Physics 2 Marks Questions

→Micro waves (short-wavelength radio waves), with frequencies in the giga hertz (GHz) range, are produced by special vacuum tubes (called klystrons, magnetrons and Gunn diodes).
→They are used in radar systems used in aircraft navigation, due to their short wave lengths.
→Radar also provides the basis for the speed guns used to time fast balls, tennis serves and automobiles.
→Microwave ovens are an interesting domestic application of these waves. In such ovens, the frequency of the microwaves is selected to match the resonant frequency of water molecules so that energy from the waves is transferred efficiently to the kinetic energy of the molecules.
→This raises the temperature of any food containing water.

→Radio waves are produced by the accelerated motion of charges in conducting wires.
→They are used in radio and television communication systems.
→Frequency range: from 500 kHz to about 1000 MHz.
→Range of frequencies for different bands:
(i) AM (Amplitude Modulated) band: from 530 kHz to 1710 kHz
(ii) SW band: Up to 54 MHz
(iii) TV waves band: 54 MHz to 890 MHz
(iv) FM (Frequency Modulated) radio band: 88 MHz to 108 MHz
(v) UHF (Ultra High Frequency band: 300 MHz to 3000 MHz.

→At the time when Maxwell predicted the existence of EM waves, the only familiar EM waves were the visible light waves.
→The existence of ultraviolet waves (u.v.) and infrared (I.R.) waves was hardly established. By the end of the nineteenth century, x-rays and gamma rays had also been discovered.
→We now know that, EM waves include visible light waves, x-rays, gamma rays, radio waves, micro waves, ultraviolet and infrared waves.
→The classification of EM waves according to frequency is called Electro magnetic spectrum, which is shown in the figure.

→After Hertz's experiment, other scientists were also successful in generating electromagnetic waves of different frequencies.
→Seven years after Hertz, Jagdish Chandra Bose, working at Calcutta succeeded in producing and observing electro magnetic waves of shorter wave length (5 mm to 25 mm).
→At around the same time, Marconi in Italy succeeded in transmitting electromagnetic waves over distances of many kilometres.
→Marconi's experiment was the beginning of the field of communication using EM waves.

→Neither stationary charges nor charges in uniform motion (steady currents) can create electromagnetic waves.
→Because the static electric charges produce only electro static fields, while charges in uniform motion produce magnetic fields.
→As per Maxwell's theory, accelerated charges radiate electromagnetic waves.
→An oscillating charge, which is an example of an accelerating charge, produces an oscillating electric field in space, which produces an oscillating magnetic field, which in turn, is a source of oscillating electric field and so on.
→The oscillating electric and magnetic fields thus regenerate each other, as the wave propogates through the space.
→The frequency of the EM wave is equal to the frequency of oscillation of the charge.
→An electro magnetic wave propogates in space with speed of light. In an electro magnetic wave, the Electric field, Magnetic field and direction of propogation all three are mutually perpendicular.

→The displacement current has some far reaching consequences.
→One key thing that can be noticed is that the laws of electricity and magnetism now seem (and can be represented) more symmetrical, though they are not perfectly symmetrical.
→Because the source of electric field are electric charges, which exist independently. Electric Mono poles (either positive or negative) exist independently. Unlike electric monopoles, magnetic mono poles do not exist. The source of Magnetic field are magnetic poles (North-N or South-S), but independent North pole or South pole doesn't exist.
→Faraday's law of induction states that there is an induced emf equal to the rate of change of magnetic flux. Now, since the emf between two points 1 and 2 is the work done per unit charge in taking it from 1 to 2, the existence of emf implies the existence of electric field.
→So, we can rephrase Faraday's law of electromagnetic induction by saying that, 'A magnetic field, changing with time, gives rise to an electric field. Then the fact that an electric field changing with time gives rise to a magnetic field, is the symmetrical counter part, and is a consequence of the displacement current being a source of magnetic field. Thus, time-dependent electric and magnetic fields give rise to each other.
→Faraday's law of electromagnetic induction and Ampere-Maxwell law give a quantitative expression of this statement.
→One very important consequence of this symmetry is the existence of electro magnetic waves.

→The displacement current has the same physical effect(s) as the conduction current.
→In some cases, for example steady electric fields in a conducting wire, the displacement current will be zero, since the electric field E does not change with time.
→In other cases, for example the charging capacitor above, both conduction and displacement currents may be present in different regions of space.
→In most of the cases, they both may be present in the same region of space, as there exists no perfectly conducting or perfectly insulating medium.
→Most interesting, there may be large regions of space where there is no conduction current, but there is only a displacement current due to time-varying electric fields. In such a region,we expect a magnetic field, through there is no (conduction) current source nearby.
→For example, a magnetic field at a point between the plates of a capacitor is seen to be the same as field at a point just outside the capacitor plates.