Question

MCAT Physics Mini-Tutorial: Capacitors & Dialectrics

Answer 1

\({\bf{Capacitors:}}\)
- for the MCAT you will mostly be concerned with parallel-plate capacitors which are typically two metal plates connected to a voltage source and separated by some distance
- capacitance: Q/V basically, stored charge/voltage
- units are the Farad but are often given as microFarads since 1 Farad is a very large amount
- for parallel plate capacitors:

C = ε_0 * (A/d) where ε_0 is the permittivity of free space (given on the constants sheet), A is the area of the capacitor surface, and d is the distance between them

- capacitors also produce a uniform electric field given by

E = V/d with the field lines pointing from the positive plate to the negative plate

- capacitors also store potential energy U = (1/2)CV^2

Answer 2

\({\bf{Dialectrics:}}\)
- dialectric material: an insulating material placed between the plates of a capacitor to increase the capacitance
- increases by a factor κ (kappa). new capacitance = κ * old capacitance
- isolated capacitors (aka not connected to a circuit): dialecctric decreases the voltage while charge stays the same
- circuit capacitors: dialectric increases the stored charge while voltage stays the same
- the energy from a capcitor can discharge across the plates or by adding a conductive material

Answer 3

\({\bf{Capacitor~Calculations:}}\)
- for capacitors in series: 1/total capacitance = sum of 1/C for each capacitor
- for capacitors in parallel: total capacitance = sum of all capacitances

note: this rule is the opposite of the rules for resistors

Answer 4

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the Third Edition Kaplan Physics & Mathematics Prep Book for the new MCAT