Revision: Electrostatics >> Capacitors and Dielectrics Physics (Theory) ISC (Science) ISC Class 12 CISCE

Substances whose resistance to the movement of charges is intermediate between conductors and insulators, are called semiconductors.

Conductors are those through which electric charge can easily flow. Metals, human body, earth, mercury and electrolytes are conductors of electricity.

OR

Substances which offer high resistance to the passage of electricity and do not allow electricity to pass through them easily, are called insulators.

Those substances in which electric charge cannot flow are called ‘insulators' (or dielectrics). Glass, hard-rubber, plastics and dry wood are insulators. Insulators have practically no free electrons.

OR

Substances which allow electricity to pass through them easily are called conductors.

A capacitor is connected across the terminals of a d.c. battery.

The energy stored on a capacitor is equal to the work done by the battery.

The work required to transport a small amount of charge (dQ) from the negative to positive plates of a capacitor is equal to V dQ, where V represents the voltage across the capacitor.

dU = V dQ

= `Q/C dQ`

∴ Energy stored (U) = ∫V dQ

= `1/C int Q dQ`

= `1/2 Q^2/C`

= `1/2 CV^2`    ...(i)

Energy density is defined as the total energy per unit volume of the capacitor.

For a parallel plate capacitor,

C = `(A epsilon_0)/d`

Putting in eqn. (i),

U = `1/2 (A epsilon_0)/d V^2`

= `epsilon_0/2 Ad(V/d)^2`

= `epsilon_0/2 Ad E^2`    ...[Putting `V/d` = E]

A × d = Volume of space between plates

So, energy is stored per unit volume.

The Molecules in which the centres of positive and negative charges coincide and so the molecules have zero electric dipole moment. Such molecules are called ‘non-polar' molecules.

OR

A molecule in which the centres of positive and negative charges coincide and has no permanent dipole moment, is called a non-polar molecule.

Dielectric strength is defined as the maximum value of the electric field that it can tolerate without its electric breakdown.

The capacitance of a capacitor is defined as the ratio of the charge given to a plate of the capacitor to the potential difference produced between the plates.

The capacitance of a conductor is defined as the ratio of the charge given to the rise in the potential of the conductor.

Mathematical definition: C = \[\frac {Q}{V}\]

A capacitor consisting of two large parallel conducting plates separated by a small distance is called a parallel plate capacitor.

A capacitor is a pair of two conductors of any shape which are close to each other and have equal and opposite charges. These conductors are called the 'plates' of the capacitor.

OR

A system of two conductors separated by an insulator, is called a capacitor.

To sum up, an electric field produces in a dielectric (non-polar or polar) a net dipole moment in the direction of the field. This phenomenon is known as 'dielectric polarisation' or 'electric polarisation of matter'.

A ‘polar' molecule is one in which the centre of gravity of the positive charges (protons) is separated from the centre of gravity of the negative charges (electrons) by a finite distance.

OR

A molecule in which the centres of positive and negative charges are separated, giving it a permanent dipole moment, is called a polar molecule.

The dielectric constant (or specific inductive capacity) of a material is the ratio of the capacitance of a given capacitor completely filled with that material to the capacitance of the same capacitor in vacuum.

OR

A non-conducting substance that has no (or negligible) free charge carriers and can be polarised in an external electric field, is called a dielectric.

“The total amount of work in charging the capacitor is stored up in the capacitor in the form of electric potential energy.”

\[U=\frac{1}{2}\frac{Q^{2}}{C}=\frac{1}{2}CV^{2}joule.\]

C = \[\frac{K\varepsilon_0A}{d}\] farad

If there is vacuum (or air) between the plates, then K = 1

C₀ = \[\frac {ε_0 A}{d}\] farad

C = 4 π ε_0​ a farad

U = \[\frac {1}{2}\] C V²

• Capacitance is directly proportional to the area of the plates
  C ∝ A
  Increasing the effective overlapping area increases capacitance.
• Capacitance is inversely proportional to the distance between the plates
  C ∝ \[\frac {1}{d}\]​
  Reducing the separation between plates increases capacitance.
• Capacitance depends on the medium between the plates
  It increases when a dielectric is introduced and is directly proportional to the dielectric constant K:
  C ∝ K

• An electric field produces dipoles in non-polar dielectrics and aligns them in polar dielectrics, resulting in a net dipole moment along the field.
• Polarisation causes bound charges to appear only on the surfaces of the dielectric slab; the interior remains electrically neutral.
• The polarisation charges create an electric field opposite to the applied field, reducing the field inside the dielectric.
• When a dielectric is inserted in an isolated capacitor, the electric field and potential difference decrease, while capacitance increases.
• A dielectric can withstand the electric field only up to a certain limit, beyond which electrical breakdown occurs.

• In metals, electric current is due to the drift of free electrons; positive ions remain fixed in the lattice and do not move.
• Valence electrons in the outermost orbit are loosely bound and can become free (conduction) electrons, especially at room temperature.
• When an external electric field is applied to a conductor, free electrons acquire a drift velocity opposite to the field, producing current.
• The electrical conductivity of a solid depends on the number of free electrons available for conduction.
• In dielectrics, an applied electric field causes electric polarisation; charges appear on the surface, but no charge flows through the material.

• Series combination: All capacitors connected in series carry the same charge Q, while the total potential difference is the sum of individual potential differences.
• Equivalent capacitance in series is given by​
  \[\begin{aligned}
  \frac{1}{C}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}
  \end{aligned}\]
  and is less than the smallest individual capacitance.
• In a series combination, the potential difference across each capacitor is inversely proportional to its capacitance, and the capacitor with the least capacitance has the highest voltage.
• Parallel combination: All capacitors connected in parallel have the same potential difference, while the charge distributes according to capacitance.
• Equivalent capacitance in parallel is given by​
  C = C₁ ​+ C₂​ + C₃​
  and is greater than any individual capacitance.