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48. (a)
The variations in electric and magnetic fields in an electromagnetic wave are perpendicular to each other and to the direction of propagation of the wave. As said earlier electromagnetic waves do not require any material medium and they can propagate in free space (i.e. in vaccum) with the velocity of light. Ibid., p. 579, 582
"Electromagnetic Induction
As said earlier, in 1820, Oersted discovered the magnetic effect of electric current i.e. when a steady current flows through a conductor, a magnetic field is produced around it. In 1831, Michel Faraday discovered the effect, called electromagnetic induction, just converse to the magnetic effect of electric current. When a coil made of copper wire is placed inside a magnetic field, magnetic flux is linked with the coil. Faraday found that when the magnetic flux linked with the coil is changed, an electric current starts flowing in the coil, provided the coil is a closed one. The charge in magnetic flux linked with a coil may be caused by varying the strength of magnetic field or by the relative motion between the source of magnetic field and the coil. In case the coil is open, an e.m.f. is set up across the two ends of the coil. The current and the e.m.f. so produced, are called induced current and induced e.m.f. respectively. The induced current and the e.m.f. in the coil last only so long as the magnetic flux linked with the coil keeps on changing. Thus, electromagnetic induction is the phenomenon of production of electric current (or e.m.f.) in a coil when the magnetic flux linked with the coil is changed.
Magnetic Flux
The magnetic flux linked with surface held in a magnetic field is defined as the number of magnetic lines of force crossing the surface normally.
Faraday's Laws of Electromagnetic Induction
The results of Faraday's experiments on electromagnetic induction are known as Faraday's electromagnetic induction. These laws are stated as below:
1.
Whenever magnetic flux linked with a circuit (a loop of wire or a coil or an electric circuit) changes, induced e.m.f. is produced.
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