Capacitors: Combinations, Energy and the Dielectric Table

In the paper

Unit 5 carries 4 of the 50 physics marks; with three chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). It is the most formulaic chapter of the three and the easiest to bank, because almost every question reduces to C = Q/V, a combination rule, an energy expression, or the dielectric table at the end of this note — which is the single most asked item in the unit.

The MEC scope lines are: Principle of a capacitor; parallel-plate capacitor; Combinations of capacitors and energy density; Effect of a dielectric. The three headings below are those points, in MEC's order.

Three ways it comes:

• Recall — the unit of capacitance, the formula for a parallel-plate capacitor, which quantity is the same for capacitors in series, the expression for energy density.
• Understanding — why the equivalent capacitance in series is smaller than the smallest capacitor, why energy is always lost when a charged capacitor is connected to an uncharged one, why inserting a dielectric raises C.
• Application — the equivalent capacitance of a network, the charge and voltage on each capacitor of a series pair, the energy stored and the energy lost, the new C when a slab fills part of the gap.