Conductors & Insulators - Physics for JEE Main & Advanced PDF Download

Any object can be broadly classified in either of the following two categories on the basis of its electrical properties:
(i) Conductors
(ii) Insulators

Conductors

Definition: Materials that allow electric charge (usually electrons) to move through them easily are called conductors. Metals such as copper, aluminium and iron are typical electrical conductors because they contain mobile (free) electrons.

Basic microscopic picture

In a conductor, some electrons are not bound to individual atoms but can move throughout the material. When an electric field is applied, these free electrons acquire an average velocity called drift velocity and produce an electric current.

The relation between current density and microscopic quantities is

J = nqv_d

where J is current density, n is the number density of charge carriers, q is the charge of each carrier (for electrons q = -e) and v_d is the drift velocity.

Ohm's law (microscopic form) and conductivity

In many conductors under steady conditions the current density is proportional to the applied electric field:

J = σE

where σ is the electrical conductivity (SI unit S m^-1). The reciprocal of conductivity is resistivity ρ, so ρ = 1/σ. For a uniform wire of length L and cross-sectional area A, the resistance is

R = ρ L / A

A simple microscopic model (Drude model) gives conductivity as

σ = ne²τ / m

where n is carrier density, e the electronic charge, m the electron mass and τ the mean time between collisions (relaxation time).

Temperature dependence

For metals, resistivity typically increases with temperature. Close to room temperature the dependence can be approximated by

ρ(T) ≈ ρ₀[1 + α(T - T₀)]

where α is the temperature coefficient of resistivity. For semiconductors and intrinsic insulators, conductivity increases strongly with temperature because thermal energy excites more charge carriers across the band gap.

Electrostatic properties of conductors

• In electrostatic equilibrium the electric field inside a perfect conductor is zero.
• Excess charge resides on the surface of a conductor.
• Conductors provide electrostatic shielding: the interior of a closed conducting shell is unaffected by external static electric fields.

Examples (electrical)

• Metals: copper, aluminium, iron, silver, gold.
• Liquid metal: mercury (used in some electrical contacts and thermometers).
• Non-metal conductor: carbon in the form of graphite (graphite is a good electrical conductor along its planes).

Applications of conductors

Fig: Use of conductors in lighting a bulb

• Electrical wiring in buildings, appliances and electronic devices (copper, aluminium).
• Contacts and connectors in circuits (gold, silver or copper plating).
• Heat transfer applications where both thermal and electrical conduction are useful: e.g., radiator fins, cooking utensils (aluminium, steel).
• Lightning conductors and earthing conductors which safely carry large transient currents to ground.
• Components such as resistors, wires, heating elements (materials chosen depending on conductivity and temperature behaviour).

Insulators

Fig: Insulators

Definition: Materials that do not allow free flow of electric charge under normal conditions are called insulators or dielectrics. In insulators electrons remain tightly bound to atoms; there are very few free charge carriers.

Microscopic and band-theory picture

In band theory terms, an insulator has a completely filled valence band and a large energy gap to the conduction band (large band gap). Thermal excitation across this gap is negligible at ordinary temperatures, so electrical conductivity is extremely low. Typical insulators have band gaps of several electronvolts (eV).

Dielectric behaviour and polarisation

When an electric field is applied to an insulator, bound charges shift slightly producing electric dipoles; this phenomenon is called polarisation. Polarisation leads to a reduction of the internal field and is quantified by the dielectric constant (relative permittivity) ε_r. Dielectrics are important in capacitors and insulating supports.

Breakdown

If the applied electric field exceeds a material-dependent critical value called the dielectric strength, the insulator may undergo electrical breakdown and start conducting (sparking or damaging the material). This is important for high-voltage design.

Examples

• Glass - a very common insulator used in windows, bulbs and as electrical insulation for wires in some applications.
• Plastic - insulating coverings and components in electrical devices and cables.
• Rubber - used for insulating handles, gloves, cable sheathing and tyres (good electrical insulator).
• Mica - used where high-temperature electrical insulation is needed.
• Diamond - an excellent electrical insulator (but thermally conductive).

Applications of insulators

Fig: An insulator is used to protect wire opening

• Coating of electric wires and cables to prevent short circuits and protect users from electric shock.
• Insulators supporting transmission-line conductors (porcelain, glass, polymer insulators).
• Dielectric material in capacitors to store electric energy.
• Thermal insulation in buildings, refrigerators and clothing (materials chosen for low thermal conductivity).
• Sound insulation in auditoria and studios (materials chosen for sound absorption properties).

Distinction between electrical and thermal conduction

Electrical conductivity and thermal conductivity are related in metals (Wiedemann-Franz law) because electrons carry both charge and heat. However, a material may be a good electrical conductor but a poor thermal conductor or vice versa in non-metals. For example, diamond is an electrical insulator but an excellent thermal conductor.

Useful formulae and relations

• Current density: J = nqv_d
• Macroscopic Ohm's law: J = σE
• Resistivity and conductivity: ρ = 1/σ
• Resistance of wire: R = ρ L / A
• Drude conductivity: σ = ne²τ / m
• Temperature dependence (metals): ρ(T) ≈ ρ₀[1 + α(T - T₀)]

Practical notes and safety

• Proper choice of conductor (material, cross-section) is essential to limit heating (I²R losses) and voltage drop in power systems.
• Insulation quality and adequate dielectric strength are critical for safe operation of high-voltage equipment.
• Earthing (grounding) combined with conductors provides a safe path for fault currents; insulators prevent unintended current paths.

Summary: Conductors permit easy flow of electric charge because of mobile carriers; they are described by conductivity, resistivity and microscopic carrier parameters. Insulators lack free carriers, exhibit polarisation as dielectrics and have a large band gap; they are essential for safety and device operation. Understanding their microscopic behaviour, temperature dependence and practical limitations is important for circuit design, high-voltage systems and many everyday applications.