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Unit 4: Electricity & Magnetism

Unit 4: Electricity & Magnetism

Electrostatics

  • AKA: Static energy; the study of fixed or stationary electric charges.
  • Definition: An object is considered electrified if it possesses a surplus or deficiency of electrons.
  • Electron Mobility: Electrons are movable, which is why negative charges are primarily discussed in the study of electrostatics and electrodynamics.
  • Electrification Methods: Can occur by:
    • Contact
    • Friction
    • Induction
  • Unit of Charge: Coulomb (C)
    • 1~Coulomb = 6.25 \times 10^{18} electron charges.
  • Electric Ground: A universal location for stray electric charges, most commonly the Earth.

Laws of Electrostatics

  1. Charge Interaction: Like charges repel one another; opposite charges attract one another.
  2. Coulomb's Law: The force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
  3. Distribution of Charges:
    • Conductor: Charges distribute uniformly on the external surface of the object.
    • Nonconductor: Charges distribute throughout the object.
  4. Concentration: Charges concentrate on the sharpest curvature of the object's surface.
  5. Electron Movement: Only electrons are free to move on solid conductors.

Electrodynamics

  • Description: The study of electric charges in motion.
  • Potential Energy: Electrical charges possess potential energy due to the force between charged particles (electrons).
    • Electromotive Force (EMF): This potential energy is also described as electromotive force.
    • Voltage: This potential or EMF is most commonly known as voltage, measured in volts (V).
  • Electric Current: Charges will move due to a potential/voltage, which constitutes electricity. Charges in motion are defined as electric current.
    • Measurement: Electric current is measured in Amperes (or Amps, A).
    • 1~Ampere = 1~Coulomb of charges moving per second.
  • Conductor: A substance in which electrons easily move.
    • Conduction Band: An energy level just above the valence shell where electrons can move very easily. In conductors, this band is very close to the valence shell.
  • Semiconductor: Materials whose conductivity depends on specific conditions (e.g., temperature, impurities) of the material. These are crucial to modern electronic circuits.
  • Superconductor: Materials that exhibit no resistance to electron flow below a certain critical temperature. They are important in applications such as MRI main magnet systems.
  • Insulator: A substance that does not allow electrons to move.
    • Has little to no ability for electrons to move; the conduction band is too far away from the valence band.

Electric Circuits

  • Definition: A closed path over which electric charges move.
  • Potential (Voltage): A build-up of charges with the potential to cause them to move.
    • Units: Measured in Volts (V).
    • Relationship: 1~V causes 1~amp of charges to move in a resistance of 1~ohm.
  • Current (Amperes): The flow of charges.
    • Units: Measured in Amperes (or Amps, A).
    • Definition: 1~Amp = 1~Coulomb of charges moving per second.
  • Resistance: Inhibits electron movement, often measured in Ohms (\Omega).
    • Resistance depends on:
      1. Material of conductor: Different materials have different inherent resistances.
      2. Length of conductor: The more length (more distance), the greater the resistance.
      3. Diameter (cross-section) of conductor: An increase in diameter generally decreases resistance.
      4. Temperature of conductor: Generally, an increase in temperature leads to an increase in resistance.
    • These properties are related by Ohm's Law (V = IR).
  • Power (P): The rate at which energy is used or produced in an electrical circuit, measured in watts (W).
    • Formulas:
      • P = IV (Power = Current imes Voltage)
      • P = I^2 R (derived from P = I(IR) by substituting V = IR from Ohm's Law)
    • This is used to calculate the power of x-ray generators and will be explained further in later discussions.

Types of Current

  • Direct Current (DC):
    • Current flows in only one direction, specifically from the positive pole/electrode to the negative pole/electrode.
    • Typically produced by batteries.
  • Alternating Current (AC):
    • The current flow changes direction in a regular pattern of cycles.
    • In the USA, the frequency of AC units is 60~Hz (cycles per second).
    • Produced by generators and transmitted via power lines into homes.

Circuit Configurations

  • Series Circuits: All parts of the circuit are connected in a single, continuous line path of the same conductor.
  • Parallel Circuits: Parts of the circuit are connected in a parallel manner, with the conductor spanning the ends of the parts.

Electronic Components

  • Battery: Stores electrical charges and releases them via a chemical reaction to provide electric potential.
  • Capacitor: Temporarily stores electrical charges on metal plates; cannot produce electrical charges.
  • Diode/Solid-state rectifier: Allows electrons to flow in only one direction.
  • Circuit breakers/Fuses: Protective devices that