Electromagnetic Waves and Communication System — Physics STD 11 Science — Question
Maharashtra BoardEnglish MediumSTD 11 SciencePhysicsElectromagnetic Waves and Communication System4 Marks
Question
State the main characteristics of EM waves.
✓
Answer
i. The electric and magnetic fields, $\vec{E}$ and $\vec{B}$ are always perpendicular to each other and also to the direction of propagation of the EM wave. Thus, the EM waves are transverse waves. ii. The cross product $(\vec{E} \times \vec{B})$ gives the direction in which the EM wave travels. $(\vec{E} \times \vec{B})$ also gives the energy carried by EM wave. iii. The $\vec{E}$ and $\vec{B}$ fields vary sinusoidally and are in phase. iv. EM waves are produced by accelerated electric charges. v. EM waves can travel through free space as well as through solids, liquids and gases. vi. In free space, EM waves travel with velocity c, equal to that of light in free space. $c=\frac{1}{\sqrt{\mu_0 \varepsilon_0}}=3 \times 10^8 m / s$ where $\mu_0$ is permeability and $\varepsilon_0$ is permittivity of free space. vii. In a given material medium, the velocity $\left(v_m\right)$ of $E M$ waves is given by $v_m=\frac{1}{\sqrt{\mu \varepsilon}}$ where $\mu$ is the permeability and $\varepsilon$ is the permittivity of the given medium. viii. The EM waves obey the principle of superposition. ix. The ratio of the amplitudes of electric and magnetic fields is constant at any point and is equal to the velocity of the EM wave. $\left|\overrightarrow{ E }_0\right|= c \left|\overrightarrow{ B }_0\right| \text { or } \frac{\left|\overrightarrow{ E }_0\right|}{\left|\overrightarrow{ B }_0\right|}= c =\frac{1}{\sqrt{\mu_0 \varepsilon_0}}$ where, $\left|\overrightarrow{E_0}\right|$ and $\left|\overrightarrow{B_0}\right|$ are the amplitudes of $\vec{E}$ and $\vec{B}$ respectively. x. As the electric field vector $\left(\overrightarrow{E_0}\right)$ is more prominent than the magnetic field vector $\left(\overrightarrow{B_0}\right)$, it is responsible for optical effects due to EM waves. For this reason, electric vector is called light vector. xi. The intensity of a wave is proportional to the square of its amplitude and is given by the equations $I _{ E }=\frac{1}{2} \varepsilon_0 E _0^2, I _{ B }=\frac{1}{2} \frac{ B _0^2}{\mu_0}$ xii. The energy of EM waves is distributed equally between the electric and magnetic fields. $I _{ E }= I _{ B }$.
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