Class 12 - Physics - Electromagnetic Waves


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 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.

Physics Class 12 Electromagnetic Spectrum Capacitor



Radius of each circular plate, r = 12 cm = 0.12 m

Distance between the plates, d = 5 cm = 0.05 m

Charging current, I = 0.15 A

Permittivity of free space ε0, = 8.85 × 10−12 C2 N−1 m−2

(a) Capacitance between the two plates is given by the relation,

C = (ε0A)/ (d)


A = Area of each plate= πr2

C= (ε0 πr2)/ (d)

= (8.85x10-12πx0.12x0.12)/ (0.05) =8.0032 x10-12 F  =80.032pF

Charge on each plate, q = CV


V = Potential difference across the plates

Differentiation on both sides with respect to time (t) gives:

(dq/dt)=C (dV/dt)

Bur (dq/dt) =current (I)

Therefore (dV/dt) = (I/C)

=> (0.15)/ (80.032x10-12) =1.87 ×109 V/s

Therefore, the change in potential difference between the plates is

(1.87 ×109) V/s

(b) The displacement current across the plates is the same as the conduction current.

Hence, the displacement current, id is 0.15 A.

(c) Yes

Kirchhoff’s first rule is valid at each plate of the capacitor provided that we take the sum of conduction and displacement for current.


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.

Physics Class 12 Electromagnetic Waves ParallelPlateCapacitor



Radius of each circular plate, R = 6.0 cm = 0.06 m

Capacitance of a parallel plate capacitor, C = 100 pF = 100 × 10−12 F

Supply voltage, V = 230 V

Angular frequency, ω = 300 rad s−1

(a) Rms value of conduction current, I


XC = Capacitive reactance

=1/ (ωC)

Therefore, I = (V × ωC)

= (230 × 300 × 100 × 10−12)

= 6.9 × 10−6 A

= 6.9 μA

Hence, the rms value of conduction current is 6.9 μA.

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

(c) Magnetic field is given as:

B = (μ0r)/ (2 R2) I0 Where,

μ0 = Free space permeability = (4x π x10-7 ) NA-2

I0 = Maximum value of current =√2 I

r = Distance between the plates from the axis = 3.0 cm = 0.03 m

Therefore B = (4x π x10-7x0.03x√2x6.9x10-6)/ (2x π x0.06)2

= 1.63 × 10−11 T

Hence, the magnetic field at that point is 1.63 × 10−11 T.

Question 3.

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 speed of light (3 × 108 m/s) in a vacuum is the same for all wavelengths. It is independent of the wavelength in the vacuum.

 Question 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?


The electromagnetic wave travels in a vacuum along the z-direction. The electric field (E) and the magnetic field (H) are in the x-y plane.

They are mutually perpendicular.


Frequency of the wave, ν = 30 MHz = 30 × 106s−1

Speed of light in a vacuum, c = 3 × 108 m/s Wavelength of a wave is given as:

λ =(c/ ν)

= (3x108)/ (30x106)

λ =10m

Question 5.

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



A radio can tune to minimum frequency, ν1 = 7.5 MHz= 7.5 × 106 Hz

Maximum frequency, ν2 = 12 MHz = 12 × 106 Hz

Speed of light, c = 3 × 108 m/s

Corresponding wavelength for ν1 can be calculated as:

λ1 = (c/v1) = (3x108)/ (7.5x106) = 40m

Corresponding wavelength for ν2 can be calculated as:

λ2 = (c/v2) = (3x108)/ (12x106) =25m

Thus, the wavelength band of the radio is 40 m to 25 m.

Question 6.

A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz.

What is the frequency of the electromagnetic waves produced by the oscillator?


The frequency of an electromagnetic wave produced by the oscillator is the same as that of a charged particle oscillating about its mean position i.e,

 109 Hz.


The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B0 = 510 nT.

What is the amplitude of the electric field part of the wave?


