EC-5.5 Dielectric and Magnetic Materials

What this is and why it exists

Every capacitor and every inductor is a material choice as much as a shape. A component picked without understanding its material is a component that will fail in a way nobody predicted.

This topic covers what happens inside an insulator when a field is applied, and why that lets it store more charge. Then why the effect weakens at high frequency, where the heat comes from, and what eventually breaks. The magnetic counterpart follows the same shape.

It is also where the transistor in the next module gets its gate physics. And where the mysterious extra loss terms in a real component stop being mysterious.

The vocabulary

• Dielectric — an insulator, considered for what it does inside a field rather than for not conducting.
• Polarisation — the slight separation of positive and negative charge inside a material under a field.
• Permittivity — how much more charge a material lets you store than a vacuum would.
• Dielectric loss — energy turned to heat each cycle because polarisation lags the field.
• Breakdown — the field strength at which an insulator stops insulating.
• Permeability — the magnetic counterpart of permittivity.
• Ferromagnetic — describing the few materials whose atomic moments align with each other.
• Saturation — the point beyond which a magnetic material cannot respond any further.
• Hysteresis — the lag that makes the response depend on what came before.

The mental model

Apply a field across an insulator. Nothing can flow, but charge inside can shift slightly: positive one way, negative the other. That is polarisation, and the separated charge sets up its own field opposing the applied one.

Here is why that matters. The opposing field reduces the net field for a given amount of stored charge, so you can store more charge before reaching the same voltage. More charge at the same voltage is more capacitance. That is the whole reason a capacitor has a dielectric in it rather than air.

Permittivity is the number that says how much more. Choosing a material with a higher one makes a capacitor larger without making it bigger. It is one of only three levers you have; the other two are area and thickness.

Now the part that catches people out. There are four separate mechanisms that produce polarisation, and they respond at different speeds. Electrons shifting within an atom respond almost instantly. Whole ions moving are slower. Molecules rotating are slower still. Charge piling up at internal boundaries is slowest of all.

Each one stops contributing above a frequency it cannot keep up with. So permittivity falls as frequency rises, in steps. A capacitor is genuinely a different capacitor at one megahertz than at one kilohertz. A datasheet quoting one figure is quoting it at one frequency.

Polarisation that lags behind the applied field turns some energy into heat every cycle. That is dielectric loss, and it is why a real capacitor has an equivalent series resistance and why it warms up when driven hard. It is not a manufacturing defect; it is the material.

Breakdown is the limit. Above a certain field strength the material can no longer hold its charge apart, it conducts, and it usually destroys itself doing so. The voltage rating on a part is that field multiplied by the thickness, with a margin. Exceeding it briefly is often enough to ruin the part.

The magnetic side runs in parallel. Most materials barely react to a magnetic field at all. In a few, the atomic magnetic moments line up with each other and reinforce, giving a response thousands of times larger than the applied field. Those are the materials used for transformer and inductor cores.

Two things make them awkward. The response saturates — beyond a point everything is already aligned and there is nothing left to gain. And it shows hysteresis — the response lags, so where the material is now depends on where it has been. Plot the response against the applied field and you get a loop rather than a line. The area of that loop is energy lost as heat on every cycle.

What you should now be able to explain or do

• Explain why a dielectric increases capacitance, in terms of what happens inside it.
• Say why permittivity falls with frequency, and what that means for a datasheet figure.
• Explain where the heat in a hard-driven capacitor comes from.
• Say what a voltage rating physically represents.
• Describe why a few materials respond enormously to a magnetic field, and what saturation and hysteresis cost you.

Check yourself