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Study material for SSE/JE examination by RRB on Electrical Engineering Part I

By on October 6, 2014

Basics of Capacitor

A capacitor is a device having two conductive plates separated by an insulator medium. As soon as electric potential is applied across the capacitor plates, electric field sets in, making the charge to flow. Due to the presence of insulator media, the current cannot flow through,  and thus the charge keeps accumulating at the conductive plates. The two plates always occupies opposite charge because of gain of charge at one plate  always results in   the loss of charge on the other plate.


Image from Electrical-tutorial

The plates accumulate charge till such time the opposing potential developed across it stops the flow of charge. Now if the applied potential drops, the accumulated charge starts flow in the reverse direction to balance the voltage across it equal  to the applied voltage. Thus the function of a capacitor is to store and release charge as per the requirement of the circuit. It is a temporary storage device. Charge Q is given as a multiple of Capacitance and Voltage across the capacitor i.e Q=CV. As the charge is held in a electric field, it posses electrostatic energy. Therefore, capacitor is an energy storage device.

A capacitor stores charge in an electric field. The insulating material provided in between the plates plays an important role, as it holds the potential across the capacitor. The insulating material is also called die-electric and its strength, say Ud with unit V/m, is vital in the design of capacitors.

The unit of capacitor is Farad and generally in the range of pF to mF only. The capacitance depends on its ability to accept charge. This is directly proportional to the area of the plates and dielectric constant of the medium, whereas inversely proportional to thickness between the plates.

  1. More area provides the ability to hold more charge, therefore directly proportional
  2. Less thickness results in the opposite polarity plates closure thus attracting the charge.
  3. Higher the dielectric constant more is the polarization of the dipole which produces its own electric field opposite to the applied field. This reduces resultant voltage across the capacitor and as we know C is inversely proportional to V, thus C increases. Dielectric constant of important materials is Vacuum =1, Glass = 5 to 10, Mica = 3 to 6, Plastic = 2 to 4, Al2O3 = 10, Tantalum Oxide TA2O5 = 26, Barium Titanate = BATiO3 = 1000 – 30000

Important relations to  remembered about capacitors are as follows:

  1. Q = C*V
  2. C = ɛr* ɛ0*A/d
  3. E = ½ CV2 or ½ ɛ*A*d*Ud2 thus energy storage depends on the Dielectric volume and square of dielectric strength.
  4. The equivalent value of two or more capacitors connected in series or parallel is given by

Parallel  Combincation Ceq = C1+ C2+ C3

Series Combination       1/Ceq= 1/C1+1/C2 + 1/C3

Application of Capacitor in Electrical Engineering

Capacitor is having wide range of application in the management of electrical power

  1. Power factor compensation: This is the most important application of capacitor in electrical circuits. A part of the electrical load is always inductive and draws current from the utility but without requiring doing any useful work. By providing a capacitor network, the needofreactivecurrentfortheinductiveloadis met locally. The utility imposes penalty for low power factor also.Important formulas in this regard are
    • Total Power drawn from utility – kVA = V*A
    • Total Active power from utility –  kW = V*A*Cos φ
    • Total Reactive Power from utility – kVAh  = V*A*Sin φ
    • Power factor = kW/kVA = Cos φ  = PF
    • Capacitor bank required to compensate and improving power factor angle from φ1 to φ2 is given by = kW(Tan φ1-Tan φ2)
    • A reactor is also used along with capacitor bank to control switching surges.
  2. Due to its property of blocking DC and passing through AC by offering lower impedance, it is widely used for  filtering and coupling in electrical and electronic circuits.
  3. The most important application is providing stabilised DC link voltage to get  any combination of several phases, variable voltage, variable frequency, controlling power factor etc. This conversion would not have been possible without the help of capacitor. This has changed the complete domain of electrical drives bringing efficiency, simplicity, improvements, size reduction etc.
  4. Transient storage device of energy and release at the instant demand in electric power application.
  5. Indirect Capacitance: Capacitance arise accumulation of charge on any metallic surface. Such situation exists in all scenarios and therefore, stray capacitance. Cables, long transmission lines etc. always has stray capacitance.

Maintenance, Reliability and Life of Capacitance

Capacitor is filled with a dielectric and its performance is largely dependent on voltage applied and temperature. Both are detrimental to the life of the dielectric.

  1. Maintenance: Capacitor is a passive item with the application of electrical stress. There is no natural maintenance need except check on the capacitance at the interval. The interval for check shall be decided based on the application and implication for a reduction in capacitance value and expected life.
  2. Reliability: The reliability of capacitor is generally  very high except if it has to work in the harsh environment of voltage, frequency and temperature. The harsh environment shall be compensated by over rating the capacitance the capacitor to take care.
  3. Life: The life is generally given as about 5-8 years but found lasting more than 10 years.
  4. Modern days capacitors are provided with over pressure dis connector switch, which disconnects the capacitor from the circuit when there is building up of pressure.

Safety with Capacitors

  1. In a circuit, the capacitor remains charged and disconnected, some charge always remains and can cause shock to human beings. It is important to discharge all high value capacitors for safe touch.
  2. The human body always has some charge and transfers to printed circuit board if touched with bare hands requiring safety measures while working with PCBs.
    More pages on this topic:

DC Motor (Page 3)

Pages: 1 2 3 4 5 6

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There Are 25 Comments

  1. Shashank says:

    Thanks a lot for this. Waiting eagerly for Mechanical and Civil too..

