CBSE Physics

Questions asked in Physics Viva Voce for Class 12 Physics Practical


Q1. Why are the resistances used in a resistance box is like 1,2,2,5,10,20,20,50,100,200,200,500,1000,2000,2000,5000 etc?

Ans: In this arrangement all natural numbers upto 10000 ohms (and above) can be realized.

1, 2, 2+1=3,2+2=4,5,5+1=6,5+2=7,5+2+1=8,5+2+2=9 etc

Try it yourself to get all numbers from 1 to 10000

Double Folded Resistance coil

Double Folded Resistance coil

Q2. Why wire is double folded in resistance box?

Ans: The resistance wires in resistance box and in standard resistances are double folded and then coiled so as to nullify the effect inductance. When double folded and coiled, in one half the current will be clockwise and in next half the current will be anticlockwise. Therefore, the magnetic induction created by one half will be cancelled by the other half.

Q3. we make coil of wire when finding resistance of wire through metre bridge ?

Ans: This is not correct. If it is to be coiled, it should be done as in Q1.

Normally the wire is stretched on a half meter scale and fixed with terminals (or arranged in some other manner)

If you feel that the wire need to be coiled, then take  care to avoid the magnetic effect.


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Quick Revision for Class X Physics SA1




  1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
  2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.
  3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.
  4. Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.
  5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.
  6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.                                                                    

       Electric current = Charge/Time     or        I = Q/t

Electric current is a scalar quantity.

  1. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.                                                                                                                         1 ampere = 1 coulomb/1 second    or      1 A = 1C/1s = 1Cs-1                                                                                                                                                                                                              1 milliampere =    1 mA = 10-3 A                                                                                                                                                                                 1 microampere = 1µA = 10-6 A
  2. Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.
  3. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.
  4. Electric field: It is the region around a charged body within which its influence can be experienced. (more…)

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Fiber optic communication


Fiber optic communication refers to the method of sending information from one

place to another, with the help of pulses or a beam of light with the help of optical fibers. The light forms the electromagnetic carrier wave which carries the information. This concept of communicating has revolutionized the communication industry. Optical communication being more advantageous over the electrical transmission has replaced the copper wire communications with optical fibers largely.

The following steps are involved to communicate using an optical fiber:

    • Creating an optical signal and using transmitter to send the signal across the optical fiber.
    • Make certain that the information in the signal is not lost and the signal is not weak or distorted.
    • Receiving the optical signal by the receiver
    • Conversion of optical signal to electrical signal as instruments and electronic appliances work upon electrical signals only.


Total Internal Reflection – Principle behind Optical Fiber

Today telephone signals, internet communication and digital television signals are transmitted using optical fiber cables. Optical fibers work on the principle of Total internal reflection.  Total internal reflection says that when the light strikes the boundary of the optical fiber at an angle greater than the critical angle, the light is totally reflected.  Critical angle refers to the angle at which the light is totally reflected and no refraction takes place. Light which enters the fiber at angle greater than the critical angle is only able to propagate through the optical fiber. The light strikes the boundaries of the cable and bounces back and forth and reaches the destination.

Construction of the Optical Fibers

Optical fibers are made up of two concentric cylindrical glasses. The inner core is surrounded by a concentric core made up of glass and of lower refractive index known as cladding. Protective layer with which the cladding is surrounded is called as protective sheath. The total internal reflection takes place at the cladding – core interface. The core diameter ranges in a few microns and is not much larger than the wavelength of light used.  When high data transmission rates are not required, core with comparatively large diameters are used which may be of a few hundred microns.





English: Optical cable Nederlands: glasvezel k...



      1. Cheap: As it is mainly made up of glass which is manufactured from sand, there is a huge supply of sand available in the world in comparison to copper or aluminum. Hence they are cheap.
      2. Safety: The medium of transportation of data is light and hence it is safer as electricity is not used and there is no chance for sparking to occur.
      3. Security: Criminal intrusions and routing of the information to unwanted receivers can be reduced as electrical signals are not used.
      4. Small size and light weight and hence the optical fibers can be used easily.
      5. Temperature: Melting point of glass is much higher than copper and thus they do not melt easily.
      6. Deterioration: Glass is immune to corrosive substances and can withstand even the harsh environment.   Moisture, Acids and toxic vapors do not harm optical fibers.
      7. Long Life: Optical fibers usually live long for about more than 100 years.



      1. Limited Application: Can only be used on ground, cannot leave the ground or be associated with the mobile communication.
      2. Nuclear Radiations: on exposure to the nuclear radiations the glass darken and the harder the glass is easily it’ll lose its color.
      3. Low Power: Light emitting sources are limited to low power and tough high power emitters are available but are costly.
      4. Fragility : The optical fibers can be broken easily due to age or vibrations
      5. Distance: The distance between the transmitter and receiver must be short or if it is long signal repeaters are used to ensure the signals are not weak.



      • In the telecommunication industry to transmit and receive telephone, internet and television signals.
      • It is used in various surgeries and operations and to illuminate the parts of the human body which are generally inaccessible. It is also used as lasers for surgeries.
      • Used in hydrophones for seismic and sonar uses, as wiring in aircrafts and in submarines.
      • For imaging and lighting in hard to reach areas.

A nationwide competition for aspiring entrepreneurs



The Indian School of Business and The Indus Entrepreneurs (TiE-ISB Connect) have
Indian School Of Business - Hyderabad

launched a nationwide competition named Biz Quest for start-ups. To be held from September 20 to 22 at the ISB Hyderabad campus, this initiative will also support aspiring entrepreneurs, early stage and growth stage entrepreneurs to showcase their business potential among venture capitalists and angel investors.

The winner of the competition has a chance to grab up to Rs 1 crore.
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