Electromagnetic Waves
- A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to displacement current is : Answer: 3. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates Key idea: a changing e... 1
- The electric field in a plane electromagnetic wave is given by E_z = 60(5x + 1.5×10^9 t) V/m. Then expression for the corresponding magnetic field is (here subscripts denote the direction of the field) : Answer: 1. B_y = 2×10⁻⁷cos(5x+1.5×10⁹ t)T Key idea: in an EM wave, B₀=E₀/c , and vec E×vec B points along the direction of propagation. The... 1
- A parallel plate capacitor is charged by connecting it to a battery through a resistor. If I is the current in the circuit, then in the gap between the plates : Answer: 3. displacement current of magnitude equal to I flows in the same direction as I. Key idea: Maxwell's displacement current I_d=ε₀dΦ_... 1
- The property which is not of an electromagnetic wave travelling in free space is that : Answer: 1. they originate from charges moving with uniform speed. Key idea: Electromagnetic waves are produced by accelerated (oscillating)... 1
- In a plane electromagnetic wave travelling in free space, the electric field component oscillates sinusoidally at a frequency of 2.0×10^10 Hz and amplitude 48 V m^-1. Then the amplitude of oscillating magnetic field is : (Speed of light in free space… Answer: 4. 1.6×10⁻⁷ T In an electromagnetic wave in free space, E₀ = cB₀ . B₀=E₀/c=dfrac483×10⁸=1.6×10⁻⁷ T. The frequency is not needed. The... 1
- When light propagates through a material medium of relative permittivity ε_r and relative permeability μ_r, the velocity of light, v is given by : (c - velocity of light in vacuum) Answer: 1. v = c/√(ε_r μ_r) In a medium, v=1/√(με) with μ=μ₀μ_r and ε=ε₀ε_r . Since c=dfrac1√(μ₀ε₀) , we get v=c/√(ε_rμ_r) . Option (1) is c... 1
- The energy that will be ideally radiated by a 100 kW transmitter in 1 hour is : Answer: 2. 36 × 10⁷ J Energy radiated equals power times time. E=Pt=100×10³\ W×3600\ s=3.6×10⁸\ J=36×10⁷\ J Option (2) is correct. 1
- Match List - I with List - II :List - I (Electromagnetic waves): (a) AM radio waves (b) Microwaves (c) Infrared radiations (d) X-raysList - II (Wavelength): (i) 10^-10 m (ii) 10^2 m (iii) 10^-2 m (iv) 10^-4 mChoose the correct answer from the options… Answer: 1. (a) - (ii), (b) - (iii), (c) - (iv), (d) - (i) Recall typical wavelength ranges of the electromagnetic spectrum. AM radio waves:... 1
- A capacitor of capacitance 'C', is connected across an ac source of voltage V, given byV = V_0 ω tThe displacement current between the plates of the capacitor, would then be given by : Answer: 1. I_d = V₀ω Ccosω t Key idea: displacement current equals the conduction current that charges the capacitor. q = CV = CV₀sinω t I_d... 1
- For a plane electromagnetic wave propagating in x-direction, which one of the following combination gives the correct possible directions for electric field (E) and magnetic field (B) respectively ? Answer: 2. -j+k,\ -j-k Key idea: vec E , vec B and the direction of propagation are mutually perpendicular, and vec E×vec B points along pro... 1
- The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is : (c = speed of electromagnetic waves) Answer: 2. 1 : 1 In an electromagnetic wave the energy is shared equally between the electric and magnetic fields. Electric energy density =... 1
- Light with an average flux of 20 W/cm^2 falls on a non-reflecting surface at normal incidence having surface area 20 cm^2. The energy received by the surface during time span of 1 minute is : Answer: 3. 24×10³ J Energy received = flux × area × time. E=20\ W/cm²×20\ cm²×60\ s=24000\ J=24×10³\ J Answer: option 3. 1
- A parallel plate capacitor of capacitance 20 μF is being charged by a voltage source whose potential is changing at the rate of 3 V/s. The conduction current through the connecting wires, and the displacement current through the plates of the capacit… Answer: 2. 60 μ A, 60 μ A In a charging capacitor the conduction current in the wires equals the displacement current between the plates, so... 1
- Which colour of the light has the longest wavelength ? Answer: 1. red In the visible spectrum wavelength decreases from red to violet (about 700 nm for red, 400 nm for violet). So red has the lon... 1
- An em wave is propagating in a medium with a velocity V=V i. The instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along Answer: 3. + z direction In an em wave, vec E , vec B and the direction of propagation are mutually perpendicular, and vec E×vec B points al... 1
- In an electromagnetic wave in free space the root mean square value of the electric field is E_rms = 6 V/m. The peak value of the magnetic field is : Answer: 1. 2.83×10⁻⁸ T Key idea: peak value =sqrt2× rms value, and B₀=E₀/c . E₀=6sqrt2=8.49 V/m. B₀=8.49/3×10⁸=2.83×10⁻⁸ T. 1
- Out of the following options which one can be used to produce a propagating electromagnetic wave ? Answer: 4. An accelerating charge Key idea: electromagnetic waves are produced by accelerated (time-varying) charges. A charge at constant v... 1