Electrostatic Potential and Capacitance — Part 2: Conductors, dielectrics and capacitors: visual lesson, CBSE Class 12 Physics

Lesson

Conductors, dielectrics and capacitors

Part 2 — how charge behaves on conductors and inside insulators, and how a capacitor stores charge and energy.

In this lesson you will

Explain the electrostatic properties of conductors and the polarisation of a dielectric
Derive the capacitance of a parallel-plate capacitor with and without a dielectric
Combine capacitors in series and in parallel
Use the formulas for the energy stored in a capacitor

Concept

Electrostatics of conductors

In a conductor in electrostatic equilibrium:

The field inside the material is zero.
Any excess charge resides on the outer surface.
Just outside, the field is normal to the surface, with $E = \sigma/\varepsilon_0$ .
The whole conductor (surface and inside) is at one potential.
A charge-free cavity inside a conductor has zero field: electrostatic shielding.

Diagram

Potential of a charged conducting sphere

inside: V = kq/R
outside: V = kq/r

Inside, E = 0, so V does not change.

For a sphere (or thin shell) of radius $R$ with charge $q$ : outside $V = kq/r$ ; on and inside the surface $V = kq/R$ , constant, because $E = 0$ inside.

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