An oscillating charge is an example of accelerating charge

Chapter 8: Electromagnetic waves

Oscillating Electric Field

An oscillating charge is an example of accelerating charge. It produces an oscillating electric field in space, which produces an oscillating magnetic field, which in turn produces an oscillating electric fields and so on. The oscillating electric and magnetic fields regenrate each other as a wave which propagetes through space.

(i) Electromagnetic waves can be deflected by (1)

(a) Only electric field

(b) Only magnetic field

(d) None of these

(ii) Total energy density of electromagnetic waves in vacuum is given by the relation (1)

(a) $\dfrac{1}{2}.\dfrac{E^2}{\epsilon_0}+\dfrac{B^2}{2 \mu_0}$

(b) $\dfrac{1}{2}.\epsilon_0.E^2+\dfrac{1}{2}. \mu_0.B^2$

(d) $\dfrac{1}{2}.\epsilon_0.E^2+\dfrac{B^2}{2 \mu_0}$

(iii) The speed of electromagnetic wave in vacuum depends upon the source of radiation (1)

(a) Increase as we move from γ-rays to radio waves

(b) Decrease as we move from γ-rays to radio waves

(d) None of the above

(iv) Solar radiation is (1)

(a) Transverse electromagnetic wave

(b) Longitudinal electromagnetic wave

(d) None of the above

(v) A plane electromagnetic wave of frequency 25 MHz travels in free space along the x-direction. At a particular point in space and time , E = 6.3 J V/m. The corresponding magnetic field at that point will be (1)

(a) $2.1 \times 10^{-8} \hat{k}T$

(b) $2.1 \times 10^{8} \hat{k}T$

(d) $3.0 \times 10^{5} \hat{k}T$

Solution:

Answer:- (i) (d)

Explanation:- Electromagnetic waves are neither deflected by magnetic field nor by electric field.

Answer:- (ii) (d)

Explanation :- Total energy density of electromagnetic waves in vacuum is equal to sum of energy density of electric field E and magnetic field B.

i.e. $u = u_E + u_B = \dfrac{1}{2}\epsilon_0E^2 + \dfrac{B^2}{2\mu_0}$

Answer:- (iii) (c)

Explanation:- Since, radiation from γ-rays to radio waves, all belong in category of electromagnetic wave, hence speed of all waves remains same as $3 \times 10^8$ m/s

Answer:- (iv) (a)

Explanation:- Solar radiation is transverse electromagnetic wave because its electric and magnetic field vector vibrate perpendicular to each other and also perpendicular to direction of propogation of wave.

Answer:- (v) (a)

Explanation:- Given. $E = 6.3 \times \hat{J}$ V/m