The word ‘Dielectric’ comes from the Greek prefix ‘di’ or ‘dia’ meaning ‘across’. That is, this material which is placed across the plates of a capacitor, just like a non-conducting bridge. Dielectric materials are basically plain and simple electrical insulators. By the peripheral application of electrical field, these electrical insulators get polarised.
Now, let us discuss what polarisation is. It is actually the alignment of the dipole moments (measurement of the breakup of two oppositely charged charges) of the permanent or induced dipole in the direction of the peripheral electric field. This is explained below. Dielectric materials have no free charges because, all the electrons are bound and associated with the nearest atom. The polar molecules in the material will be in random alignment when there is no peripheral electric field as shown in figure 1.
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Now, when an electric field is implemented to this material, it will get polarised by aligning the dipole moments of polar molecules. The positive charges within the material are shifted slightly in the direction of electric field and the negative charges in the direction opposing the direction of electric field.
When we place a dielectric material in an electric field, practically no current is flowing through them, rather polarisation of molecules happens. It transfers electrical energy through the shifting of current and not through the process of conduction. This is shown in Figure 2.
The dielectric material which is used in capacitors fulfils the following functions.
- Decreases the useful electric field between the capacitor plates.
- Boosts the capacitance of the capacitor plate structure.
- Keep the conducting plates from coming in contact.
- Reduce the possibility of shorting out by sparking during high voltages.
As said above, the presence of dielectric material will decrease the electric field produced by the charge density. The effective electric field which is produced by the charge density can be given by σ - Charge density in A/m2 κ - Dielectric constant. ε0 - Permittivity of free space in F/m = 8.854 × 10-12 F/m. The effective field is decreased by polarisation of the dielectric by a factor κ. It is called the dielectric constant of the material. For a vacuum, it is equal to 1. The capacitance of the normal capacitor; The capacitance of the capacitor which is filled with the dielectric will be increased by the factor called the dielectric constant, κ which is given by κ’ is defined as the relative permittivity of a dielectric material and it is an important parameter in deciding the capacitor characteristics. Every insulator can be forced to conduct electric current or electricity. This phenomenon is known as dielectric breakdown. During high voltages, a capacitor with a dielectric will reduce the chance of short circuit by sparking (that particular voltage which cause breakdown is called dielectric breakdown voltage and its field strength is known as dielectric strength). Another important property of a dielectric is its ability to support an electric field with dissipating minimum energy as heat. A good dielectric should have minimum dielectric loss.