  2. Abhishek Bhardwaj says:

    Thank you Sir.
    Awaiting mech and civil….

    • Mahesh Kumar Jain says:

      Abhishek, I am a free lancer and feel happy in helping others with what little I know or I can do. all this material is not available readily with me. I am preparing, understanding and con-sizing to benefit you all. So first I have to complete electrical engineering which will take another week and than I proceed on other content. Please don’t depend on me alone for the content on other subject. I am not very sure if I will complete. However, I will be posting mock drill questions for you all shortly.

      • Sudhir Kamat says:

        Thanks a lot for your help sir.

      • vino says:

        thank u so much sir…from the starting i dont know where to start..but when i came to this site..i am getting ideas of where to start..thank u!!!

        • Mahesh Kumar Jain says:

          Thanks vino for the complement. I will request if you to share with your friends as well so that they are benefited. I am writing and posting study material and multiple choice questions to benefit candidates. Keep in touch with the website for new posts.

      • nitin says:

        Mahesh sir, your effort is very appreciable…i know sir its very difficult for you to focus on other mainstream as well…but sir plzz can we have any sort of guidelines regarding SSE rrb civil..any course material?? becuse syllabus is too wide in civil engineeing..
        if u could help sir..mail me at [email protected]

        very eager to hear from u…thank you

      • jagadeesh says:


  3. nitin says:

    everyone is posting for all other branches except civil engineering…can anyone help regarding course material for civil engineering..plzz mail me at [email protected]

    • Mahesh Kumar Jain says:

      I am an electrical engineer by profession and therefore posted the content of study material sufficient for objective type questions. I hope you know that you have to study electrical as well for civil engineering job exam as well.

  4. sumit butman says:

    Hell sir,
    I just know about you, that u help everyone for his carrier..
    So I need your help.
    (my education qualifications is- 10th passed under WBBSE, after that I am going to complete my diploma course in electrical engg in 2016,)
    I decide to sit in upcoming RRB’s ALP and JE exam..
    Will you please help me to complete the large GK syllabus, because I don’t know from where or which topic of gk comes in rrb exam, or is their any book for rrb’s gk preparation which is recently popular in market? This ‘gk’ is my problem to crack the exam.
    I am so asking because I forget every thing after learning, I like to read by understanding..

    I want make my carrier in Indian railway, pls help me..

    • Mahesh Kumar Jain says:

      It is good that you have decided to prepare on the subject now itself. At this stage, just do following things: 1. Read newspaper daily and makes notes of interested developments. There are many website making questions on the current affairs. Generally current affairs is given importance in any examination. 2. Read your class 8th maths and 9th-10th science books for fundamentals. Develop skills in multiplication tables. Prepare technical notes of your diploma classes which can fit in for multiple choice questions. I will be posting multiple choice questions mock drill tests for people like you to test in between your preparedness.

      • Sumit Burma says:

        Thank you sir,
        Thanks for advise…but according to new syllabus of RRB JE, not only from electrical trade but all trade’s question will be set in the technical part of question. Then my questions is that, how I prepare the other trades question?
        I mean the question will be set from Either all the 3 year’s syllabus of other trades Or only few important subjects of other trades.. Please help me sir.. I have no idea in it..

        If you have any recent RRB JE question paper then please attached it on your website..

        • Mahesh Kumar Jain says:

          The syllabus for JE and SSE as well as Assistant Loco Pilot is same irrespective of the branch he has done his ITI, diploma or Degree. The questions are generally such which one has studied of all the branches. For example while doing BE every one studies some part of the engineering of other branches also. I will be posting the question bank of RRB JE after some time.

  5. Sanket Sharma says:

    Mr.Mahesh Kumar jain could you please tell me which publication or author book is best for preparing RRB sse & je examination for technical & non-technical.

    • Mahesh Kumar Jain says:

      I don’t think any one book can be useful. I suggest you go through the Class IX and X books for JE Science, and Class XI and XII science books for SSE. Read and familiarize with the concept given Class VI and VIII arithmetic. For Technical part, go through your notes prepared during I and II year of study. For other subjects, many books are available. This should be more than sufficient.

  6. Rajesh says:

    hi sir, i would like to know the cutoff marks based on last year notification for SCR SSE & JE.
    And also the inhand salary of them.

    • Mahesh Kumar Jain says:

      I don’t have the cut off of SCR with me. But in any case, you should work of minimum 70% to ensure your making a place in the panel.

  7. Syed Faaique says:

    Sir what was the cut off marks for SSE under general category in 2014 for RRB Secunderabad.

  8. prasanna kumar semili says:

    sir plz can I have last 5 year question paper on following

    • Mahesh Kumar Jain says:

      The examination is not held every year. The question paper is not given to candidate after the examination. During the 2014 examination, the RRB has uploaded the paper on their website. The same are available on this website.

  9. kajal12 says:

    hello sir .. i m a b.e electrical passout. wantd help in tech sction of competative exams. pls help sir. or tell me if there is any you tube channel to help me.