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Electrostatics : Electric Charge and properties of electric charge



1. Electric Charge

  • Electric charge is a fundamental property like mass, length etc associated with elementary particles for example electron, proton and many more.
  • Electric charge is the property responsible for electric forces which acts between nucleus and electron to bind the atom together.
  • Charges are of two kinds
    (i) negative charge
    (ii) positive charge
  • Electrons are negatively charged particles and protons, of which nucleus is made of, are positively charged particles. Actually nucleus is made of protons and neutrons but neutrons are uncharged particles.
  • electric force between two electrons is same as electric force between two protons kept at same distance apart i. e., both set repel each other but electric force between an electron and proton placed at same distance apart is not repulsive but attractive in nature.
    Conclusion
    (a) Like charges repel each other



    (b) Unlike charges attract each other



  • Assignment of negative charge on electron and positive charge on proton is purely conventional , it does not mean that charge on electron is less than that on proton.
  • Importance of electric forces is that it encompasses almost each and every field associated with our life; being it matter made up of atoms or molecules in which electric charges are exactly balanced or adhesive forces of glue associated with surface tension, all are electric in nature.

Unit of charge

  • Charge on a system can be measured by comparing it with the charge on a standard body.
  • SI unit of charge is Coulomb written as C.
  • 1 Coulomb is the charge flowing through the wire in 1 second if the electric current in it is 1A.
  • Charge on electron is -1.602 × 10 -19 C and charge on proton is positive of this value.

2. Basic properties of electric charge


(i) Additivity of charges

  • Charges adds up like real numbers i. e., they are Scalars more clearly if any system has n number of charges q1, q2, q3, qn then total charge of the system is
    q = q1 + q2 + q3 + ................ qn
  • Proper sign have to be used while adding the charges for example if
    q1 = +1C
    q2 = -2C
    q3 = +4C
    then total charge of the system is
    q = q1 + q2 + q3
    q = (+1) + (-2) + (+4) C
    q = (+3) C

(ii) conservation of charge : Charge is conserved

  • Charge of an isolated system is conserved.
  • Charge can not be created or destroyed but charged particles can be created or destroyed.

(iii) Quantization of charge


  • All free charges are integral multiples of a unit of charge e, where e = -1.602 × 10 -19 C i. e., charge on an electron or proton.
  • Thus charge q on a body is always denoted by
    q = ne
    where n = any integer positive or negative

Solved Example

These are some solved examples related to electric charge and properties of electric charge given in this page.
Question 1 Is there any transfer of mass when electrons are transferred from one substance to another?

Question 2 (NCERT)  A polythene piece rubbed with wool is found to have a negative charge of 3.2 × 10–7 C.
(a) Estimate the number of electrons transferred (from which to which?)
(b) Is there a transfer of mass from wool to polythene?

Question 3  When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Question 4  (a) Explain the meaning of the statement ‘electric charge of a body is quantized’.
(b) Why can one ignore quantization of electric charge when dealing with macroscopic i.e., large scale charge?

Problem solving tips

  • Firstly read your problem carefully. Reading and understanding the problem is the first step towards solving it.
  • Once you have read and understood the problem then try to analyze it using your previous knowledge of the topic.
  • Comprehensive knowledge of the topic is required and it is prerequisite for solving any problem.
  • Now look for the known and unknown quantities.
  • Try to find the formula that can be applied to the problem based on your known and unknown quantities.
  • Now apply the formula, perform the desired calculation and obtain the results.
  • Try to interpret your results.


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