A parallel plate capacitor has plate area A, separation d, and dielectric constant K. Its capacitance is:

The correct answer is B: Kε₀A/d. The capacitance of a parallel plate capacitor with dielectric is C = Kε₀A/d, where K is the dielectric constant.

A straight wire carrying current I is placed in a uniform magnetic field B at an angle θ to the magnetic field. If the length of the wire is L, the magnetic force on the wire is:

The correct answer is B: BIL sinθ. The magnetic force on a current-carrying conductor is F = BIL sinθ, where θ is the angle between the current direction and the magnetic field. When θ = 90°, force is maximum (BIL), and when θ = 0°, force is zero.

A biconvex lens (n = 1.5) has both radii of curvature equal to 20 cm. What is its focal length?

The correct answer is B: 20 cm. Using lens maker's formula: 1/f = (n-1)[1/R₁ + 1/R₂] = (0.5)[1/20 + 1/20] = (0.5)(2/20) = 1/20. Therefore f = 20 cm.

A concave mirror has a focal length of 20 cm. An object is placed at the center of curvature. Where will the image form?

The correct answer is A: At the center of curvature. When object is at center of curvature (u = R = 2f), image forms at the same position (v = R). Magnification = -1

Channel length modulation in a MOSFET leads to:

The correct answer is B: Decreased output impedance. Channel length modulation occurs when the depletion region at the drain expands with increasing Vds, shortening the effective channel length. This causes output current to increase with voltage, reducing output impedance (ro decreases).

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JEE Physics
Electrostatics

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

13 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 11–13 of 13

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.11 Hard Electrostatics

The self-energy of a uniformly charged sphere of radius R and total charge Q is:

A U = 3kQ²/(5R)

B U = kQ²/(2R)

C U = kQ²/R

D U = 3kQ²/(2R)

Correct Answer: A. U = 3kQ²/(5R)

EXPLANATION

Self-energy of uniformly charged sphere: U = 3Q²/(20πε₀R) = 3kQ²/(5R)

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Q.12 Hard Electrostatics

A point charge q is placed at distance r from an infinite grounded conducting plane. The force on the charge is:

A F = q²/(16πε₀r²)

B F = q²/(8πε₀r²)

C F = q²/(4πε₀r²)

D F = q²/(32πε₀r²)

Correct Answer: A. F = q²/(16πε₀r²)

EXPLANATION

By method of images, image charge -q is at distance r behind plane. Total distance = 2r. F = kq²/(2r)² = q²/(16πε₀r²)

Test

Q.13 Hard Electrostatics

A charged soap bubble of radius R has surface charge density σ. The excess pressure inside the bubble due to electrostatic force is:

A p = σ²/(2ε₀)

B p = σ²/(ε₀)

C p = 2σ²/ε₀

D p = σ²/(4πε₀)

Correct Answer: A. p = σ²/(2ε₀)

EXPLANATION

Electrostatic pressure = ε₀E²/2 at surface. E = σ/ε₀ just outside. Excess pressure p = σ²/(2ε₀)

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JEE Physics Electrostatics

JEE Physics
Electrostatics