Chapter 8: Electromagnetic Waves Short Questions | physics Class 12 CBSE

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Chapter 8: Electromagnetic Waves Short Questions | physics Class 12 CBSE | ByteNotes.in

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Question 1

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What inconsistency did Maxwell notice in Ampere's circuital law, and what did he propose to remove it?

When Ampere's law was applied to a charging capacitor using different surfaces with the same boundary, it gave different values of magnetic field at the same external point — a clear contradiction.
Maxwell proposed an additional term called displacement current (iₐ = ε₀ dΦ_E/dt), arising from a time-varying electric flux, to be added to the total current in Ampere's law.
With this addition, the total current (conduction + displacement) gives the same consistent magnetic field regardless of which surface is chosen, removing the inconsistency.

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Question 2

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Define displacement current and write its mathematical expression.

Displacement current is the current equivalent due to a time-varying electric flux; it is not caused by the movement of charges but by a changing electric field.
Its mathematical expression is iₐ = ε₀(dΦ_E/dt), where ε₀ is the permittivity of free space and Φ_E is the electric flux through the chosen surface.
Displacement current has the same physical effect as conduction current — it acts as a source of magnetic field in exactly the same way.

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Question 3

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Write the Ampere-Maxwell law and explain the physical significance of each term.

The Ampere-Maxwell law is: ∮B·dl = µ₀iₓ + µ₀ε₀(dΦ_E/dt), where the line integral of B is taken over a closed loop.
The term µ₀iₓ represents the contribution to the magnetic field from the conduction current — the actual flow of charges through a conductor.
The term µ₀ε₀(dΦ_E/dt) represents the contribution from the displacement current — the magnetic field generated by a time-varying electric flux even in the absence of real charges.

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Question 4

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State the condition under which a source can produce electromagnetic waves.

Neither stationary charges nor charges in uniform motion (steady currents) can produce electromagnetic waves.
Only accelerated charges can radiate electromagnetic waves, as stated by Maxwell's theory.
An oscillating charge is a common example of an accelerating charge and serves as a basic source of electromagnetic waves, radiating waves of the same frequency as the oscillation.

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Question 5

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Describe the orientation of electric field, magnetic field, and direction of propagation in an electromagnetic wave.

In an electromagnetic wave, the electric field (E) and the magnetic field (B) are perpendicular to each other.
Both E and B are also perpendicular to the direction of propagation of the wave, making electromagnetic waves transverse in nature.
For example, for a wave propagating along the z-axis, E may lie along the x-axis and B along the y-axis, as shown by the equations Eₓ = E₀ sin(kz−ωt) and B_y = B₀ sin(kz−ωt).

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Question 6

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Write the relation between the amplitudes of the electric and magnetic fields in an electromagnetic wave in free space.

The amplitudes of the electric field (E₀) and magnetic field (B₀) in an electromagnetic wave are related by the equation B₀ = E₀/c.
Here, c = 3 × 10⁸ m/s is the speed of electromagnetic waves in vacuum.
This relation is derived directly from Maxwell's equations and holds for all electromagnetic waves propagating in free space.

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Question 7

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Express the speed of electromagnetic waves in vacuum in terms of fundamental constants and state its value.

The speed of electromagnetic waves in vacuum is given by c = 1/√(µ₀ε₀), where µ₀ is the permeability and ε₀ is the permittivity of free space.
Its numerical value is approximately 3 × 10⁸ m/s, which matches the speed of light obtained from optical measurements.
This equality led Maxwell to conclude that light is itself an electromagnetic wave, unifying optics with electromagnetism.

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Question 8

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How does the speed of electromagnetic waves change when they travel through a material medium?

In a material medium with permittivity ε and magnetic permeability µ, the speed of electromagnetic waves becomes v = 1/√(µε).
Since ε > ε₀ and µ > µ₀ for most materials, the speed v is less than the speed in vacuum (c = 1/√(µ₀ε₀)).
This reduction in speed in a medium is the basis of the concept of refractive index, where the refractive index equals the ratio of c to v.

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Question 9

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Why are electromagnetic waves said to be self-sustaining in free space?

A time-varying electric field produces a magnetic field (Ampere-Maxwell law), and a time-varying magnetic field in turn produces an electric field (Faraday's law).
These two fields continuously regenerate each other as the wave propagates through space, requiring no material medium or external energy source.
This self-sustaining oscillation of mutually perpendicular E and B fields propagating through vacuum is precisely what defines an electromagnetic wave.

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Question 10

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What is the frequency range of radio waves and what are their main uses?

Radio waves are generally in the frequency range from 500 kHz to about 1000 MHz and are produced by the accelerated motion of charges in conducting wires.
The AM band spans 530 kHz to 1710 kHz, while the FM band extends from 88 MHz to 108 MHz; TV waves range from 54 MHz to 890 MHz.
Radio waves are used in radio and television communication systems, and cellular phones use them (in the UHF band) for voice communication.

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Question 11

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Why are microwaves used in radar systems, and how do microwave ovens work?

Microwaves have short wavelengths (frequencies in the GHz range), making them suitable for radar systems used in aircraft navigation and speed measurement using speed guns.
In microwave ovens, the frequency is chosen to match the resonant frequency of water molecules, enabling efficient energy transfer from the microwaves to the kinetic energy of the molecules.
This transfer of energy raises the temperature of any food that contains water, making microwave cooking possible.

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Question 12

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Why are infrared waves called heat waves?

Infrared waves are absorbed by water molecules and many other molecules (such as CO₂ and NH₃) present in most materials.
After absorption, the thermal motion of these molecules increases, raising the internal energy and temperature of the substance.
Also, infrared waves vibrate not just electrons but entire atoms or molecules, increasing internal energy — which is why they are often called heat waves.

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Question 13

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State the wavelength and frequency range of visible light and explain why our eyes are sensitive to this range.

Visible light spans a wavelength range of approximately 400 nm to 700 nm, corresponding to a frequency range of about 4 × 10¹⁴ Hz to 7 × 10¹⁴ Hz.
The human eye is sensitive to this range because the centre of its sensitivity coincides with the centre of the wavelength distribution of sunlight.
Humans have evolved with vision most sensitive to the wavelengths that are most strongly emitted by the sun, making visible light the ecologically dominant part of the EM spectrum for vision.

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Chapter 8: Electromagnetic Waves | Short Questions

What inconsistency did Maxwell notice in Ampere's circuital law, and what did he propose to remove it?

Define displacement current and write its mathematical expression.

Write the Ampere-Maxwell law and explain the physical significance of each term.

State the condition under which a source can produce electromagnetic waves.

Describe the orientation of electric field, magnetic field, and direction of propagation in an electromagnetic wave.

Write the relation between the amplitudes of the electric and magnetic fields in an electromagnetic wave in free space.

Express the speed of electromagnetic waves in vacuum in terms of fundamental constants and state its value.

How does the speed of electromagnetic waves change when they travel through a material medium?

Why are electromagnetic waves said to be self-sustaining in free space?

What is the frequency range of radio waves and what are their main uses?

Why are microwaves used in radar systems, and how do microwave ovens work?

Why are infrared waves called heat waves?

State the wavelength and frequency range of visible light and explain why our eyes are sensitive to this range.

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