Technology / Technical guide
Capacitor Dielectrics: Polarisation, Construction and Energy
A capacitor stores electric-field energy between conductive regions separated by a dielectric. Material, geometry and construction influence its behaviour. Names such as film, ceramic and electrolytic describe different aspects of that construction; they are not interchangeable claims about efficiency or the components installed in a converter.
The dielectric changes the field relationship, while the boundary fixes the energy comparison.
Compare charge and voltage conditions before interpreting what a higher capacitance means.
01 / Material role
Insulating material and conductive structure have different jobs.
The conductive regions carry free charge.
The conductive regions carry free charge. The dielectric supports an electric field and can polarise in response to it. Polarisation involves a material response, not free current passing through an ideal dielectric as it would through a resistor.
TDK’s capacitor primer distinguishes dielectric-based constructions. Plastic film can form the dielectric in a film capacitor, and ceramic material in a ceramic capacitor. In an electrolytic construction, a thin oxide layer provides the dielectric; the electrolyte is associated with the conductive counter-electrode structure rather than replacing that insulating oxide.
02 / Geometry model
A simple capacitance formula has explicit assumptions.
For ideal parallel plates with uniform linear dielectric, negligible fringing, plate area A and separation d, capacitance is C = εA/d.
For ideal parallel plates with uniform linear dielectric, negligible fringing, plate area A and separation d, capacitance is C = εA/d. Here ε is the absolute permittivity in F/m, A is in m² and d in metres. Real wound or multilayer constructions require their applicable geometry.
Permittivity concerns the relationship between electric field and displacement within the declared model. A higher permittivity can increase capacitance at unchanged ideal geometry. It does not alone establish a permissible field, leakage behaviour, temperature range or energy density for a complete component.
03 / Polarisation effect
The result differs when charge cannot change.
With fixed free charge on an isolated ideal capacitor, dielectric polarisation reduces the internal field and voltage relative to the same empty geometry.
With fixed free charge on an isolated ideal capacitor, dielectric polarisation reduces the internal field and voltage relative to the same empty geometry. The increased capacitance follows C = Q/V. OpenStax explains this fixed-charge comparison without implying that every practical capacitor is an isolated system.
If an ideal voltage source instead maintains voltage, charge can change as capacitance changes. The external source then participates in the energy balance. Comparing the two experiments without stating the electrical boundary can produce apparently contradictory claims about what dielectric insertion does to stored energy.
04 / Energy comparison
Keep either voltage or charge fixed explicitly.
For constant linear capacitance, W = ½CV² = Q²/(2C).
For constant linear capacitance, W = ½CV² = Q²/(2C). At fixed voltage, doubling C doubles stored energy. At fixed charge, doubling C halves it. These statements concern different neighbouring states, so neither disproves the other.
Hypothetically take C = 1 µF and V = 10 V, giving 50 µJ. Doubling capacitance at the same voltage gives 100 µJ. Retaining the original 10 µC charge with the doubled capacitance instead gives 25 µJ. These invented values describe no actual converter capacitor or operating voltage.
05 / Construction effects
One material label does not describe every nonideality.
Electrode arrangement, material response and internal connections contribute to practical frequency and temperature behaviour.
Electrode arrangement, material response and internal connections contribute to practical frequency and temperature behaviour. A material family may include several constructions with different dependencies. Ranking every film, ceramic and electrolytic component with one universal performance order would discard those differences.
The loss-tangent guide concerns phase-related dissipation. The ripple and ESR guide concerns an equivalent resistive contribution. Neither is a substitute for the dielectric, geometric and electrical-state distinction developed here.
06 / Product scope
An energy formula does not identify installed materials.
The ideal expressions describe state relationships, rather than capacitor life, accepted transient stress or component suitability.
The ideal expressions describe state relationships, rather than capacitor life, accepted transient stress or component suitability. A constant C approximation can also fail when capacitance depends strongly on voltage, temperature or frequency; the applicable state relation is then needed.
No film, ceramic, electrolytic or other capacitor construction is specified for a MARSTON converter by this page. It provides no replacement choice or connection instructions. A generator’s permanent-magnet architecture likewise does not identify the dielectric materials used by separate power electronics.
Related technical guides: Dielectric Loss Tangent: Phase, Frequency and Equivalent Models · Filter-Capacitor Reactive Power and Cyclic Stored Energy · Inductor Air Gaps: Magnetic Reluctance and Stored Energy.
General background: TDK: Dielectric Materials in Capacitors · OpenStax: Capacitor with a Dielectric. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Dielectric | Insulating, polarising region | Distinct from conductive electrode structure. |
| Capacitance | C = εA/d | Uniform linear parallel-plate idealisation. |
| Fixed voltage | W = ½CV² | Source can supply changing charge. |
| Fixed charge | W = Q²/(2C) | Isolated ideal state comparison. |
| Material label | Film, ceramic or oxide construction | Does not establish a complete performance ranking. |
Engineering questions.
Is the electrolyte the insulating dielectric?
In the described electrolytic construction, the oxide layer provides the dielectric.
Does higher capacitance always mean more stored energy?
Only with the comparison condition specified; fixed voltage and fixed charge give different trends.
Can C = εA/d describe every capacitor exactly?
No. It is a uniform parallel-plate idealisation.
Does dielectric polarisation mean ideal resistive conduction?
No. Polarisation and conductive current are different material responses.
Are the microfarad and voltage values real product parameters?
No. They are hypothetical quantities for comparing energy states.
Does this guide identify MARSTON capacitor materials?
No. Component constructions are not inferred from the general converter context.
Technical enquiry
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A capacitor stores electric-field energy between conductive regions separated by a dielectric. Share the model and applicable operating or design information for an engineering discussion.