NCERT Physics Class 12 Exemplar Ch 8 Electromagnetic Waves Part 1

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Very Short Answer

Q 14 Why is the orientation of the portable radio with respect to broadcasting station important?

Ans:

As electromagnetic waves are plane polarized, so the receiving antenna should be parallel to electric/magnetic part of the wave.

Q 15 Why does microwave oven heats up a food item containing water molecules most efficiently?

Ans:

Frequency of the microwave matches the resonant frequency of water molecules.

Q 16 The charge on a parallel plate capacitor varies as . The plates are very large and close together (area , separation ). Neglecting the edge effects, find the displacement current through the capacitor?

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Q 17 A variable frequency a.c source is connected to a capacitor. How will the displacement current change with decrease in frequency?

Ans:

  • On decreasing the frequency, reactance will increase which will lead to decrease in conduction current.

  • In this case ; hence displacement current will decrease.

Q 18 The magnetic field of a beam emerging from a filter facing a floodlight is given by . What is the average intensity of the beam?

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Q 19 Poynting vectors S is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propagation. Mathematically, it is given by . Show the nature of S vs t graph.

Ans:

Poynting vectors S Graph

Poynting Vectors S Graph

Poynting vectors S Graph

Q 20 Professor C.V Raman surprised his students by suspending freely a tiny light ball in a transparent vacuum chamber by shining a laser beam on it. Which property of EM waves was he exhibiting? Give one more example of this property

Ans:

EM waves exert radiation pressure. Tails of comets are due to solar solar radiation.