Table of Contents
Electromagnetic Waves Notes Class 12 Physics Chapter 8
→ Displacement current is a 1 ways equal to charging (for discharging) current and lasts so long as the capacitor (producing varying electric field) is charged or discharged.
→ An accelerated charged particle emits e.m. waves.
→ S→ = E→ × B→ is called Poynting vector acts in a direction perpendicular to the plane of E→ and B→ .
→ The displacement current is named so because it is produced by the displacement of electrons caused by changing electric fields.
→ X-rays have the shortest wavelength (≈ 1 Å).
→ The charging or discharging current is called conduction current.
→ The amplitude of electric and magnetic fields in free space in e.m. waves are related as E = CB
→ Electric vector is called light vector as it is responsible for the optical effect of e.m. wave.
→ The energy of the e.m. wave is shared equally between the electric field vector and the magnetic field vector.
→ Microwaves are very commonly used in radar to locate flying objects like airplanes, jet planes, etc.
→ Tire earth’s atmosphere produces Green House effect. In the absence of the earth’s atmosphere, the temperature on earth during the day will increase and during the night it would decrease.
→ The ozone layer which is present in the stratosphere protects the earth from high-energy radiations coming from outer space.
→ The velocity of em. waves in a medium is given by
v = 1μ0ε0μrεr√=Cμrεr√
→ There is no conduction current in a traveling e.m. wave.
→ Earth’s atmosphere is transparent to visible light and most of the infrared rays are absorbed by the atmosphere.
→ Radio waves were discovered by Hertz and are used in communication.
→ e.m. waves are transverse in nature.
→ e.m. waves exert pressure on the objects on which they fall as they carry energy and momentum.
→ The wavelength range of em. waves are from 10-15 m to 109 m and the frequency range is 1024 Hz to 1 Hz.
→ Green House Effect takes place due to the heating of the earth’s atmosphere due to the trapping of infrared rays by the CO2 layer in the atmosphere.
→ Modified Ampere Circuital law: It states that the line integral of the magnetic field around a closed path is always equal to μ0 times the sum of the conduction dnd displacement currents i.e.,
→ Displacement Current: It is defined as the current produced in a region where a change of electric flux takes place due to the change in electric field intensity in that region.
Important Formulae
→ Amper’s circuital law states that
∫ B→.dl→ = μ0 IC
where IC = conduction current Displacement current is given by
→ Displacement current is given by
ID = ε0 dϕEdt
→ C = E0B0=1μ0ε0√
→ Energy density of electric field, UE = 12 ε0 E2
→ Energy density of electric field, UB = B22μ20
→ Intensity of e.m. wave is given by
I = average energy density × speed of e.m. wave
= 12 ε0E2 × C = ρ/4πr²
→ B→ at a point between the plates of the capacitor at a distance r from its axis is given by.
B = μ0Ir2πR2
Where R = radius of each circular plate of the capacitor.
→ Velocity of e.m. waves is
C = vλ
→ An electromagnetic wave of frequency v, wavelength λ propagating along the z-axis, we have
→ The speed of light or of electromagnetic waves in a material medium is given by
υ = 1με√
where μ is the permeability of the medium and ε is its permittivity.
→ Bmax = μ0ID2πR
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