---
title: "Dielectrics and the Polarisation of Matter"
description: "An insulator in a field develops aligned dipoles whose own field opposes the applied one, cutting the field inside by the dielectric constant. That is why filling a capacitor with plastic raises its c"
canonical: https://lightmysky.com/learn/science/dielectrics-and-the-polarisation-of-matter-mt_OYRr_k3IW6
source: https://lightmysky.com/learn/science/dielectrics-and-the-polarisation-of-matter-mt_OYRr_k3IW6.md
retrieved: 2026-09-12
---

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# Dielectrics and the Polarisation of Matter

An insulator in a field develops aligned dipoles whose own field opposes the applied one, cutting the field inside by the dielectric constant. That is why filling a capacitor with plastic raises its capacitance.

Subject: Science · Area: Electricity & Magnetism · Ages 19 to 20
Page: https://lightmysky.com/learn/science/dielectrics-and-the-polarisation-of-matter-mt_OYRr_k3IW6

## Ready when they can

- Explains polarisation for polar and non-polar molecules and the field it produces
- Relates the dielectric constant to the reduction of the field inside the material
- Predicts how capacitance, stored charge and voltage change when a dielectric is inserted

## Lesson: The insulator that boosts a capacitor

An insulator in a field still responds: its molecules become aligned dipoles whose own field opposes the applied one. The net field inside drops by the dielectric constant kappa. Polar molecules already own dipoles that swing into line, while nonpolar ones grow induced dipoles as their electron clouds shift. No free charges move, yet the slab reshapes the whole field.

**Example.** Slide plastic into a capacitor and the capacitance multiplies by kappa. With charge fixed and lonely, the same Q needs less voltage: V drops by kappa and the stored energy drops too. With a battery holding voltage fixed, extra charge flows on instead, raising stored charge and energy. A slab of kappa 2 on an isolated capacitor halves its voltage. Either way the bound charge layer on the slab faces does the work.

The molecular picture explains the faces: aligned dipoles leave bound charge on the slab surfaces, and that layer makes the opposing field. Shape sets the base capacitance, since bigger plates or a smaller gap raise it, and the dielectric multiplies whatever shape gives. Capacitance is charge per volt, and stored energy grows with the square of the voltage.

**Tip.** Predict before computing: name what is fixed, Q or V. Fixed Q means V falls by kappa; fixed V means Q climbs by kappa. Mixing up the two cases is the classic slip. Kim's version, where capacitance falls by kappa, gets it backwards: capacitance multiplies while voltage falls.

**Recap.** Aligned dipoles oppose the field, capacitance grows by kappa, and the fixed quantity decides what changes.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Recovering the Field from the Potential Gradient](https://lightmysky.com/learn/science/recovering-the-field-from-the-potential-gradient-mt_EqCLB9bqG1)
- [Capacitance and the Energy Stored in a Capacitor](https://lightmysky.com/learn/science/capacitance-and-the-energy-stored-in-a-capacitor-mt_j1Eltr9z6l)

## Opens up

- [Magnetisation: Diamagnetism, Paramagnetism and Ferromagnetism](https://lightmysky.com/learn/science/magnetisation-diamagnetism-paramagnetism-and-ferromagnetism-mt_1apyQ1_1gg)
- [Current Density, Drift Velocity and the Microscopic Ohm's Law](https://lightmysky.com/learn/science/current-density-drift-velocity-and-the-microscopic-ohms-law-mt_QU-HkQrkFV)