EC-5.5 Dielectric and Magnetic Materials
You can explain what a dielectric does inside a capacitor, what limits how much voltage it will stand, and why some materials respond strongly to a magnetic field and most do not.
Before:nothing requiredUnlocks:PE2-5. MEMSEC-6. Semiconductor Devices and Device Models
Every capacitor and every inductor is a material choice as much as a geometry, and a component chosen without understanding the material is a component that fails in a way nobody predicted. This topic covers what happens inside the insulator when a field is applied, why that increases capacitance, what eventually breaks, and the magnetic counterpart. It is also where the MOS capacitor in the devices module gets its physics, and where loss in a real component stops being a mysterious extra term.
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Polarisation: what a field does inside an insulator
An applied field pulls positive and negative charge in a material slightly apart without letting either flow, and the separated charge sets up a field opposing the applied one. That opposition is what lets more charge be stored for the same voltage.
NPTEL: Fundamentals of Electronic Materials and Devices · CoursePermittivity, and why it multiplies capacitance
Relative permittivity says how much more charge a material holds than a vacuum would for the same voltage and shape. Choosing a material with a higher value is one of the three ways to make a capacitor larger without making it bigger.
The four polarisation mechanisms, and why permittivity falls with frequency
Electronic, ionic, dipolar and interfacial polarisation each respond at their own speed, and each stops contributing above a frequency it cannot follow. A capacitor is therefore a different capacitor at one megahertz than at one kilohertz.
NPTEL: Physics of Materials · CourseDielectric loss, and where the heat in a capacitor comes from
Polarisation that lags behind the applied field turns some energy into heat every cycle, which appears as a loss term rather than as ideal storage. This is why a capacitor has an equivalent series resistance and why it warms up.
Breakdown, and what the voltage rating on a part actually means
Above a field strength the material can no longer hold its charge apart and it conducts, usually destroying itself. The rating is that field multiplied by the thickness, with a margin, and exceeding it briefly is often enough.
NPTEL: Fundamentals of Electronic Materials and Devices · CourseMagnetic materials, and why a few respond enormously
Most materials barely react to a magnetic field, but in a few the atomic moments align with each other and reinforce, giving a response thousands of times larger. Those are the materials used for cores, and their response saturates and lags rather than staying proportional.
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