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

The physical quantity speed is same for X-ray, red light and radio waves in vacuum and is c = 3 × 108ms-1.
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# Figure shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 mm. 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 charge 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.

(a) Given,
Radius of capacitor plates, r = 12 cm = 0.12 m

distance between the plates, d = 5.0 mm = 5 × 10-3m

Charge carried, I = 0.15 A

Permittivity of medium, $\mathrm{\epsilon }$0= 8.85 × 10-12C2N-1m2

∴ Area of cross-section of plates, A =$\mathrm{\pi }$R2= 3.14 × (0.12)2m2

Capacitance of parallel plate capacitor is given by

Now, charge on capacitor plate,

$⇒$

$⇒$

$⇒$

(b) Displacement current is equal to the conduction current i.e., 0.15 A.

(c) Yes, Kirchhoff's first rule is valid at each plate of the capacitor provided. We take the current to be the sum of the conduction and displacement currents.

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A radio can tune in to any station in the7.5 MHz to 12 MHz band. What is the corresponding wavelength band?

Given,
Frequency band of radio = 7.5 MHz - 12 MHz

Therefore, the corresponding wavelength band is 40 m - 25 m.

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A parallel plate capacitor (Fig.) 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.

Given, a parallel plate capacitor made of circular plates.

(a) Here,
Radius of the circular plate, R = 6.0 cm

Therefore,

$\therefore$

(b) Yes, the conduction current is equal to the displacement current.

$⇒$

$⇒$
(c) We know that,

This formula goes through even if displacement current, ID(and therefore magnetic field B) oscillates in time. The formula above shows that they oscillate in phase.
Since ID= I, we have

If I = I0, the maximum value of current, then
Amplitude of B = maximum value of B

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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?

Given,
Plane EM wave travels in vacuum along z-direction.
Frequency of the wave = 30 MHz

lie in x-y plane and are mutually perpendicular.

Since,we have, Wavelength of the EM wave,

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