Because dielectric materials can store electrical energy without conducting current, they have a specific role in the study of electrostatics and capacitors. The dielectric strength of these materials is one of their most vital characteristics. It is also widely used in electrical devices including transformers, insulators and capacitors.
What is Dielectric Strength?
The highest electric field that a dielectric material can sustain before degrading and turning electrically conductive is known as its dielectric strength. Put more simply, it is the maximum electric field intensity that a substance can withstand before losing its ability to act as an insulator and beginning to permit current to flow through it.
It is generally expressed in volts per unit thickness, i.e., volts per meter (V/m) or kilovolts per centimeter (kV/cm).
For example:
Air has a dielectric strength of about 3 × 10⁶ V/m.
Mica has a dielectric strength of around 100 × 10⁶ V/m.
Glass has a dielectric strength of about 40 × 10⁶ V/m.
This means mica can resist a much stronger electric field compared to air or glass before breakdown occurs.
Explanation of Breakdown
Since a dielectric has no free charges, it is typically an insulator. However, the electrons attached to atoms inside the dielectric may be torn away by an extremely high electric field.
A chain reaction results when these liberated electrons begin to move, clash with other atoms, and release even more electrons. The material stops being an insulator and turns conductive once this process, called dielectric breakdown, starts.
The electric field strength at which this happens is the dielectric strength of that material.
Factors Affecting Dielectric Strength
The dielectric strength of a material is not always fixed it depends on several physical and environmental conditions.
Material Purity
Pure materials have higher dielectric strength, while impurities and defects lower it by creating weak spots.
Thickness of Material
Thin samples generally break down more easily because the applied voltage creates a stronger field across them.
Temperature
High temperatures reduce dielectric strength, as atoms vibrate more and release electrons easily.
Moisture and Humidity
Presence of moisture lowers dielectric strength because water conducts electricity slightly.
Frequency of Applied Voltage
Alternating fields at high frequencies may reduce dielectric strength due to continuous polarisation and heating.
Units of Dielectric Strength
The most commonly used unit is volt per meter (V/m). But in practical engineering, values are often very large, so multiples like kV/cm or MV/m are used. For example:
If air has a dielectric strength of 3 × 10⁶ V/m, this means that air will break down if the electric field exceeds this limit.
Importance in Capacitors
Capacitors use a dielectric substance to keep two conducting plates apart in order to store charge. The dielectric strength of the insulator between the plates directly affects the capacitor’s capacity to tolerate a given voltage.
The capacitor operates correctly and retains energy if the applied voltage is less than the dielectric strength.
The capacitor breaks down, short-circuits, and releases the stored charge if the applied voltage is higher than the dielectric strength.
To guarantee safe operation, engineers carefully select dielectrics with the right dielectric strength values when building capacitors for electronics.
Applications of Dielectric Strength
High-Voltage Equipment
Insulators in transformers, circuit breakers, and transmission lines must have high dielectric strength to withstand high voltages.
Capacitors
Different dielectric materials like mica, ceramic, and plastics are chosen based on their dielectric strength for manufacturing capacitors of different ratings.
Cables and Insulation
Power cables are covered with insulating materials such as rubber or PVC to ensure safe transmission of electricity without leakage.
Electronic Devices
Devices like televisions, radios, and computers use capacitors and other components where dielectric strength is a critical factor.
Practical Examples
Air as Dielectric: Lightning is a natural example of dielectric breakdown of air. When the electric field in the atmosphere exceeds the dielectric strength of air, a massive discharge occurs in the form of lightning.
Mica in Capacitors: Mica is widely used in capacitors because of its very high dielectric strength, which allows them to operate at high voltages without breakdown.
Dielectric constants and dielectric strength
Dielectric | Dielectric constant | Dielectric strength in kv mm-1 |
Vaccume | 1.00000 | ∞ |
Air | 1.00054 | 0.8 |
Water | 81 | – |
Paper | 3.5 | 14 |
Pyrex glass | 4.5 | 13 |
Mica | 5.4 | 160 |
porcelain | 6.5 | 4 |
Conclusion
One essential characteristic of insulating materials that determines their resistance to electric breakdown is their dielectric strength. It is quantified as the highest electric field that a dielectric material can withstand before turning conductive.
It is greatly impacted by a number of variables, including moisture, temperature, thickness, and purity. Its practical significance is found in high-voltage equipment design, electrical insulation, and capacitor design.
Summary
The limit of safe use of insulating materials in the presence of strong electric fields is determined by their dielectric strength. It guarantees that equipment in huge power systems as well as home gadgets operates effectively, safely, and dependably.
Dielectric strength is the maximum electric field a dielectric material can withstand without losing its insulating property and undergoing electrical breakdown. It is measured in volts per unit length, such as V/m or kV/cm.
Permittivity determines how easily a dielectric gets polarised in an electric field, affecting capacitance.
Dielectric strength indicates the limit of the electric field the dielectric can endure before breakdown.
Thus, permittivity relates to storage capacity, while dielectric strength relates to safety against breakdown.