Saturday, April 8, 2017

Dielectric Strength

      We know that,  E = V/d
      So, as the voltage on the capacitor is increased with a given thickness (d) or the thickness (d) is reduced with a given voltage (V), the electric intensity E increases.
      This intensity represents the force exerted on the charge on the molecules or the dielectric material.
      As E is increased, the centre of the positive charges is pushed in the direction of E and centre of the negative charges in the opposite direction.

      Now, every dielectric medium has its capacity to withstand the increasing E. If the applied voltage and hence E is increased beyond a certain limit, then forces on the molecules become sufficiently large. The electrons break away from the molecules causing ionization and free charges.
      The material then conducts due to ionization and the charge recombine, thereby vanish from the capacitor plates. The capacitor can no more hold the charge and is said to be breakdown. The dielectric medium is said to be punctured and becomes useless from using it as a dielectric.
      The ability of an insulating medium to resist its breakdown when a voltage is increased across it, is called its dielectric strength.
      This depends upon the temperature of the material and presence of air pockets and imperfections in the molecules arrangement of that material. It is generally expressed in kV/cm of kV/mm.
Note: The voltage at which the dielectric medium of the capacitor breakdown is known as breakdown voltage of the capacitor.
      The factors affecting the dielectric strength are,
1. Temperature
2. Type of material
3. Size, thickness and shape of the plates.
4. Presence of air pockets in the material.
5. Moisture content of the material.
6. Molecules arrangement of the material.
      Dielectric strength and dielectric constants of some materials are quoted below from published literature.
     The dielectric strength varies as thickness of dielectric material hence the range of values are given in the table . the value indicates that if material is subjected to electric field more than specified dielectric strength then it will breakdown.

1.1 Dielectric Leakage and Losses
      If there is no leakage of current in the dielectric and the insulation is perfect, then the charge on the capacitor plates can be held on for hours.
      The fact however remains that the insulation resistance of most of the dielectric materials is only of the order of megaohms and hence charge on the capacitor leaks leaks away through the insulating material in a few minutes.
Note: In any case, it is dangerous to touch a charge capacitor even after it is disconnect from the supply.
      In case of d.c. a practical capacitor is considered to be a charge strong device in parallel with a leakage resistance(R) as shown in the Fig 2.

     Further, when the voltage applied to the capacitor is alternating, due to molecular friction of dipoles created in the material, the value of R becomes frequency dependent. The loss due to molecular friction is called dielectric loss.

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