Definition of Dielectric
Dielectric is an electrical insulator that is polarized by an applied electric field. When a dielectric material is kept in an electric field, electric charges do not flow through the material as they flow in an electrical conductor. They only slightly shift from their average equilibrium positions and cause dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the direction which is opposite to the field. This further creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric composes of weakly bonded molecules, those molecules become polarized, then reorient.
What are Capacitors?
Capacitors can also be called Electric-condensers. A capacitor is a two-terminal electronic component that has the capacity to store energy in the form of an electric charge. They are usually designed to enhance and increase the effect of capacitance. So, they take into account properties like size and shape. The storage capacity of capacitance varies from small to high storage.
Use of Dielectric in Capacitors
Manufactured capacitors use a solid dielectric material as the intervening medium between the stored positive and negative charges. The advantage of using such a dielectric material is that it prevents the conducting plate from coming into direct electrical contact. However, a high permittivity can allow a greater stored charge at a given voltage. It can be seen by treating the case of a linear dielectric with permittivity ε and thickness d between two conducting plates with uniform charge density σε. Here, the charge density is given by
\[\sigma \epsilon = \frac{\epsilon V}{d}\]
And the capacitance per unit area by
\[c = \frac{\sigma \epsilon}{V} = \frac{\epsilon}{d}\]
Through this, it can be seen that a larger ε leads to the greater charge stored and so, greater capacitance.
Dielectric materials used in capacitors are also chosen according to their resistance to ionization. This helps the capacitor to operate at higher voltages before the insulating dielectric ionizes and begins to allow undesirable current.
Effect of Dielectric on Capacitance
When a dielectric is placed between the plates of a parallel plate capacitor occupying the region, the dielectric is polarized by the electric field. The surface charge densities are called σp and - σp.
The dielectric constant of a substance is the phenomenon by which the capacitance increases from its vacuum value when the dielectric is completely inserted in between the plates of the capacitor
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The Capacitance of Parallel Plate Capacitor with Dielectric Slab
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On the two plates, the microscopic dipole moment of the material will shield the charges. So, it will alter the effect of dielectric material that is inserted between the two plates. Materials have a permeability that is given by the relative permeability K
The capacitance is thus given by:
\[C = \frac{\epsilon A}{d} = k\frac{\epsilon_{0}A}{d}\]
By placing the dielectric between the plates, the parallel plate capacitor’s capacitance can be increased because the dielectric has a relative permittivity k greater than 1.k is also sometimes known as Dielectric Constant.
Some Practical Dielectrics
Dielectric materials can be solids, liquids, or gases. Solid dielectrics are most commonly used in electrical engineering, and they are very good insulators. Examples include porcelain, glass, and plastics. Air, nitrogen, and sulfur hexafluoride are three commonly used gaseous dielectrics.
Industrial coatings provide a dielectric barrier between the substrate and its environment. For example- Parylene.
Mineral oil is used inside electrical transformers as a fluid dielectric and to assist in cooling. Dielectric fluids that have higher dielectric constants are often used in high voltage capacitors to help prevent corona discharge and increase the capacitance.
As dielectrics resist the flow of electricity, the surface of a dielectric can retain stranded excess electrical charges. This may occur when the dielectric is rubbed. This can be useful or it can be potentially destructive as in the case of electrostatic discharge.
Did You Know?
The value of the static dielectric constant of all materials is greater than one, its value for a vacuum.
The value of the dielectric constant at room temperature (25°C, or 77°F) is 1.00059 for air, 2.25 for paraffin, 78.2 for water, and about 2,000 for barium titanate (BaTiO3) when the electric field is applied perpendicularly to the principal axis of the crystal.
As the value of the dielectric constant for air is nearly the same as a vacuum, air does not increase the capacitance of a capacitor for all practical purposes.
Dielectric constants of liquids and solids are determined by comparing the value of capacitance when the dielectric is in place to its value when the capacitor is filled with air.
The presence of dielectric material affects all other electrical phenomena.
The capacitance of a capacitor filled with a dielectric is greater than it is in a vacuum.
FAQs on Effect of Dielectric Capacitance
1. What is the main effect of inserting a dielectric material into a capacitor?
The primary effect of inserting a dielectric material, such as glass or mica, between the plates of a capacitor is that it increases its capacitance. The new capacitance (C) becomes 'K' times the original capacitance (C₀), where K is the dielectric constant of the material. This allows the capacitor to store more charge for the same voltage.
2. What is the formula for the capacitance of a parallel plate capacitor with a dielectric slab?
According to the CBSE Class 12 syllabus for 2025-26, the formula for the capacitance (C) of a parallel plate capacitor completely filled with a dielectric material is given by:
C = K * ε₀ * (A / d)
Where:
- K is the dielectric constant of the material (K > 1).
- ε₀ is the permittivity of free space.
- A is the area of the capacitor plates.
- d is the distance between the plates.
3. Why does introducing a dielectric slab increase a capacitor's capacitance?
A dielectric increases capacitance due to a phenomenon called dielectric polarization. When placed in the capacitor's electric field (E₀), the molecules of the dielectric material align themselves to create an internal, opposing electric field (Eᵢ). This opposing field reduces the net electric field (E = E₀ - Eᵢ) between the plates. Since voltage (V) is the product of the electric field and distance (V = E * d), a lower electric field results in a lower potential difference across the plates for the same amount of charge, thereby increasing the capacitance (C = Q/V).
4. What are some common examples of dielectric materials used in capacitors?
Dielectric materials are essentially insulators. Common examples used in practical applications include:
- Solids: Mica, paper, ceramic, glass, and various plastics like polyester.
- Liquids: Mineral oil, often used in high-power transformers and capacitors for cooling and insulation.
- Gases: Air, nitrogen, and sulfur hexafluoride are used in specific types of capacitors and high-voltage equipment.
5. What is the difference between dielectric constant and dielectric strength?
These are two distinct properties of a dielectric material.
- The dielectric constant (K) measures the ability of a material to store electrical energy by increasing capacitance. A higher K means higher capacitance.
- The dielectric strength measures the maximum electric field a material can withstand without breaking down and starting to conduct electricity. It is a measure of the material's insulating capability under high voltage.
6. How does the effect of a dielectric change if the battery remains connected versus being disconnected before insertion?
This is a crucial concept in electrostatics. The effect on the capacitor's parameters depends entirely on the connection to the battery:
- Battery Disconnected (Isolated Capacitor): The charge (Q) on the plates remains constant. When the dielectric is inserted, the capacitance (C) increases, and as a result, the potential difference (V = Q/C) and the stored energy (U = Q²/2C) both decrease.
- Battery Remains Connected: The potential difference (V) across the plates is held constant by the battery. When the dielectric is inserted, the capacitance (C) increases. To maintain the constant voltage, more charge (Q = CV) flows from the battery to the plates, causing both the stored charge (Q) and stored energy (U = ½CV²) to increase.
7. If a dielectric slab is inserted into a charged, isolated capacitor, what happens to the net electric field?
When a dielectric is inserted into an isolated (charged and disconnected) capacitor, the net electric field between the plates decreases. The original electric field (E₀) due to the charge on the plates polarizes the dielectric, creating a smaller, opposing internal field (Eᵢ). The net electric field becomes the vector sum of these two, which is E_net = E₀ - Eᵢ. The net field is reduced by a factor of the dielectric constant, K, so E_net = E₀ / K.
