CBSE Class 12 Physics VBQs Electromagnetic Waves

ONE MARK QUESTIONS

Question. Identify the part of the electromagnetic spectrum to which the following wavelengths belong:
(i) 10-1m (ii) 10-12 m
Answer : (i) short radio waves (ii) gamma rays

Question. Identify the part of the electromagnetic spectrum to which the following wavelengths belong:
(i) 1 mm (ii) 10-11 m
Answer : 6. (i) microwaves (ii) gamma rays.

Question. Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiations. Name the radiations and write the range of their frequency.
Answer : Ultraviolet radiations, frequency range: 1015-1o17 Hz 

Question. The charging current for a capacitor is 0.25A.What is the displacement current across its plates?
Answer : 0.25A

Question. A variable frequency a.c source is connected to a capacitor. Will the displacement current increase or decrease with increasing frequency?
Answer : Increases

TWO MARKS

Question. A capacitor of capacitance of ‘C’ is being charged by connecting it across a dc source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuity of current in the circuit? Write the expression for the current inside the capacitor.
Answer : Yes, because of the production of displacement current between the plates of capacitor on account of changing electric field.
Current inside the capacitor
Id= ε0 (dφE/dt)

Question. What is meant by the transverse nature of electromagnetic waves? Draw a diagram showing the propagation of an electromagnetic wave along X-direction, indicating clearly the directions of oscillating electric and magnetic fields associated with it.
Answer : In an electromagnetic wave, the electric and magnetic field vectors oscillate, perpendicular to the direction of propagation of wave. This is called transverse nature of electromagnetic wave. (Image 4)
Accordingly if a wave is propagating along z-axis, the electric vector oscillates along x- axis and magnetic field vector oscillates along y-axis.

Question. Identify the following electromagnetic radiations as per the wavelengths given below.
(a) 10-3 nm (b) 10-3m (c) 1 nm
Write one application of each.
Answer : (a) gamma radiation : Radio therapy or to initiate nuclear reactions.
(b) Microwaves : In radar for aircraft navigation.
(c) X-ray : In medical science for detection of fractures, stones in kidneys, gall bladder etc.

Question. A capacitor, made of two parallel plates each of plate area A and separation d, is being charged by an external ac source. Show that the displacement current inside the capacitor is the same as the current charging the capacitor.
Answer :  

Ic = dq/dtIcis the conduction current.
Id= ε0 (dφE/dt)
φE is the electric flux
φE= q/ε0, so Id = dq/dt Id is the displacement current.
Both conduction current and displacement current are equal.

Question. Considering the case of a parallel plate capacitor being charged, show how one is required to generalize Ampere’s circuital law to include the term due to displacement current.
Answer : ∮ B.dl= µ0IC + µ0ε0dɸE/dt
Here Id= ε0 (dφE/dt) = displacement current
IC = conduction current.

Question 1. Clinical microscopes are used to diagnose diseases based on blood and urine samples. Mr. Bajaj does not believe in such tests. He prefers to go to doctors who diagnose on the basis of pulse check only. He fell ill and his temperature persisted for more than a month. Anurag a student of class twelfth resides near Mr. Bajaj house, convinced Mr. Bajaj and got his examination conducted. How X ray is produced? What are the values exhibited by Anurag?
Answer; X-rays are produced by accelerated charged particles.
Values; Caring nature, sympathy, general awareness.

Question 2. Sushma’s mother suffers from cancer of third stage. She has been advised a therapy in which cancerous growth will be burnt by atomic radiations. She is told that her beautiful hair will fall in this therapy and she is liable to become bald. Sushma’s mother refuses the therapy which is otherwise must for her. Sushma talked to her mother explaining the need of the therapy and could convince her. What are the values exhibited by Sushma? Which electromagnetic radiation is used in cancer treatment?
Answer; Sympathy, caring nature, ability to react to situations.
Radition used; Gamma radiation.

Question 3. As water nowadays is more contaminated Ritesh’s father decided to install a water purifier. When the technician installed the purifier, Ritesh asked him whether it is capable of killing the bacteria and other germs also apart from suspended particles.The technician replied that they are killed by a radiation which is emitted from one of the compartments and explained the whole process of purification.
a) What kind of quality is shown by Ritesh?
b) What radiation does the technician refer to? Mention two more applications of such a radiation.
Answer; UVRadition. Also used in crack detection, forensic lab for detection of finger prints, in sterilization of surgical instruments.

Question 4. Pramila was gifted with a box like oven on her birthday and she began to cook food extremely fast. Her son Ramu got surprised by the speed with which the food items getting cooked and he enquired about the working of the device as her mother is an engineer. Then his mother explained the importance of a particular type of radiation employed in the system and the principle behind it.
a) What was the quality exhibited by Ramu.
Answer; Curiosity, Scientific temper

b) What kind of radiation does Pramila refer to and mention one more application of that radiation?

Answer; Microwaves. Also used in radar operations and in Communication.