CBSE Class 12 Physics Electrostatic Potential And Capacitance Notes

CBSE Class 12 Physics Notes - Electrostatic Potential and capacitance. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations. 

 Class XII Physics Ch 2: Electrostatic Potential and capacitance 

Chapter Notes

Top Concepts

1. Potential at a point is the work done by per unit charge by an external agency, in bringing a charge from infinity to that point.

2. Equipotential surface: Definition: An equipotential surface is a surface over which potential has a constant value. Imp:

a. For a point charge, concentric spheres centered at a location of the charge are equipotential surfaces.

b. The electric field E at a point is perpendicular to the equipotential surface through the point.

c. E is in the direction of the steepest decrease of potential.

3. Electric field E along the outward normal to the surface is zero and σ is the surface charge density. Charges in a conductor can reside only at its surface. Potential is constant within and on the surface of a conductor. In a cavity within a conductor (with no charges), the electric field is zero.

4. Capacitance is determined purely geometrically, by the shapes, sizes, and relative positions of the two conductors.

5. Changes observed when the medium between the plates of a capacitor is filled with an insulating substance (dielectric).

i. polarization of the medium gives rise to a field in the opposite direction. The net electric field inside the insulating medium is reduced.

ii. potential difference between the plates is thus reduced.

iii. capacitance C increases from its value C0 when there is no medium

(vacuum). C = KC₀ 

where K is the dielectric constant of the insulating substance.

6. A conductor has a cavity with no charge inside the cavity, then E = 0 inside, no matter what happens outside the conductor.

Therefore, even if there are intense electric fields outside the conductor, the

cavity inside has E= 0 , shielding whatever is inside the cavity from whatever is outside the cavity. This is called electrostatic shielding.

Top Formulae

1.    

1. Potential due to a charge at a point is given by V(r)= 2/ 4πε_ο Q/r

2. The electrostatic potential at a point with position vector r due to a point dipole of dipole moment p place at the origin is

V(r)= 2/ 4πε_ο  pr /r²

The result is true also for a dipole (with charges –q and q separated by 2a) for r >> a.

3. For a charge configuration q₁, q₂, …… qn with position vectors r1, r2, ….rn, the potential at a point P is given by the superposition principle

V=1 /4πε_ο(q₁ r_{1p +} q₁ r_2p+..... +qn / rnp ) 

where r_1p is the distance between q₁, and P, as and so on.

4. Potential energy stored in a system of charges is the work done (by an external agency) in assembling the charges at their locations. Potential energy of two charges q₁, q₂, at r₁, r₂ is given by

 where r12 is distance between q1 and q2

5. PE of a charge q in an external potential, V(r)= qV(r). PE of a dipole of dipole moment p in a uniform electric field E = –p.E.

6. Capacitance C of a system of two conductors separated by an insulator is defined by C = Q/ V

where Q and – Q are the charges on the two conductors V is the potential difference between them.

7. Capacitance C of a parallel plate capacitor (with vacuum between the plates) is given by 

where A is the area of each plate and d the separation between them.

8. For capacitors in the series combination, the total capacitance C is given by:

For capacitors in the parallel combination, the total capacitance C is given by:

C = C1 + C2 + C3 + …..

where C1, C2, C3... are individual capacitances

9. The energy U stored in a capacitor of capacitance C, with charge Q and voltage V is

The electric energy density (energy per unit volume) in a region with electric field is (1/2) ε0E2

10. The potential difference between the conductor (radius ro) inside & outside spherical shell (radius R) is

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