5-6

• Capacitance


  A property of an electric non-conductor that permits the storage of energy
• Capacitance


  The ratio of the magnitude of the charge per volts
• Capacitance formula


  C = q/V
• Farad (F)


  The SI unit of capacitance
• Capacitance will always be a positive quantity
• The capacitance of a given capacitor is constant
• Capacitance


  A measure of the capacitor's ability to store charge
• The farad is a large unit, typically you will see microfarads (mF) and picofarads (pF)
• Compute the capacitance

  1. Value of charge stored

  2. Volt supply
• Determine the volt supply

  1. Capacitance value

  2. Charge stored
• Capacitors


  Devices that store electric charge
• Where capacitors are used

  • Radio receivers
  • Filters in power supplies
  • Energy-storing devices in electronic flashes
• Capacitor


  Consists of two conductors called plates
• When the conductor is charged, the plates carry charges of equal magnitude and opposite directions

  
• A potential difference exists between the plates due to the charge
• Dielectric


  A non-conducting material that, when placed between the plates of a capacitor, increases the capacitance
• Dielectrics


  • Rubber
  • Glass
  • Waxed paper
• Area of conducting plate

  Increase capacitance
• Distance between the conducting plates

  Increase capacitance
• Type of Dielectrics


  • More conducting - Decrease capacitance
  • Less conducting - Increase capacitance
• Role of Dielectrics


  • Impede or block the charges passing through the material
  • Allows the storage of electric current on the material (capacitor)
  • The less conducting/more insulating the dielectrics are, the greater is the capacitance
  • Leakage Current: small amount of current that manage to pass through even with the dielectrics blocking their way
• Shapes of Capacitors


  • Parallel-plate
  • Cylindrical
  • Spherical
• Parallel Plate Capacitor


  • Each plate is connected to a terminal of the battery
  • The battery is a source of potential difference
  • If the capacitor is initially uncharged, the battery establishes an electric field in the connecting wires

  
• Cylindrical Capacitor


  • The inner and outer cylinder structures correspond to the plates
  • The value of the capacitance is directly proportional to each length and cross-sectional area
• Spherical Capacitor


  • Almost similar to the construction of the cylindrical capacitor
  • The value of the capacitance is directly proportional to the overall radius
  • Voltage on this type is also intensified by increasing the radius
• Capacitors in a Circuit


  • Series connection
  • Parallel connection
• Series connection


  1. Total charge stored is constant

  2. Total voltage varies

  3. Total capacitance is reciprocal of sum of reciprocals

  
• Parallel connection


  1. Total charge is sum of individual charges

  2. Total voltage is constant

  3. Total capacitance is sum of individual capacitances

  
• Compute the total values

  1. Capacitance

  2. Voltage

  3. Charge
• Applications of Capacitance


  • Charged Parallel-plate Capacitors
  • Batteries and Electroscopes
  • Camera Flash Lamps
  • Geiger Counters
  • Coaxial Cables
• Charged Parallel-plate Capacitor


  • Dependent on the area and the distance between charged plates
  • C = K.ε0.A/d
  • K = constant; dielectric value of a specific material
  • ε0 = constant; 8.85 x 10^-12 F/m
  • A = area of one of the charged plates
  • d = distance between the plates

  
• Compute the capacitance


  1. Area of parallel plates

  2. Distance between plates
• Batteries and Electroscopes


  • Convenient source of electric energy
  • Effective storage of electrical charges
• Camera Flash Lamps


  • Provide an artificial source of light
  • Connected in an electric circuit that charges a capacitor
• Geiger Counter


  • Used to measure ionizing radiation
  • Made up of a tube filled with noble gases
  • High voltage is applied to the tube
  • An electric current is conducted inside the tube when incident radiation ionizes the gas
• Coaxial Cables


  • Has cylindrical capacitor structure
  • Primarily used in local area networking (LAN)
• Energy is neither created nor destroyed; it can only be transformed from one form to another
• Electric Power


  • The rate of energy conversion
  • P = VI
  • P = V^2/R
• Compute the current


  1. Power

  2. Voltage
• Compute the power


  1. Current

  2. Voltage