# PSEB 12th Class Physics Important Questions Chapter 8 Electromagnetic Waves

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 8 Electromagnetic Waves Important Questions and Answers.

## PSEB 12th Class Physics Important Questions Chapter 8 Electromagnetic Waves

Question 1.
They are produced by rapid accelerations and deaccelerations of electrons in aerials.

Question 2.
How are microwaves produced?
By using a magnetron. Question 3.
Write two uses of microwaves.
Uses of Microwaves

• In analysis of molecular and atomic structure.

Question 4.
To which part of the electromagnetic spectrum does a wave of frequency 3 × 1013 Hz belong?
The frequency of 3 × 1013 Hz belongs to the infrared waves.

Question 5.
Name the electromagnetic waves, which (i) maintain the Earth’s warmth and (ii) are used in aircraft navigation.
(i) Infrared rays
(ii) Microwaves

Question 6.
Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiation. Name the radiations and write the range of their frequency.
Welders wear special goggles or face mask with glass windows to protect their eyes from ultraviolet rays. The range of UV rays is 4 × 10-7 m (400 nm) to 6 x 10-10 m (0.6 nm).

Question 7.
How are X-rays produced?
X-rays are produced when high energetic electron beam is made incident on a metallic target of high melting point and high atomic weight. Question 8.
Write two uses of X-rays.
Uses of X-rays

• In medical diagnosis as they pass through the muscles not through the bones.
• In detecting faults, cracks, etc. in metal products.

Question 9.
A variable frequency AC source is connected to a capacitor. How will the displacement current change with decrease in frequency? (NCERT Exemplar)
On decreasing the frequency, reactance XC = $$\frac{1}{\omega C}$$ will increase which will lead to decrease in conduction current. In this case Id = Ic, hence displacement current will decrease.

Question 10.
Do electromagnetic waves carry energy and momentum?
Yes. Electromagnetic waves carry energy and momentum.

Question 11.
Why is the orientation of the portable radio with respect to broadcasting station important? (NCERT Exemplar)
As electromagnetic waves are plane polarised, so the receiving antenna should be parallel to electric/magnetic part of the wave.

Question 12.
The charge on a parallel plate capacitor varies as q = q0 cos 2πvt. The plates are very large and close together (area = A, separation = d). Neglecting the edge effects, find the displacement current through the capacitor? (NCERT Exemplar)
Conduction current IC = Displacement current ID
IC = ID = $$\frac{d q}{d t}$$ = $$\frac{d}{d t}$$ (q0 cos 2π vt) = -2πcq0vsin2πvt Question 13.
Professor C.V. Raman surprised his students by suspending freely a tiny light ball in a transparent vacuum chamber by shining a laser beam on it. Which property of electromagnetic waves was he exhibiting? Give one more example of this property. (NCERT Exemplar)
Electromagnetic waves exert radiation pressure. Tails of comets are due to solar radiation.

Question 1.
Write the generalised expression for the Ampere’s circuital law in terms of the conduction current and the displacement current. Mention the situation when there is
(i) only conduction current and no displacement current,
(ii) only displacement current and no conduction current.
Generalised Ampere’s Circuital Law
$$\oint \vec{B} \cdot \overrightarrow{d l}$$ = μ0Ic + μ0ε0$$\frac{d \phi_{E}}{d t}$$
Line integral of magnetic field over closed loop is equal to p 0 times sum of conduction current and displacement current.

(i) In case of steady electric field in a conducting wire, electric field does not change with time, conduction current exists in the wire but displacement current may be zero.
So $$\oint \vec{B} \cdot \overrightarrow{d l}$$ = μ0Ic

(ii) In large region of space, where there is no conduction current, but there is only a displacement current due to time varying electric field (or flux).
So Φ $$\vec{B} \cdot \overrightarrow{d l}$$ = μ0 ε0 $$\frac{d \phi_{E}}{d t}$$

Question 2.
How are infrared waves produced? Why are these referred to as ‘heat waves’? Write their one important use.
Infrared waves are produced by hot bodies and molecules. Infrared waves are sometimes referred to as heatwaves. This is because water molecules present in most materials readily absorb infrared waves. After absorption, their thermal motion increases, that is they heat up and heat their surroundings.
Infrared lamps are used in physical therapy and in remote control of devices. Question 3.
(i) Arrange the following electromagnetic waves in the descending order of their wavelength.
(a) Microwaves
(b) Infrared rays
(d) γ-rays
(ii) Write one use each of any two of them.
(i) The decreasing order ofwavelength of electromagnetic waves are Microwaves > Infrared > Ultraviolet > y-rays

(ii) Microwaves: They are used in RADAR devices,
γ-rays: It is used in radio therapy.

