Electromagnetic Waves

Class 12

12 previous-year questions from this chapter every option and the correct answer, free · where the marks are

1. Displacement current and Maxwell's equations (qualitative)

Exam focus: Displacement current isn't an actual flow of charge — it's the term Maxwell added so Ampère's law gives a consistent answer for any surface, and the textbook example is a charging capacitor's gap, where conduction current is zero but displacement current isn't.

Displacement Current and Maxwell's Equations

Physics Wallah - Alakh Pandey

Opening lecture of the EM Waves series, directly matching the subtopic.

PhysicsNEET 2025show ▾

A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to the displacement current is:

A.Zero at all places
B.Constant between the plates and zero outside the plates
C.Non-zero everywhere, with a maximum at the imaginary cylindrical surface connecting the peripheries of the plates✓
D.Zero between the plates and non-zero outside

Solution

The growing electric field between the plates is a displacement current, and by Ampère–Maxwell it produces a magnetic field that grows with distance from the axis up to the plate edge and then falls as $1/ r$ outside — non-zero everywhere, maximum at the rim.

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PhysicsNEET 2024show ▾

A parallel plate capacitor is charged by connecting it to a battery through a resistor. If

I

is the current in the circuit, then in the gap between the plates:

A.Displacement current of magnitude equal to $I$ flows in the same direction as $I$ ✓
B.Displacement current of magnitude equal to $I$ flows in a direction opposite to that of $I$
C.Displacement current of magnitude greater than $I$ flows but can be in any direction
D.There is no current

Solution

Between the plates the changing electric field constitutes a displacement current $I_{d} = ε_{0} \frac{d Φ _{E}}{d t}$ , equal in magnitude to the conduction current and in the same direction, so the current is continuous around the circuit.

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PhysicsNEET 2022show ▾

When light propagates through a material medium of relative permittivity

ε_{r}

and relative permeability

μ_{r}

, the velocity of light

v

is given by (

c

= velocity of light in vacuum):

A. $v = \frac{c}{ε _{r} μ _{r}}$ ✓
B. $v = c$
C. $v = \frac{μ _{r}}{ε _{r}}$
D. $v = \frac{ε _{r}}{μ _{r}}$

The worked solution is part of a paid pack.

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PhysicsNEET 2021show ▾

A capacitor of capacitance

C

is connected across an ac source of voltage

V

, given by

V = V_{0} sin ω t

. The displacement current between the plates of the capacitor would then be given by:

A. $I_{d} = V_{0} ω C cos ω t$ ✓
B. $I_{d} = \frac{V _{0}}{ω C} cos ω t$
C. $I_{d} = \frac{V _{0}}{ω C} sin ω t$
D. $I_{d} = V_{0} ω C sin ω t$

The worked solution is part of a paid pack.

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PhysicsNEET 2021show ▾

For a plane electromagnetic wave propagating in the

x

-direction, which one of the following combinations gives the correct possible directions for the electric field (

E

) and magnetic field (

B

) respectively?

A. $\hat{j} + \hat{k}, \hat{j} + \hat{k}$
B. $- \hat{j} + \hat{k}, - \hat{j} - \hat{k}$ ✓
C. $\hat{j} + \hat{k}, - \hat{j} - \hat{k}$
D. $- \hat{j} + \hat{k}, - \hat{j} + \hat{k}$

The worked solution is part of a paid pack.

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PhysicsNEET 2019show ▾

A parallel plate capacitor of capacitance

20 μ F

is being charged by a voltage source whose potential is changing at the rate of 3 V/s. The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively:

A.Zero, $60 μ A$
B. $60 μ A, 60 μ A$ ✓
C. $60 μ A$ , zero
D.Zero, zero

The worked solution is part of a paid pack.

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2. Electromagnetic spectrum

Exam focus: Every part of the electromagnetic spectrum, from gamma rays to radio waves, travels at exactly the same speed c in vacuum — only wavelength and frequency change across it, contrary to a common trap implying higher-energy radiation 'travels faster.'

Electromagnetic Spectrum

JEE Wallah

Full-chapter EM Waves one-shot; the spectrum is covered as the closing section alongside displacement current, Maxwell's equations and wave properties. A PW-family channel used because the main channel's dedicated spectrum episode could only be found as a third-party re-upload.

PhysicsNEET 2025show ▾

The electric field in a plane electromagnetic wave is given by

E_{z} = 60 cos (5 x + 1.5 \times 1 0^{9} t)

V/m. Then the expression for the corresponding magnetic field is (here the subscripts denote the direction of the field):

A. $B_{y} = 2 \times 1 0^{- 7} cos (5 x + 1.5 \times 1 0^{9} t)$ T✓
B. $B_{x} = 2 \times 1 0^{- 7} cos (5 x + 1.5 \times 1 0^{9} t)$ T
C. $B_{z} = 60 cos (5 x + 1.5 \times 1 0^{9} t)$ T
D. $B_{y} = 60 sin (5 x + 1.5 \times 1 0^{9} t)$ T

Solution

$B_{0} = \frac{E _{0}}{c} = \frac{60}{3 \times 1 0 ^{8}} = 2 \times 1 0^{- 7}$ T. The wave travels along $- x$ (phase $5 x + ω t$ ); with $E$ along $z$ , $E \times B$ must point along $- x$ , which requires $B$ along $+ y$ . So $B_{y} = 2 \times 1 0^{- 7} cos (5 x + 1.5 \times 1 0^{9} t)$ T.

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PhysicsNEET 2024show ▾

The property which is not of an electromagnetic wave travelling in free space is that:

A.The energy density in the electric field is equal to the energy density in the magnetic field
B.They travel with a speed equal to $\frac{1}{μ _{0} ε _{0}}$
C.They originate from charges moving with uniform speed✓
D.They are transverse in nature

Solution

Electromagnetic waves are produced by accelerating charges; a charge moving with uniform velocity does not radiate. The other three statements are standard properties of EM waves in free space.

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PhysicsNEET 2023show ▾

In a plane electromagnetic wave travelling in free space, the electric field component oscillates sinusoidally at a frequency of