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Electricity · Physics

Dielectrics reduce fields by polarizing.

In an electric field, dielectric materials polarize. The induced bound charges reshape the field, changing capacitance and stored energy in predictable ways.

This topic

Dielectrics

Understand polarization, dielectric constant, how dielectrics change capacitance, and how energy depends on constraints.

Mechanism
Polarization of dielectrics
A dielectric contains bound charges that can shift slightly in response to an electric field. This creates induced dipoles and surface bound charge, altering the net field.
  • Microscopic picture: induced dipoles
  • Bound vs free charge (conceptual distinction)
  • Polarization reduces the field inside the material
  • Why dielectrics are insulators (charges do not flow freely)
Start nano-lesson Bound charge reshapes the field.
Parameter
Dielectric constant
The dielectric constant (relative permittivity) describes how a material modifies electric fields and capacitance compared with vacuum.
  • κ (kappa) as a material factor
  • Effective reduction of E for the same free charge
  • Why κ > 1 for ordinary dielectrics
  • Limits: breakdown and non-ideal behavior (preview)
Start nano-lesson κ summarizes the material response.
Effect
Effect on capacitance
Inserting a dielectric between capacitor plates increases capacitance because the same voltage can support more free charge when the field is reduced by polarization.
  • Parallel plates: C increases by a factor κ (ideal)
  • If connected to a battery: V fixed, Q increases
  • If isolated: Q fixed, V decreases
  • Key idea: constraint determines what changes
Start nano-lesson Always ask: is V fixed or Q fixed?
Energy
Energy considerations with dielectrics
Energy changes depend on constraints. With fixed voltage, increased capacitance lowers stored energy; with fixed charge, increased capacitance also lowers energy, but the physical explanation differs.
  • Energy in terms of C, Q, and V
  • Battery-connected vs isolated capacitor cases
  • Where the energy difference goes (conceptual)
  • Why dielectrics can be pulled into capacitors (preview)
Start nano-lesson Energy depends on what is held fixed.
Practice
Practice & Exercises
Practice polarization ideas, how κ changes capacitance, and energy changes under different constraints.
  • Identify free vs bound charge qualitatively
  • Compute new C after inserting a dielectric (ideal)
  • Battery-connected: find Q, E, and energy changes
  • Isolated: find V, E, and energy changes
  • Exam-style dielectric sets (conceptual + computational)
Open practice sets State the constraint before you compute.