Dielectric

Also called Dielectric constant, kappa

A dielectric is an insulating material placed between the plates of a capacitor. It polarizes in the applied field and sets up a field opposing it, which weakens the net field in the gap and multiplies the capacitance by the dielectric constant kappa.

A dielectric is an insulator doing a particular job. Its charges are not free to move as they are in a conductor, so the material polarizes instead, creating a field opposite in direction to the external one. The net field in the gap is weaker, the same charge sits at a smaller potential difference, and by $C = Q/\Delta V$ the capacitance is larger.

The dielectric constant $\kappa$ is the factor. It relates the permittivity of the material to the permittivity of free space, $\kappa = \varepsilon / \varepsilon_0$ , it is a pure number with no units, and it is 1 for vacuum. Both the AP Physics 2 and AP Physics C: E&M sheets print $C = \kappa \varepsilon_0 A / d$ .

Inserting a slab multiplies $C$ by $\kappa$ either way. Everything else turns on one question: is the battery still connected?

Quantity	Battery attached, $\Delta V$ held fixed	Battery removed first, $Q$ held fixed
Capacitance $C$	$\times \kappa$	$\times \kappa$
Charge $Q$	$\times \kappa$	unchanged
Potential difference $\Delta V$	unchanged	$\div \kappa$
Field in the gap	unchanged	$\div \kappa$

The CED states the right-hand column outright: the field between the plates of an isolated parallel-plate capacitor decreases when a dielectric is inserted. Isolated is the word that means the charge is stuck.

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