Electric Charges and FieldsNEET Physics · Class 12 · NCERT Chapter 1

You can expect 1 to 2 questions from this chapter in NEET 2027. The chapter has high PYQ frequency. Coulomb's law for a system of charges, electric field at a point, dipole field, electric flux, and Gauss's law applications (sphere, infinite sheet, line) are the most repeated topics.

The force between two point charges q_1 and q_2 separated by distance r is F equals (1 over 4 pi epsilon_0) times (q_1 q_2 over r squared), directed along the line joining them. Like charges repel, unlike charges attract. The constant 1 over 4 pi epsilon_0 equals 9 times 10 to the 9 N m squared per C squared.

Electric field E at a point is force per unit positive test charge: E equals F over q. Field is a property of the source charge configuration; force depends on which charge you place at the test point. Once you know E at a point, force on any charge q there is simply F equals q E.

An electric dipole is a pair of equal and opposite charges plus q and minus q separated by a small distance 2 a. The dipole moment p equals q times 2 a, directed from the negative to the positive charge. On the axis at distance r far from the dipole, E equals 2 k p over r cubed; on the equator, E equals k p over r cubed (half as strong, opposite direction).

Electric flux phi_E through a surface is the dot product E dot dA summed over the surface. For a uniform field E and a flat area A at angle theta to the field, phi_E equals E A cos theta. Flux measures the number of field lines passing through the surface.

The total electric flux through any closed surface equals the total charge enclosed divided by epsilon_0. In symbols, integral over the closed surface of E dot dA equals q_enclosed over epsilon_0. Gauss's law works best for highly symmetric charge distributions where E can be pulled out of the integral.

For a line with linear charge density lambda (charge per unit length), pick a cylindrical Gaussian surface around it. Field is radial. By Gauss's law, E equals lambda over (2 pi epsilon_0 r), where r is the distance from the line. Note that this falls off as 1 over r, not 1 over r squared.