Amplitude of magnetic field of an electromagnetic wave in a vacuum,

B0 = 510 nT = 510 × 10−9 T

Speed of light in a vacuum, c = 3 × 108 m/s

Amplitude of electric field of the electromagnetic wave is given by the relation,

E = cB0

= 3 × 108 × 510 × 10−9

= 153 N/C

Therefore, the electric field part of the wave is 153 N/C.

Question 8.

Suppose that the electric field amplitude of an electromagnetic wave is E0 = 120 N/C and that its frequency is ν = 50.0 MHz

 (a) Determine, B0,ω, k, and λ.

(b) Find expressions for E and B.



Electric field amplitude, E0 = 120 N/C

Frequency of source, ν = 50.0 MHz = 50 × 106 Hz

Speed of light, c = 3 × 108 m/s

  • Magnitude of magnetic field strength is given as:

B0= (E0/c)

= (120)/ (3x108)

=4 x10-7 T


Angular frequency of source is given as:

ω = 2πν

= 2π ×50 × 106

= 3.14 × 108 rad/s

Propagation constant is given as:

k= (ω/c)

= (3.14 × 108)/ (3x108)


Wavelength of wave is given as:


 = (3x108)/ (50x106)

 = 6.0m

  • Suppose the wave is propagating in the positive x direction. Then, the electric field vector will be in the positive y direction and the magnetic field vector will be in the positive z direction. This is because all three vectors are mutually perpendicular.

Equation of electric field vector is given as:

E=E0 sin (kx – ωt) ˆj

= (120) sin [(1.05) x – (3.14x108) t] ˆj

And, magnetic field vector is given as:

B=B0 sin (kx – ωt) ˆk

= ((4 x10-7) sin1.05 x – (3.14x108) t)] ˆk


Question 9.

The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula E = hν (for energy of a quantum of radiation: photon)

and obtain the photon energy in units of eV for different parts of the electromagnetic spectrum.

In what way are the different scales of photon energies that you obtain related to the sources of electromagnetic radiation?



Energy of photon E = (h (c/ λ))

Where, h = (6.63 x 10-34) js and c = (3 x 108) m/s, λ = wavelength of radiation

Therefore, E = (hc)/ (λ)

E = (6.63 x 10-34 x 3 x 108)/ (λ)

= (19.8 x 10-26)/ (λ) J

= (19.8 x 10-26)/ (λ x 1.6 x 10-19) J

= (12.375 x 10-7)/ (λ) eV

The given table lists the photon energies for different parts of an electromagnetic spectrum for different λ.

The photon energies for the different parts of the spectrum of a source indicate the spacing of the relevant energy levels of the source.

 Physics Class 12 Electromagnetic Waves Spectrum

Question 10.

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of 2.0 × 1010 Hz and amplitude 48 V m–1.

(a) What is the wavelength of the wave?

(b) What is the amplitude of the oscillating magnetic field?

(c) Show that the average energy density of the E field equals the average energy density of the B field. [c = 3 × 108 m s–1.]



Frequency of the electromagnetic wave, ν = 2.0 × 1010 Hz

Electric field amplitude, E0 = 48 Vm−1

Speed of light, c = 3 × 108 m/s

(a) Wavelength of a wave is given as:

λ=(c/ ν)

= (3 × 108)/ (2.0 × 1010)


(b) Magnetic field strength is given as:

B0 = (E0/c)

= (48)/ (3 × 108)

=1.6x10-7 T

(c) Energy density of the electric field is given as:

UE = (1/2) (E2 ε0)

And, energy density of the magnetic field is given as:

UB = (B2 ((1/2) μ0))


ε0 = Permittivity of free space

μ0 = Permeability of free space

We have the relation connecting E and B as:

E = cB ... (1)


c= (1/√ μ0ε0)   … (2)

Putting equation (2) in equation (1), we get

E= (1/√ μ0ε0)   B

Squaring both sides, we get

E2= (1/ (μ0ε0) B2)

ε0 E2 = (B2 / μ0)

((1/2) ε0 E2) = (1/2) μ 0 B2


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