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Physics 2 Marks Questions

Electromagnetic Waves · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 11 questions.

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2 Marks Questions

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Question 12 Marks
(a) An em wave is travelling in a medium with a velocity $\vec{v}=v\hat{i}$. Draw a sketch showing the propagation of the em wave, indicating the direction of oscillating electric and magnetic fields. (b) How are the magnitudes of the electric and magnetic fields related to the velocity of em wave.
Answer
(a) $\vec{v}=v\hat{i}$ Direction of propagation of em waves
Image
(b) $v=\frac{E}{B}$ where $v=c$ i.e, the velocity of light.
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Question 22 Marks
Identify the electromagnetic waves whose wavelengths vary as :
(a) $10^{-12} m<\lambda<10^{-8}m$ (b) $10^{-3} m<\lambda<10^{-1}m$
Write one use of each.
Answer
(a) X-rays: In medical science in detection of fractures, stones in kidney, gall bladder etc.
(b) Micro waves: In radar for aircraft navigation.
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Question 32 Marks
Identify the electromagnetic waves whose wavelengths lie in the range :
(a) $10^{-11} m<\lambda<10^{-14}m$, (b) $10^{-4}m<\lambda<10^{-6}m$
Write one use of each.
Answer
(a) Gamma rays: In knowing facts about structure of the nucleus.
(b) Ultraviolet rays: In keeping the plants warm in green houses.
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Question 42 Marks
Why are infrared waves often called heat waves ? Explain.
Answer
Infrared waves have frequencies lower than those of visible light they have ability to vibrate not only the electrons but the entrire atom on molecules of a body this vibration increases the internal energy and temperature of the body.
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Question 52 Marks
What is electromagnetic wave? How is its speed is related with permittivity and permeability of the medium?
Answer
An oscillating or accelerated charge produces an oscillating electric field in space which further produces an oscillating magnetic field which in turn is a source of electric field. These oscillating electric and magnetic field keep on regarding each other and a wave is produced which is known as electromagnetic wave.
Speed of electromagnetic waves in a medium
$v=\frac{1}{\sqrt{\mu\in}}$
where $\mu$ = permeability of the medium and
$\in$ = permittivity of the medium
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Question 62 Marks
State four properties of electro magnetic waves.
Answer
(i) They do not require any material medium to travel.
(ii) They are transverse in nature i.e., electric and magnetic fields are perpendicular to each other and also to the direction of the propagation of the wave.
(iii) The energy of the wave is divided equally amongst the electric and the magnetic field.
(iv) They travel, in free space, with a velocity of $3 \times 10^{8} ms^{-1}$.
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Question 72 Marks
Why red signals are used an danger sign? Explain.
Answer
Because red light is scattered the least by air molecules. The effect of scattering is inversely related to the fourth power of the wavelingth of a colour. Red colour has the highest wavelength, so it is able to trave the longest distance through fog, rain and the alike.
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Question 82 Marks
Out of electric field vector $\vec{E}$ and magnetic field vector $\vec{B}$ in an electromagnetic wave, which is more effective and why ?
Answer
Although electric field vector $\vec{E}$ and magnetic field vector B represents characteristic properties of wave equally, but even then $\vec{E}$ is more effective than $\vec{B}$ because it is only the vector by which photographic plate and retina of our eye are affected.
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Question 92 Marks
What do you mean by electromagnetic wave? Write the name of all the electromagnetic waves from gamma rays to radio waves in ascending order of wavelength.
Answer
Electromagnetic Waves : An oscillating or accelerated charge produces an oscillating electric field in space which further produces an oscillating magnetic field which in turn is a source of electric field. These oscillating electric and magnetic field keep on regarding each other and a wave is produced which is known as electromagnetic wave.
Name of electromagnetic waves in ascending order of wavelength : Gamma rays, X-rays, ultraviolet rays, visible radiation, infrared radiation, microwaves and radio waves.
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Question 102 Marks
How was Ampere's circuital law modified by Maxwell ? Explain.
Answer
Modification of Ampere's Circuital Law given by Maxwell : A magnetic field is produced around a current carrying conductor. Ampere's circuital law gives the qualitative relationship between the current and the magnetic field. "According to this law the line integral of the magnetic field $\vec{B}$ around any closed loop is $\mu_{0}$ times the net current I threading through the area enclosed by the loop", i.e. $\int\vec{B}.\vec{d}l=\mu_{0}I$
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Question 112 Marks
The maximum value of electric field vector $\overrightarrow{ E }$ in a electromagnetic wave is 0.3 mV metre$^{-1}$. Calculate the maximum value of magnetic field vector $\overrightarrow{ B }$.
Answer
Here $E_{0}=0.3\times10^{-3}V~m^{-1}$
∴ $\frac{E_{0}}{B_{0}}=c\Rightarrow B_{0}=\frac{E_{0}}{c}=\frac{0.3\times10^{-3}Vm^{-1}}{3\times10^{8}ms^{-1}}$
$=1.0\times10^{-12}$ Tesla
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