes excluding ends = n-1. For third harmonic, nodes = 3-1 = 2

Two resistors of 4Ω and 6Ω are connected in series with a 10V battery. The current through the circuit and power dissipated are respectively:

The correct answer is C: 1A, 10W. Total R = 4 + 6 = 10Ω. I = V/R = 10/10 = 1A. P = V×I = 10×1 = 10W

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JEE Physics
Electrostatics

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

50 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–30 of 50

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.21 Medium Electrostatics

A point charge Q is placed at the center of a cubic Gaussian surface of side a. The electric flux through one face is:

A Q/(6ε₀)

B Q/(ε₀)

C 6Q/ε₀

D Q/(24ε₀)

Correct Answer: A. Q/(6ε₀)

EXPLANATION

Total flux = Q/ε₀. By symmetry, distributed equally over 6 faces: flux per face = Q/(6ε₀).

Test

Q.22 Medium Electrostatics

A conducting plane carries uniform surface charge density σ. The electric field just outside the surface is:

A σ/(2ε₀)

B σ/ε₀

C 2σ/ε₀

D σε₀

Correct Answer: B. σ/ε₀

EXPLANATION

Using Gauss's law for an infinite charged plane: E = σ/ε₀ (one side only, applies just outside conductor).

Test

Q.23 Medium Electrostatics

An electron enters a uniform electric field of strength E with initial velocity perpendicular to the field. Its trajectory is:

A Circular

B Parabolic

C Straight line

D Elliptical

Correct Answer: B. Parabolic

EXPLANATION

Motion is analogous to projectile motion: uniform motion perpendicular to field, accelerated motion along field direction → parabola.

Test

Q.24 Medium Electrostatics

A parallel plate capacitor is filled partially with dielectric (dielectric constant K, thickness d/2) and partially with air (thickness d/2). Its capacitance is:

A 2ε₀A(K+1)/(d(K+1))

B 2ε₀AK/(d(K+1))

C ε₀AK/d

D ε₀A(K+1)/(2d)

Correct Answer: B. 2ε₀AK/(d(K+1))

EXPLANATION

Capacitors in series: 1/C_total = d/(2ε₀AK) + d/(2ε₀A). Solving: C = 2ε₀AK/[d(K+1)].

Test

Q.25 Medium Electrostatics

The work done to move a charge q from point A (potential V_A) to point B (potential V_B) is:

A q(V_A - V_B)

B q(V_B - V_A)

C q·V_A·V_B

D q/(V_B - V_A)

Correct Answer: B. q(V_B - V_A)

EXPLANATION

Work done by external agent = q(V_B - V_A). If V_B > V_A, positive work is needed.

Test

Q.26 Medium Electrostatics

A non-conducting sphere of radius R is uniformly charged with charge density ρ. What is the electric field at distance r from center (r < R)?

A ρr/(3ε₀)

B ρR/(3ε₀)

C ρr²/(3ε₀)

D ρ/(3ε₀r)

Correct Answer: A. ρr/(3ε₀)

EXPLANATION

Using Gauss's law with spherical symmetry: E(4πr²) = (ρ × 4πr³/3)/ε₀, giving E = ρr/(3ε₀).

Test

Q.27 Medium Electrostatics

An electric dipole of moment p is placed in a uniform electric field E at angle θ to the field. The potential energy is:

A -pE cos θ

B pE sin θ

C pE cos θ

D pE/(cos θ)

Correct Answer: A. -pE cos θ

EXPLANATION

Potential energy of dipole in electric field: U = -p·E = -pE cos θ. Minimum at θ = 0 (aligned).

Test

Q.28 Medium Electrostatics

A uniformly charged ring of radius R carries total charge Q. What is the electric field at a point on the axis at distance x from the center?

A kQx/(x² + R²)^(3/2)

B kQ/(x² + R²)

C kQx/(x² + R²)

D kQ(x² + R²)^(3/2)/x

Correct Answer: A. kQx/(x² + R²)^(3/2)

EXPLANATION

Using Coulomb's law and symmetry, the radial components cancel. The axial component gives E = kQx/(x² + R²)^(3/2).

Test

Q.29 Medium Electrostatics

A point charge is brought from infinity to a point in an electric field. Work done depends on:

A Path taken and initial potential

B Only the final potential and charge

C Initial and final positions only

D Strength of the field at every point

Correct Answer: C. Initial and final positions only

EXPLANATION

Electric force is conservative; work depends only on initial and final positions (potentials), not on the path taken. W = q(V_initial - V_final).

Test

Q.30 Medium Electrostatics

Three capacitors of capacitance C each are connected in a combination where two are in parallel and this combination is in series with the third. The equivalent capacitance is:

A 3C

B 2C/3

C C

D 3C/2

Correct Answer: B. 2C/3

EXPLANATION

Two capacitors C in parallel: C_parallel = 2C. This in series with C: C_eq = (2C × C)/(2C + C) = 2C/3.

Test

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