Question 4.
Write Maxwell’s generalization of Ampere’s Circuital Law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is
i = ε0$$\frac{d \phi_{\boldsymbol{E}}}{d t}$$
Where ΦE is the electric flux produced during charging of the capacitor plates.
Ampere’s circuital law is given by
$$\oint \vec{B} \cdot \overrightarrow{d l}$$ = μ0Ic
For a circuit containing capacitor, during its charging or discharging the current within the plates of the capacitor varies producing displacement current Id Hence, Ampere’s circuital law is generalised by Maxwell, given as
$$\oint \vec{B} \cdot \overrightarrow{d l}$$ = μ0Ic + μ0Id
The electric flux (ΦE) between the plates of capacitor changes with time, producing current within the plates which is proportional to ($$\frac{d \phi_{E}}{d t}$$)
Thus, we get,
Ic = ε0 $$\frac{d \phi_{E}}{d t}$$ε

Question 5.
How are electromagnetic waves produced? What is the source of energy of these waves? Write mathematical expressions for electric and magnetic fields of an electromagnetic wave propagating along the z-axis. Write any two important properties of electromagnetic waves.
Electromagnetic wave produced by oscillating charged particle. Mathematical expression for electromagnetic wave travel along z-axis:
Ex = E0 sin(kz – ωt) [For electric field]
By = B0 sin(kz – ωt) [For magnetic field]
Properties
(i) Have oscillating electric perpendicular direction.
(ii) Transverse nature. Question 6.
Electromagnetic waves with wavelength
(i) λ1 is used in satellite communication.
(ii) λ2 is used to kill germs in water purifier.
(iii) λ3 is used to detect leakage of oil in underground pipelines.
(iv) λ4 is used to improve visibility in runways during fog and mist conditions.
(a) Identify and name the part of electromagnetic spectrum to which these radiations belong.
(b) Arrange these wavelengths in ascending order of their magnitude.
(c) Write one more application of each. (NCERTExemplar)
(a) λ1 → Microwave, λ2 → UV
λ3 → X rays, λ4 → Infrared

(b) λ3 < λ24 < λ1

UV-LASIK eye surgery
X-ray-Bone fracture identification (bone scanning)
Infrared-Optical communicatio

Question 1.
Draw a labelled diagram of Hertz’s experiment. Explain how electromagnetic radiations are produced using this set-up.
Hertz Experiment: Hertz’s experiment was based on the fact that an oscillating electric charge radiates electromagnetic waves and these waves carry energy which is being supplied at the cost of K.E. of the oscillating charge.

Hertz Apparatus: The experimental arrangement used by Hertz for the production and detection of electromagnetic waves in the laboratory, is shown in fig. His experimental arrangement consists of two metal sheets P1 and P2. These sheets are connected to a source of very high voltage (i.e. an induction coil, which can supply a potential difference of several thousand volts). S1 and S2 are two metal spheres connected to the metal sheets P1 and P2 . The distance between the metal sheets is kept nearly 60 cm and that between the sphere is normally from 2 cm to 2.5 cm.

The two plates P1 and P2 form a capacitor of very low capacitance (C). The circuit containing P1 and P2 (being completed by conducting wire), has also some low value of inductance L. It thus forms an LC circuit. Detector (D) consisting of a coil to the ends of which two other small metal spheres S1‘and S2‘ are connected. Working of Hertz Apparatus: Due to existence of very high voltage, air present in the gap across the plates of spheres S1 and S2 gets ionised. Due to presence of the ions or charged particles, the path between the spheres S1 and S2 become conducting. As a result of this, very high time- varying current flows across the gap between S1 and S2 (as plates P1 and P2 form an LC circuit). Due to this a spark is produced. Since, sheets P1 ,
P2 form an LC-circuit, hence, electromagnetic waves of frequency f = $$\frac{1}{2 \pi} \sqrt{\frac{1}{L C}}$$

Function of the Detector D: Hertz detected the electromagnetic waves by means of a detector D kept at suitable distance from the conducting spheres S1, S2 Detector D is made of two similar conducting spheres S1‘ and S2‘ joined to the ends of a coil to form another LC circuit. The frequency of this LC circuit is made equal to the frequency of electromagnetic waves reaching it. The frequency can be adjusted by changing the diameter of the coil of the detector and by changing the distance between S1‘ and S2‘. Hertz placed the detector in such a way that the magnetic lines of force produced by the oscillating electric field across the gap between S1‘ and S2‘ are normal to the plane of coil (C). When magnetic lines of force cut the detector coil, an emf is induced in it. Hence, air in the gap between S1‘ and S2‘ gets ionised. A conducting path becomes available for the induced current to flow across the gap. Thus, the spark is produced between S1‘ and S2‘. Hertz also observed that the spark across S1‘ and S2‘ was greatest when the S1‘ S2‘ and S1 S2 were parallel to each other. This clearly established that electromagnetic waves produced were polarise i.e., $$\vec{E}$$ and $$\vec{B}$$ always lie in one plane.