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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 8 ELECTROMAGNETIC WAVES

Displacement current - the missing term in Ampere’s circuital law

Maxwell’s generalisation of Ampere’s circuital law

Maxwell equations

Source of electromagnetic waves

Maxwell’s theory of electromagnetic radiation

Why can’t we prove that light is an electromagnetic wave?

Nature of electromagnetic waves

Properties of electromagnetic waves

Speed of electromagnetic wave

Poynting vector

Energy in electromagnetic waves

Radiation pressure

Electromagnetic spectrum

Radio waves

Microwaves

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 8 ELECTROMAGNETIC WAVES

Properties of electromagnetic waves

  1. Electromagnetic waves are produced by accelerated charged particles.

  2. These waves are transverse in nature.

  3. These waves propagate through space and do not need a medium

  4. The speed of electromagnetic wave in vacuum,

    c =1μoεo or c = EoBo

    Where Eo and Bo are maximum values of electric and magnetic field vectors.

    The velocity of light in a medium,

    v =1με 

    where, μ= relative permeability and ε = electrical permittivity of the medium.

    Thus, the velocity of light depends on electric and magnetic properties of the medium.

    The velocity of electromagnetic waves in free space or vacuum is an important fundamental constant and is same for all electromagnetic waves. Speed of light is vacuum is 3×108 m/s


  5. The rate of flow of energy in an electromagnetic wave is described by the vector S called the poynting vector, which is, defined by the expression,

     S =1μoE×B

    SI unit of S is watt/m2.

    Its magnitude S is defined as the rate at which energy is transported by a wave across a unit area at any instant.


  6. The electromagnetic waves carry energy from one place to another. The radio and TV signals from broadcasting stations carry energy. Light carries energy from the sun to the earth, thus making life possible on the earth.

  7. The energy in electromagnetic waves is divided equally between electric field and magnetic field vectors.

    The average electric energy density.

    UE = εoE2 2

    The average magnetic energy density,

    UB = B2 2μo

    The electric vector is responsible for the optical effects of an electromagnetic wave. Intensity of electromagnetic wave is defined as energy crossing per unit area per unit time perpendicular to the directions of propagation of electromagnetic wave.

  8. An electromagnetic wave (like other waves) carries both energy and momentum. Since it carries momentum, an electromagnetic wave also exerts pressure, called radiation pressure.

    If the total energy transferred to a surface in time t is U, the magnitude of the total momentum delivered to this surface (for complete absorption) is given by,

    p =Uc

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