What is the Difference between Insulators and Dielectrics?

Insulator

An insulator is a material in which electric charges do not flow freely. These materials do not allow current to pass through them. An ideal insulator is one which completely blocks movement of charges (or current) through them. It does not allow absolutely any electrical conduction (or drift of charges) even in trace amount. They do not possess any free electrons which can move under the effect of electric field. Insulation is a term generally applied for materials which inhibit the flow of charges, or transfer of electrical energy.

An insulating substance does not have loose or free electrons, which can jump from atom to atom in random movements (as in conductors) under normal conditions. Hence it is not possible to get drift of electrons under electric field. This means there is absolutely no current under an electric field, or a potential difference across two points in an ideal insulator.

There is no ideal insulator in practice, and all materials have at least a small amount of electrons which are free to move, or drift, when subjected to electric field. This is the source of stray resistance in electric circuits.

Electric dipoles

Atoms or molecules of a material may be visualized as having nuclei and electron clouds around them. Electron cloud model is a simplification of all electron orbits taken together. Center of electron cloud is same as that of nucleus. This keeps the system in neutral equilibrium. In insulators, the electrons cannot leave the atom/ molecule (or are not free to leave the electron cloud).

When an electric field is applied, electron cloud experiences a pull towards positive plate side, while the positive nucleus gets a push away from positive plate (in the direction of field). This external field results in nucleus and electron cloud center being pulled and slightly displaced in opposite directions in the field. Cloud and nucleus get displaced with respect to each other, and a distance is created between them. The distance d depends on the strength of electric field, and goes up with increasing field strength. This phenomenon, where centers of positive and negative charges are displaced, and are balanced by forces of external field and their internal forces of attraction (between nucleus and electron cloud), is polarization. The nucleus/cloud pair in this position is called a dipole.

Dielectrics

It takes energy to create this drift, which gets stored in dipoles. The dipoles get oriented in the direction of field – electron cloud in the direction of positive electrode, and nucleus towards negative electrode. The distance of cloud center and nucleus from each other depends on electric field strength. Bonds between the cloud and nucleus varies in different materials. Where this bond is weaker, drift of electron cloud is higher, or significant. The energy needed for creation of drift of electron cloud is taken from the external electric field, and gets stored in these electron clouds. When the electric field is gone, and there is no force on dipoles, they get back to original state, and the energy is returned to source of field.

Material having the property of significant polarization under electric field is called a dielectric. This property of storing energy under electric field is used in capacitors. (Although most material do have certain polarization at some field strength, it is miniscule.) All dielectrics are insulators, but all insulators are not dielectrics.

If there are conducting surfaces at the extreme ends of material thickness, they develop charges on their surface to balance the charges in dipoles (electron clouds or nuclei) facing them. The conducting surface takes these charges from supply. If the supply source is removed, these charges on conductor surfaces remain in place, bound now by the dipole clouds and nuclei. The electric field also remains unchanged. If now the conductors are connected externally, the opposite charges between them get merged into each other over time (depending on load, or connecting circuit.), till they finally become zero. Electric field is weakened, diploes vanish, and electron clouds and nuclei get back to their original positions. (This is the discharge process of a capacitor.)

In most common insulators, the drift of electron cloud is insignificant, and dipole formation can be neglected for all practical purposes (though the drift is not absolutely zero). However, these weak dipoles can be the reason for formation or occurrence of stray capacitance in real life, where the stray capacitance can become a nuisance, particularly at high frequencies.

Wood, rubber, Bakelite etc., are all are insulators, while plastic films, mica, ceramics are dielectric materials. It may be noted that dielectrics in large enough thickness often act as practical insulators. The fact that most insulation materials are not ideal insulators, is the reason for most stray capacitances in circuits.