Lakshya Education MCQs

Question: In an electromagnetic wave, the amplitude of electric field is 1 V/m. the frequency of wave is 5× 1014Hz. The wave is propagating along z-axis. The average energy density of electric field, in Joulem3, will be
Options:
A.1.1× 10−11
B.2.2× 10−12
C.3.3× 10−13
D.4.4× 10−14
Answer: Option B
: B

Average energy density of electric field is given by

ue=12ϵ0E2=12ϵ0(E02)2=14ϵ0E20

=14×8.85×1012(1)2=2.2×1012J/m3.

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More Questions on This Topic :

Question 1. A TV tower has a height of 100 m. The average population density around the tower is 1000 per km2. The radius of the earth is 6.4× 106m. The population covered by the tower is
  1.    2× 106
  2.    3× 106
  3.    4× 106
  4.    6× 106
Answer: Option C
: C

Population covered =2πhR×Population density
=2π×100×6.4×106×1000(103)2=4×106
Question 2. An LC resonant circuit contains a 400 pF capacitor and a 100 mH inductor. It is set into oscillation coupled to an antenna. The wavelength of the radiated electromagnetic waves is
  1.    377 mm
  2.    377 metre
  3.    377 cm
  4.    3.77 cm
Answer: Option B
: B

v=12πLC and λ=CV
Question 3. In an electromagnetic wave, the electric and magnetising fields are 100V m1 and 0.265 Am1. The maximum energy flow is
  1.    26.5 W/m2
  2.    36.5 W/m2
  3.    46.7 W/m2
  4.    765 W/m2
Answer: Option A
: A

Here E0=100V/m,B0=0.265 A/m.

Maximum rate of energy flow S=E0×B0
=100×.265=26.5Wm2
Question 4. A radio receiver antenna that is 2 m long is oriented along the direction of the electromagnetic wave and receives a signal of intensity 5× 1016 W/m2. The maximum instantaneous potential difference across the two ends of the antenna is
  1.    1.23 mV
  2.    1.23 mV
  3.    1.23 V
  4.    12.3 mV
Answer: Option A
: A

I=12ϵ0CE20

E0=2Iϵ0C=2×5×10168.85=0.61×106Vm

Alsop E0=V0dV0=E0d=0.61×106×2=1.23μV
Question 5. According to Maxwell’s hypothesis, a changing electric field gives rise to
  1.    An e.m.f.
  2.    Electric current
  3.    Magnetic field
  4.    Pressure radiant
Answer: Option C
: C

According to the Maxwell’s EM theory, the EM waves propagation contains electric and magnetic field vibration in mutually perpendicular direction. Thus the changing of electric field give rise to magnetic field.
Question 6. TV waves have a wavelength range of 1-10 meter. Their frequency range in MHz is
  1.    30-300
  2.    3-30
  3.    300-3000
  4.    3-3000
Answer: Option A
: A

v=Cλv1=3×1081=3×108 Hz=300 MHz
and v2=3×10810=3×107 Hz=30 MHz

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