CBSE Class 12 Physics Electromagnetic Wave Theory and Solved Examples

CBSE Class 12 Physics Electromagnetic Wave Theory and Solved Examples.Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

1. BASIC EQUATIONS OF ELECTRICITY AND MAGNETISM
The whole concept of electricity and magnetism can be explained by the four basic equations we have deal so far

2. FARADAYS LAW FOR TIME VARYING MAGNETIC FIELD
To understand the concept of faradays law we consider a circular conducting loop placed in a region where time dependent magnetic field is present

From the earlier concept we know that an induced emf will be produced in the conducting loop due to which current will flow in the loop. For current to flow a force must act on the electron which will move then from static state. This force cannot be due to magnetic field (since magnetic force does not act on stationary charge). Hence this force must be due to an electric field which has been generated due to changing Magnetic field.
Note :- This electric field is non conservative in nature.

Faraday stated this fact in his equation

3. CONCEPT OF DISPLACEMENT CURRENT (MODIFIED AMPERE’S LAW)
Maxwell tried to generalis the concept of faradays law that if changing magnetic field can produce changing electric field then the reverse should also be true i.e. changing electric field must produce magnetic fied. To understand the concept of displacement current let us try to understand this experiment when the switch was closed at t = 0 both the needles deflected

Deflection of needle (1) is under stood as M.F. is produced due to current flowing in the wire. But why did needle 2 deflect? It is lying in between the two plates of capacitor where there is no current. This magnetic field between the plates is due to the changing electric field between the plates (During charging of capacitor). Hence maxwell conducted that changing electric field produces a magnetic field For Needle (1) Amper’s law

Displacement current
Ex. 1 A parallel plate capacitor with plate area A and seperation between the plated d, is charged by a constant current i, consider a plane surface of area A/4 parallel to the plates and drawn symetrically between the plates what is the displacement current through this area.
(A) i (B) 2i (C) i/4 (D) i/2
Sol.(C) Electric field between the plates of the capacitor is given by

4. FINAL FORM OF MAXWELL’S EQUATION

The above equation is known as maxwell’s equation for time varying form. Howover for free space there are no charges and no conduction current the equations that are significant

5. TRANSEVERSE NATURE OF ELECTRO MAGNETIC WAVE AND ITS PROPERTIES
5.1 Electromagnetic waves :
The idea of electromagnetic waves was given by Maxwell and experimental verification was provided by Hertz and other scientists. A brief history of electromagnetic waves is as follows :
On the basis of experimental study of electromagnetic induction, Faraday concluded that a magnetic field changing with time at a point produces a time varying electric field at that point. Maxwell in 1864 pointed out an electric field changing with time at a point also produces a time varying magnetic field. The two fields are mutually perpendicular to each other. This idea led Maxwell to conclude that the mutually perpendicular time varying electric and magnetic fields produce electromagnetic disturbances in space. These disturbances have the properties of wave which are called as electromagnetic waves.
According to Maxwell, the electromagnetic waves are those waves in which there are sinusoidal variation of electric and magnetic field vectors at right angle to each other as well as right angles to the direction of wave propagation. An electromagnetic wave is shown in fig.

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