Q3 of 20 Page 285

What physical quantity is the same for X-rays of wavelength 10–10 m, red light of wavelength 6800 Å and radio waves of wavelength 500m?

The physical quantity which is same for X-rays of wavelength 10–10 m, red light of wavelength 6800 Å and radio waves of wavelength 500 m is the speed of light in vacuum which is equal to 3×108 m/s. The speed of light in vacuum remains same for all wavelengths. This is because speed of light is independent of wavelength in the vacuum.


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1

Figure 8.6 shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm. The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to 0.15A.

(a) Calculate the capacitance and the rate of change of potential difference between the plates.


(b) Obtain the displacement current across the plates.


(c) Is Kirchhoff’s first rule (junction rule) valid at each plate of the capacitor? Explain.


 2

A parallel plate capacitor (Fig. 8.7) made of circular plates each of radius R = 6.0 cm has a capacitance C = 100 pF. The capacitor is connected to a 230 V ac supply with a (angular) frequency of 300 rad s–1.

(a) What is the rms value of the conduction current?


(b) Is the conduction current equal to the displacement current?


(c) Determine the amplitude of B at a point 3.0 cm from the axis between the plates.


 4

A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?

 5

A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?