AP Physics C: Electricity and Magnetism College Board

This subject is broken down into 19 topics in 5 modules:

  1. Conductors, Capacitors, Dielectrics 3 topics
  2. Electric Circuits 4 topics
  3. Electromagnetism 3 topics
  4. Electrostatics 5 topics
  5. Magnetic Fields 4 topics
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This page was last modified on 28 September 2024.

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Physics C: Electricity and Magnetism

Conductors, Capacitors, Dielectrics

Capacistors

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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.

Course material for Physics C: Electricity and Magnetism, module Conductors, Capacitors, Dielectrics, topic Capacistors

Physics C: Electricity and Magnetism

Electrostatics

Electrostatics: Charge and Coulomb's Law

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Electrostatics: Charge and Coulomb's Law

Concept and Nature of Electric Charge

  • Electric charges are core attributes of the fundamental particles that make up atoms. These include negative electrons, and positive protons.
  • The charge of an electron or proton is a discrete quantity, commonly referred to as the elementary charge. An electron has a charge of -1, and a proton has a charge of +1.
    • Remember: an electron and a proton have exactly the same magnitude of charge, but with opposite signs, -1 and +1 respectively.
  • Charges of the same type repel each other while charges of opposite kind attract each other.

Conservation of Charge

  • The law of conservation of charge states that the total charge in an isolated system always remains constant unless new charge is imported or existing charge leaves.
    • In other words, charge can neither be created nor destroyed.
  • Any time a charge is thought to be 'created', in reality, it is just being separated or transferred.

Coulomb's Law

  • Coulomb's law describes the force between two charged particles.
  • The force exerted between two charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The relationship can be expressed using the formula: F = k * (q1 * q2) / r²
    • Here, F is the force, q1 and q2 are charges, r is the distance between the charges, and k is Coulomb's constant.
  • Note the following ways in which the force between two charges can be changed:
    • Increasing the magnitude of the charges will increase the force.
    • Increasing the distance between the charges will decrease the force.
  • Remember: the force is a vector quantity, meaning it has a direction - it will act along a straight line joining the charges.

The Concept of an Electric Field

  • An electric field is a region around a charged particle in which a force would be exerted on other charged particles. It thus represents a field of influence produced by the charged particle.
  • The direction of an electric field is always directed away from positive charges and towards negative charges.
  • The strength of the electric field (E) at any point is defined as the force (F) per unit of charge (q) at that point: E = F / q.
  • Field lines are used to represent the electric field, they point from positive to negative in the direction a small positive charge would move.

This revision content should help you solidify your understanding of the basics of electrostatics. Keep revising these bullet points and remember that practice is key to mastering any topic—especially for physics!

Course material for Physics C: Electricity and Magnetism, module Electrostatics, topic Electrostatics: Charge and Coulomb's Law

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