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Published on: 01/06/2021
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Questions + Answers key
Take MCQ Physics Test1.
Consider a parallel plate capacitor whose plates are closely spaced. Let R be the radius of the plates and the current in the wire connected to the plates is 5 A, calculate the displacement current through the surface passing between the plates by directly calculating the rate of change of flux of electric field through the surface.
2.
Compute the speed of the electromagnetic wave in a medium if the amplitude of electric and magnetic fields are 3 x 104 N C–1 and 2 x 10–4 T, respectively.
3.
If the relative permeability and relative permittivity of a medium are 1.0 and 2.25 respectively, find the speed of the electromagnetic wave in this medium.
4.
Let an electromagnetic wave propagate along the x-direction, the magnetic field oscillates at a frequency of 1010 Hz and has an amplitude of 10−5 T, acting along the y-direction. Then, compute the wavelength of the wave. Also write down the expression for electric field in this case.
5.
A pulse of light of duration 10−6 s is absorbed completely by a small object initially at rest. If the power of the pulse is 60\(\times\) 10−3 W. Calculate the final momentum of the object.
1.
Area of the capacitor = A
Radius = R
Current in the wire connected to the plates I = 5 A
The electric field, between the plates of a parallel plate capacitor,
\(E=\frac{\sigma}{\varepsilon_0} \)
\(E=\frac{Q}{A \varepsilon_0}\)
Q is the charge accumulated at the positive plate.
The flux of this field, \(\phi_E=\frac{Q}{A \varepsilon_0} \times A=\frac{Q}{\varepsilon_0}\)
Displacement current \(i_d=\varepsilon_0 \frac{d \phi_E}{d t}\)
\(=\varepsilon_0 \frac{d}{d t}\left(\frac{Q}{\varepsilon_0}\right)=i_c\)
\(\therefore \mathrm{i}_{\mathrm{d}}=5 \mathrm{~A} \quad\left(\because\right.\) The current through the capacitor ic = 5 A)
Displacement current = 5 A
2.
The amplitude of the electric field, E0 = 3 x 104 NC-1
The amplitude of the magnetic field, B0 = 2 x 10-4 T. Therefore, speed of the electromagnetic wave in a medium is
v = \(\frac { 3\times { 10 }^{ 4 } }{ 2\times { 10 }^{ -4 } } \) = 1.5 x 108 ms-1.
3.
μr = 1.0, εr = 2.25
Refractive index of the medium \(n= {\sqrt{\varepsilon_{r} \mu_{r}}} =\sqrt{1 \times2.25}=1.5\)
Speed of electromagnetic waves in this medium,
\(V_{m}=\frac{c}{n}=\frac{3 \times 10^{8}}{1.5}\)
Vm = 2 x 108 m/s
4.
Amplitude of magnetic field B = 10-5 T
Frequency, f = 1010 HZ
(i) Wavelength of the wave,
\({\lambda}=\frac{c}{f} =\frac{3 \times 10^8}{ 10^{10}} \)
= 3 x 108 - 10
Wavelength = 3 x 10-2 m
(ii) Electric field E(x,t)\(\hat i\)
Angular frequency \(\omega =2 \pi f \)
\(\omega=2 \times 3.14 \times 10^{10}=6.28 \times 10^{10} rads^{-1}\)
\(k=\frac{2 \pi}{\lambda} =\frac{2 \times 3.14}{3 \times 10^{-2}} \)
\(=\frac{6.28}{3 \times 10^{-2}}=\frac{628}{3}=2.09 \times 10^{2} \)
k = 2.09 x 102
(iii) Eo = BoC
Eo = 10-5 x 3 x 108
Eo = 3 x 103 V m-1
The required expression for electric field is
\(\vec{E}(x, t)=E_o \sin \left(\frac{2 \pi}{\lambda} x-2 \pi f t\right) \hat{i} N C^{-1} \)
\(\vec{E}(x , t)=3 \times 10^{3} \sin \left(2.09 \times 10^{2} \mathrm{x}-6.28 \times 10^{10} \mathrm{t}\right) \hat(-{k}) N C^{-1} \)
5.
Power of the pulse P = 60 x 10-3 W
Time internal t = 10-6 S
Energy U = Power x time
U = p x t
= 60 x 10-3 x 10-6
U = 60 x 10-9 J
Lineral momentum, \(\mathrm{P}=\frac{\text { Energy }}{\text { speed }}=\frac{U}{C} \ \)
\(\mathrm{P}=\frac{60 \times 10^{-9}}{3 \times 10^{8}} \)
P = 20 x 10-17 kg ms-1
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