Electromagnetic Waves Class 12 Notes Physics Chapter 8 - CBSE

Chapter : 8

What are Electromagnetic Waves ?

Topic Formula Symbol Representation Important Points
Displacement Current $$\text{I}_\text{D}= \epsilon_\text{0}\frac{\text{d}\phi_\text{E}}{\text{dt}}\\ \space\space\space\space\space\space\text{or} \\ \frac{\text{dq}}{\text{dt}}=\epsilon_\text{0}\frac{\text{d}\phi}{\text{dt}}$$ ID = Maxwell’s displacement current
Φ = Electric flux
(dΦ/dt) = Rate of change of electric flux
  • When electric field or electric flux change with time, ID comes in the picture.

  • ID does not exist in steady condition in a region.

  • ID does not flow through the conducting wire.

  • ID produce magnetic field in a region.
Electromagnetic waves $$\text{c}=\frac{1}{\sqrt{\mu_0\epsilon_0}}\\=3×10^8\text{ms}^{-1}\\ \oint\vec{\text{B}}.\vec{\text{dl}}=\mu_0(\text{I}+\text{I}_\text{D})\\=\mu_0(\text{I}+\frac{\epsilon_0\text{d}\phi_\epsilon}{\text{dt}})\\ (\text{Ampere-Maxwell law}) \\ \oint\vec{\text{E}}\vec{\text{ds}}=\frac{\text{q}}{\epsilon_0}\\ \text{Gauss law in electrostate}$$
$$\oint\vec{\text{B}}.\vec{\text{ds}}=0\\ \text{(Gauss law in magnetostate)} \\ \oint\vec{\text{E}}.\vec{\text{dl}}=\frac{\text{-df}}{\text{dt}}\int_s \vec{\text{B}}\vec{\text{ds}} \\ \text{(Faraday’s law of EM induction)}$$
c = speed of light
μ0 = permeability constant = 4π × 10-7H/m
ε0 = Vacuum permittivity = 8.85 × 10-12C2/N.m2
Symbol Representation
= magnetic field
∮ = integral over closed path
I = conduction current
ID = displacement current
$$\bull\space \text{Electric field} \vec{(\text{E})} \text{and magnetic field} \\\vec{(\text{B})} \text{are at right angle to each other}\\ \space \text{as well as right angle to the}\\ \text{direction of wave propagation}. $$
Example: radio waves, microwaves, infrared waves etc.
  • Speed of EM, in vacuum, is equal to speed of light.

  • Oscillating or accelerated charge produced EM.

  • EM waves are transverse in nature (Maxwell).

  • EM wave do not require any medium for propagation.

  • $$\bull\space \vec{\text{E}} \space\text{and} \space\vec{\text{B}} \space\text{are in same phase (in EM)}. $$
  • Velocity of EM wave entirely depends on electric and magnetic field of medium.

  • Energy carried by EM wave equally divide between

  • $$\space \vec{\text{E}} \space\text{and} \space\vec{\text{B}}$$
  • EM waves are not deflected by electric and magnetic field.
  • EM spectrum
  • Orderly distribution of EM radiations according to their wavelength or frequency.