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Dielectric Strength
The maximum electric field that dielectric medium can withstand without breaking done of its insulating property is called its dielectric strength. This is of great practical importance.
From V = Q/C we find that a capacitor can hold large amounts of charge Q at a relatively small potential V only when its capacity is large. High potential difference (V) implies a strong electric filed that would ionize the surrounding air, accelerate the charges so produced to oppositely charged plates, thereby neutralizing partly the charge on capacitor plates. It means the charge on capacitor plates would leak away due to reduction in insulating power of the insulating medium.
Hence for a capacitor to store large amounts of charge its capacitance should be large enough so that the potential difference and hence the electric field do not exceed the break down limits.
Dielectric constant and dielectric strength of some common dielectrics
| S. No. |
Dielectric |
Dielectric constant |
Dielectric strength (kV/mm) |
| 1. |
Natural rubber |
2.7 - 5.0 |
18 - 24 |
| 2. |
Mica |
6-7 |
80 |
| 3. |
Polystyrene |
2.55 |
20 - 28 |
| 4. |
Polythene |
2.3 |
20 - 160 |
| 5. |
Teflon |
2.1 |
16 - 20 |
| 6. |
PVC |
5-6 |
30 |
| 7. |
Quartz glass |
3.78 |
--- |
| 8. |
Pyrex glass |
4.6 |
--- |
| 9. |
Alumina |
8 - 9.5 |
--- |
| 10. |
Zircon |
7.1 - 9.1 |
--- |
| 11. |
Air |
1.0006 |
0.8 |
| 12. |
Vacuum |
1.00000 |
∞ |
For reasons of safety, the maximum electric field actually applied is only 10% of the dielectric strength of the material.
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