CBSE Class 12 Physics Electric Charges And Fields Notes Set B

Physics

Chapter 1: Electric Charges and Fields

Top Concepts

1. Like charges repel and unlike charges attract.

2. Conductors allow movement of electric charge through them, insulators do not.

3. Quantization of electric charge means that total charge (q) of a body is always an integral multiple of a basic quantum of charge (e) i.e.,q = ne, where n = 0, ±1, ±2, ±3, ….

4. Additivity of electric charges : Total charge of a system is the algebraic sum of all individual charges in the system.

5. Conservation of electric charges: Total charge of an isolated system remains uncharged with time.

6. Superposition Principle : Property that the forces with which two charges attract or repel each other are not affected by the presence of a third (or more) additional charge(s).

7. The electric field E at a point due to a charge configuration is the force on a small positive test charges q placed at the point divided by a magnitude

│ q│ / 4 π ε0 r2; it is radially outwards from q, if q is positive and radially inwards if q is negative.

8. E at a point varies inversely as the square of its distance from Q, the plot of E v/s r will look like the figure given below.

8. Coulomb’s Law: The mutual electrostatic force between two point charges q₁ and q₂ is proportional to the product q1q2 and inversely proportional to the square of the distance r₂₁ separating them. 

F₂₁ (force on q₂ due to q₁ )  k( q₁ q₂) / r²₂₁* i₂₁ 

where 21 ˆr is a unit vector in the direction from q₁ to q₂ and  k = 1/4πε₀ is the constant of proportionality.

9. An electric field line is a curve drawn in such a way that the tangent at each point on the curve gives the direction of electric field at that point.

10. Important properties of field lines are:

(i) Field lines are continuous curves without any breaks.

(ii) Two field lines cannot cross each other.

(iii) Electrostatic field lines start at positive charges and end at negative charges – they cannot form closed loops.

11. The electric flux φ = ∫dφ = ∫E.ds  is a ‘dot’ product, hence it is scalar.

Δφ is positive for all values of Θ < π/2

 is negative for all values of Θ < π/2.

12. Gauss’s law: The flux of electric field through any closed surface S is 1/ε₀ times the total charge enclosed by S.

 φ = ∫E.dS = q/ ε₀

13. Electric field outside the charged shell is as though the total charge is concentrated at the centre. The same result is true for a solid sphere of uniform volume charge density.

The electric field is zero at all points inside a charged shell.

Graphical plot of E vs R inside the spherical shell. 

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