5 Electromagnetic Waves

ELECTROMAGNETIC WAVES 1. El Elec ectr trom omag agne neti tic c Wav Waves es 2. Pro Proper pertie ties s of Ele Electr ctroma omagnet gnetic ic Wav Waves es 3. Her ertz tz Exp xpe eri rime ment nt 4. El Elec ectr trom omag agne neti tic c Sp Spec ectr trum um - Wave Wavelength length and and Frequenc Frequency y Range Range - Sou Source rces s and and Use Uses s

Trusted by over 1 million members

Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.





Electromagnetic Waves: For a region where w here there are no charges and conduction current, Faraday’s and Ampere’s laws take the symmetrical form: E . dl = -

dΦB dt

l

B . dl = - µ0ε0

and l

dΦE dt

It can also be shown that that time – varying electric electric field produces produces space – varying magnetic field and time – varying magnetic magnetic field produces space – varying electric field with the equations: jEy jx

=-

jBz jt

and

jBz jx

= - µ0ε0

Electric and magnetic fields are sources to each other.

jEy jt





Properties of Electromagnetic Waves: Y E0 0

X

B0

Z 1. Variations Variations in both both electric electric and magneti magnetic c fields fields occur simult simultaneous aneously. ly. Therefore, they attain their maxima maxima and minima at the same same place and at the same time. 2. The direction direction of electric electric and magneti magnetic c fields fields are are mutually mutually perpe perpendicul ndicular ar to each other and as well as to the direction of propagation p ropagation of wave.





4. The velo velocit city y of elect electrom romagn agneti etic c waves waves in free free spa space, ce, c = 1 / √µ0ε0 5. The vel velocity ocity of elec electroma tromagnetic gnetic waves in a mater material ial medium = 1 / √µε where µ and ε are absolute absolute permeabil permeability ity and absolute absolute permitivi permitivity ty of the material medium. 6. Electromagnetic waves obey the principle of superposition. 7. Electromagnetic waves carry energy as they propagate through space. This energy is divided equally between electric and magnetic fields. 8. Electromagnetic waves can transfer transfer energy as well as momentum to objects placed on their paths. 9. For discussi discussion on of optica opticall effects effects of EM wave, more signifi significanc cance e is give given n to Electric Field, E. Therefore, electric electric field is called called ‘light ‘light vector’. vector’. 10. Elect Electromag romagnetic netic waves waves do not require materia materiall medium to travel. travel.





Hertz Experiment: The copper or zinc plates are kept parallel separated by 60 cm. The metal spheres are slided over the metal rods to have a gap of 2 to 3 cm. Induction coil supplies high voltage of several thousand volts. The plates and the rods (with spheres) constitute an LC combination.

Copper or Zinc Plate Metal Rod P P

S S

Induction Coil Metal Spheres

S1 S2

S1’ EM Wave

Metal Rod

S2’ Ring

Copper or Zinc Plate





Due to high voltage, the air in the th e small gap between the spheres gets ionised. This provides provides the path for the discharge of the plates. A spark begins to pass between the spheres. A very high frequency oscillations oscillations of charges occur on the plates. This results in high frequency frequency oscillatin oscillating g electric electric field in the vertical vertical gap S1S2. Consequently, an oscillating magnetic field of the same frequency is set up in the horizontal plane and perpendicular to the gap between the spheres. These oscillating electric and magnetic fields f ields constitute electromagnetic waves. The electromagnetic waves produced are radiated from the spark gap. The detector is held in a position such that the magnetic field produced by the oscillating current is perpendicular to the plane of the coil. The resultant electric field induced by the oscillating magnetic field causes the ionisation of air in the gap between the the spheres. So, a conducting conducting path becomes available for the induced current to flow across the gap. This causes sparks sparks to appear at at the narrow gap.







Electromagnetic Spectrum: S. EM Wave No.

Range of λ

Range of ν

Source

Use

1

Radio Wave

A few km to 0.3 m

A few Hz to 109 Hz

Oscillating electronic circuits

Radio and TV broadcasting

2

Microwave

0.3 m to 10-3 m

109 Hz to 3 x 1011 Hz

Oscillating electronic circuits

Radar, analysis of fine details of atomic and molecular





S. No.

EM Wave

Range of λ

Range of ν

Source

Use

5

Ultra Violet Rays

3.8 x 10-7 m to 6 x 10-10 m

8 x 1014 Hz to 3 x 1017 Hz

Atoms and molecules in electrical discharges and Sun

Medical application, sterilization, killing bacteria and germs in food stuff, detection of invisible writing, forged documents, finger print, etc.

5 Electromagnetic Waves

Recommend Documents