Online Test: Electromagnetic Waves | Class 12 Physics

Question 1 of 18

The displacement current is defined as:

$\epsilon_0\frac{d\Phi_E}{dt}$

$\mu_0\frac{d\Phi_B}{dt}$

$\epsilon_0\frac{d\Phi_B}{dt}$

$\mu_0\frac{d\Phi_E}{dt}$

Explanation: Maxwell's displacement current is $I_d=\epsilon_0\frac{d\Phi_E}{dt}$, caused by a changing electric flux.

Question 2 of 18

In a charging capacitor, the displacement current between the plates is:

zero

equal to the conduction current

half the conduction current

infinite

Explanation: The displacement current in the gap equals the conduction current in the wire, keeping the Ampère–Maxwell law consistent.

Question 3 of 18

Gauss's law for magnetism, $\oint \vec{B}\cdot d\vec{A}=0$, implies:

charges create B

no isolated magnetic monopoles

B is always zero

current is conserved

Explanation: Zero net magnetic flux means field lines are closed loops, so isolated magnetic poles do not exist.

Question 4 of 18

EM waves are produced by:

a stationary charge

a charge in uniform motion

an accelerating charge

a permanent magnet

Explanation: Only accelerating (oscillating) charges radiate electromagnetic waves.

Question 5 of 18

The speed of EM waves in vacuum is:

$\sqrt{\mu_0\epsilon_0}$

$\frac{1}{\sqrt{\mu_0\epsilon_0}}$

$\mu_0\epsilon_0$

$\frac{\mu_0}{\epsilon_0}$

Explanation: $c=\frac{1}{\sqrt{\mu_0\epsilon_0}}\approx3\times10^{8}\ \text{m/s}$.

Question 6 of 18

In an EM wave, $\vec{E}$ and $\vec{B}$ are:

parallel

mutually perpendicular and both perpendicular to propagation

along the propagation direction

at 45 degrees to each other

Explanation: EM waves are transverse: E, B and the direction of travel are mutually perpendicular.

Question 7 of 18

The ratio $\frac{E_0}{B_0}$ for an EM wave in vacuum equals:

$c^2$

$c$

$\frac{1}{c}$

$\mu_0\epsilon_0$

Explanation: At every instant $\frac{E_0}{B_0}=c$.

Question 8 of 18

If $E_0=150\ \text{N/C}$, the magnetic field amplitude is:

$0.5\ \text{T}$

$5\times10^{-7}\ \text{T}$

$2\times10^{-6}\ \text{T}$

$4.5\times10^{10}\ \text{T}$

Explanation: $B_0=\frac{E_0}{c}=\frac{150}{3\times10^{8}}=5\times10^{-7}\ \text{T}$.

Question 9 of 18

The electric and magnetic energy densities in an EM wave are related by:

$u_E=2u_B$

$u_E=u_B$

$u_B=2u_E$

$u_E=4u_B$

Explanation: Since $E=cB$ and $c^2=\frac{1}{\mu_0\epsilon_0}$, the two energy densities are equal.

Question 10 of 18

EM waves carry momentum $p$ related to energy $U$ by:

$p=Uc$

$p=\frac{U}{c}$

$p=\frac{U}{c^2}$

$p=Uc^2$

Explanation: An EM wave of energy $U$ carries momentum $p=\frac{U}{c}$, which is why it exerts radiation pressure.

Question 11 of 18

The radiation pressure on a perfectly absorbing surface is:

$\frac{2I}{c}$

$\frac{I}{c}$

$\frac{I}{2c}$

$Ic$

Explanation: For full absorption $P=\frac{I}{c}$; for a perfect reflector it is $\frac{2I}{c}$.

Question 12 of 18

Which band has the longest wavelength?

gamma rays

X-rays

radio waves

ultraviolet

Explanation: Radio waves have the longest wavelength and lowest frequency.

Question 13 of 18

Microwaves are typically produced by:

radioactive nuclei

klystrons and magnetrons

hot bodies

fast electrons on metal

Explanation: Magnetrons and klystrons generate microwaves used in RADAR and ovens.

Question 14 of 18

Infrared radiation is best described as:

heat radiation from hot bodies

produced only by nuclei

shorter wavelength than X-rays

unable to travel in vacuum

Explanation: IR is emitted by hot bodies and felt as heat; it is used in night vision and remotes.

Question 15 of 18

A wave of frequency $5\times10^{14}\ \text{Hz}$ lies in which band? ($c=3\times10^{8}$)

radio

visible light

X-ray

gamma

Explanation: $\lambda=\frac{c}{f}=600\ \text{nm}$, which is visible (orange) light.

Question 16 of 18

Gamma rays are emitted by:

antennas

hot filaments

radioactive nuclei

vibrating molecules

Explanation: Gamma rays originate in nuclear transitions and radioactive decay.

Question 17 of 18

The correct order of increasing frequency is:

radio, microwave, infrared, visible, UV, X-ray, gamma

gamma, X-ray, UV, visible, infrared, microwave, radio

visible, radio, gamma, X-ray, UV, infrared, microwave

X-ray, gamma, UV, visible, radio, microwave, infrared

Explanation: Frequency increases (wavelength decreases) from radio up to gamma rays.

Question 18 of 18

A microwave of frequency $3\ \text{GHz}$ has wavelength ($c=3\times10^{8}$):

$0.1\ \text{m}$

$1\ \text{m}$

$10\ \text{m}$

$0.01\ \text{m}$

Explanation: $\lambda=\frac{c}{f}=\frac{3\times10^{8}}{3\times10^{9}}=0.1\ \text{m}$.

Online Test — Electromagnetic Waves