AP Physics C: Electricity and Magnetism College Board

Capacistors

Capacitors

Basics

• A capacitor is a two-terminal device used in electronic circuits to store electric charge.
• It is made from two conductors separated by an insulator, or dielectric.
• When a voltage difference is applied across the conductors, an electric field is created, causing positive charge to accumulate on one plate and negative charge on the other.

Charge and Discharge

• The amount of electric charge it holds for a given voltage is measured in farads (F), the SI unit of capacitance.
• Increasing the applied voltage or the surface area of the plates, or decreasing the separation distance between them, increases the capacitance.
• Capacitors can be used for energy storage, as they can rapidly charge and discharge.

Energy Storage

• The energy (E) stored in a capacitor is calculated with the equation E = 0.5CV^2, where V is the voltage across the capacitor and C is the capacitance.
• This stored energy can be released in a short time, making capacitors useful in power surges.

Dielectrics

• A dielectric is a type of insulating material between capacitor plates. It reduces the electric field and increases the capacitance.
• The ability of a dielectric to increase the capacitance of a capacitor is defined as the dielectric constant (K). The capacitance (C) of a capacitor with a dielectric can be calculated with C = K*C', where C' is the capacitance without the dielectric.

Capacitors in Series and Parallel

• Capacitors in series have a total capacitance (C_total) that is less than any of the individual capacitances: 1/C_total = 1/C1 + 1/C2 + ... + 1/Cn.
• Capacitors in parallel have a total capacitance equal to the sum of their individual capacitances: C_total = C1 + C2 + ... + Cn.
• These rules make it possible to solve complex circuits involving multiple capacitors.

Capacitive Reactance

• In an AC circuit, the opposition that a capacitor offers to current flow is called capacitive reactance.
• Capacitive reactance depends on the frequency of the applied AC and is calculated with Xc = 1/(2PifC), where Pi is a constant approximately equal to 3.14159, f is the frequency, and C is the capacitance.

Always remember that capacitors play vital roles in electronic circuits and understanding their principles is crucial to mastering advanced topics in electricity and magnetism. Practice applying these concepts to actual circuit problems can help solidify the knowledge and concepts learnt.