Electric Charge
Charge is the property associated with matter due to which it produces and experiences electrical and
magnetic effects.
Note: The excess or deficiency of electrons in a body is the causes of net charge on a body.
Types of charge :
(i) Positive charge : It is the deficiency of electrons as compared to protons.
(ii) Negative charge : It is the excess of electrons as compared to protons.
- SI unit of charge : ampere × second i.e. coulomb
- Dimension : [A T]
- 1 C = 3 × 10⁹ stat coulomb
- 1 absolute - coulomb =10 C
- 1 Faraday = 96500 C
1. Specific Properties of Charge
2. Conductors and Insulators
Conductors : Materials in which the outer electrons of each atom or molecule are weakly bound and these
electrons are almost free to move throughout the body of the material are known as conductors.
Insulators : Materials in which all the electrons are tightly bound to their respective atoms or molecules are
known as insulators. In insulators there are very few free electrons. Such materials are also called
dielectrics.
3. Method of Charging
Friction : If we rub one body with another body, electrons are transferred from one body to the other.Electrostatic Induction : If a charged body is brought near a neutral body, the charged body will attract opposite charge and repel similar charge present in the neutral body. As a result of this one side of the neutral body becomes negative while the other positive, this process is called 'electrostatic induction'. Hence induction is a phenomena of redistribution of charge on a body when any other charged body is brought near it.
Conduction : The process of transfer of charge by contact of two conducting bodies is known as conduction.
If a charged conducting body is put in contact with uncharged conducting body, the uncharged body becomes charged due to transfer of electrons from one body to the other.
The charged body loses some of its charge (which is equal to the charge gained by the uncharged body).
4. Key Points
- 1. Charge differs from mass in the following aspects :
(i) In SI units, charge is a derived physical quantity while mass is a fundamental quantity.
(ii) Charge is always conserved but mass is not.
(iii) Charge cannot exist without mass but mass can exist without charge.
(iv) Charges are of two type (positive and negative) but mass is of only one type (positive).
(v) For a moving charged body, mass increases while charge remains constant.
- True test of electrification is repulsion and not attraction as attraction can take place between a charged and an uncharged body or between two similarly charged bodies.
- Charge can be detected and/or measured with the help of gold-leaf electroscope, electrometer, voltameter etc.
- For a non relativistic (i.e. v << c) charged particle, specific charge q/m = constant.
5. Problems
Problem 1 : When a piece of polythene is rubbed with wool, a charge of – 2 × 10-⁷ C is developed on polythene. What is
the amount of mass, which is transferred to the polythene ?
Problem 2 : How many electrons should be removed from a conductor so that it acquires a positive charge of 3.5 nC ?
Problem 3 : Can a body have a charge of (a) 0.32 × 10–¹⁸ C (b) 0.64 × 10–²⁰ C (c) 4.8 × 10–²¹ C ?
Problem 4 : In a neutral sphere, 5 × 10²¹ electrons are present. If 10 percent electrons are removed, then calculate the
charge on the sphere.
Problem 5 : Two identical metal spheres A and B placed in contact are supported on insulating stand. What kinds of
charge will A and B develop when a negatively charged ebonite rod is brought near A ?
Problem 6 : 10¹² α–particles (nuclei of helium) fall per second on a neutral sphere; calculate the time in which the sphere
gets charged by 2 µC.
Coulomb's Law
The electrostatic force between two stationary point charges is directly proportional to the product of the charges, inversely proportional to the square of distance between them and acts along the straight line joining the two charges.
Let us consider two points charges q1 and q2 seperated by a distance r and F be the electrostatic force between these two charges
1. Coulomb's Law in Vector Form
2. Coulomb's in Terms of Position Vector
3. Principal of Superposition
4. Equilibrium of charged particles
- Equilibrium three point charge
- Equilibrium of symmetric geometrical point charged system
- Equilibrium of suspended point charge system
5. Key Points
6. Problems
Electric Field
1. Intensity of Electric Field due to Point Charge
2. Properties of electric field line/intensity
3. Electric Field intensities due to various charge distributions
- Discrete distribution of charge
- Continuous distribution of charge
4. Electric Field due to uniformly charged ring
- At its centre :
- At an axial point :
- Segment of ring
- Uniformly charged semicircular ring
5. Key Points
6. Problems
Electric Field Lines and Electric Flux
1. Field Lines
2. Electric Flux (ф) :
3. Gauss' Theorem
4. Characteristics of Gaussian surface and Important points regarding Gauss' law
5. Flux Calculation Using Gauss' Law
6. Application of Gauss' Law
- Electric Field due to solid or hollow conducting sphere or shell
- Electric Field due to solid non conducting sphere