Physics B

• Electric Circuit


  an interconnection of electrical elements that provides a closed-loop path for the flow of electrons
• Open, Closed, and Short

  Types of Electric Circuits
• Active and Passive


  Circuit Elements
• Active Elements


  are sources of electrical energy

  
  

  Ex. Voltage and Current source
• Passive Elements


  are elements that use or dissipate electrical energy

  
  

  Ex. Resistor, Inductor, Capacitor, and Memristor
• Voltage


  is directly proportional to the current
• Resistance


  the electrical property of a material to oppose or limit the passage of current through it

  
  

  is the measure of how much a conductor resists the flow of charges
• Conductance, G


  the reciprocal of resistance
• Current


  is the rate at which charge flows through an area

  
  

  I = Q/t = dQ/dt
• Electron flow


  actual direction of current

  
  

  charges flow from negative terminal to positive terminal of the voltage source
• Conventional flow


  direction used in circuit analysis

  
  

  charges flow from positive to negative terminal of the voltage source
• Current Density


  current per unit area

  
  

  J = I/A in Ampere / m^2

  
  

  directly proportional to the Electric Field Strength in the conductor

  
  

  J = I/A = σE

  
  

  where σ = conductivity or specific conductance of a material
• Resistivity


  is the ratio of the electric field and current density

  
  

  ρ = E/J

  
  

  where:

  ρ in V/m

  E in V/m

  J in A/m^2
• greater

  the ______ the length and resistivity and the smaller the cross-sectional area, the _______ the resistance

  
  

  R = ρL/A
• Temperature effects on resistance


  Conductors:

  increased temperature = increased resistance. conductors have a positive temperature coefficient.

  
  

  Semiconductors:

  increased temperature = decreased resistance. semiconductors have a negative temperature coefficient.

  
  

  Insulators:

  same with semiconductors. negative temperature coefficient.
• Electromotive forces


  produces a steady flow of charges to maintain a potential difference between a pair of terminals

  
  

  amount of energy provided by the device to the charges passing through it

  
  

  supplies energy and does NOT apply force
• Charge pump


  the source moves charges uphill from the lower potential to the higher
• internal resistance


  resistance inside the source of EMF which cannot be removed
• Terminal Voltage


  potential difference between two terminals of a practical source of EMF
• When no current exists, VT is equal to the EMF of the battery

  
  

  VT = ε


  Case 1 of Terminal Voltage
• When supplying or delivering power (discharging,) VT is equal to the EMF - the Voltage drop in the total internal resistance

  
  

  VT = ε - Ir


  Case 2 of Terminal Voltage
• When receiving power (charging), VT is equal to EMF plus the voltage drop in the total internal resistance

  
  

  VT = ε + Ir


  Case 3 of Terminal Voltage
• PL = I^2 x R

  
  

  can also be expressed as

  PL = εI - I^2 x r


  Current comes out of its positive terminal

  
  

  Power delivered to the load
• PL = VT x I

  
  

  can also be expressed as:

  PL = εI + I^2 x r


  Current enters its positive terminal

  
  

  Power received by the source