Chapter 27 Current Resistance

							Chapter 17

Electric Current and resistance
Batteries
 Alessandro Volta (1745-1827)
 Electrodes—dissimilar metals, “terminals of a
  battery”
   – Anode—positive terminal, loses free electrons in the
     acid
      • Manganese Oxide, Carbon, Silver, lead, Nickel
   – Cathode—negative terminal, gains free electrons in the
     acid
      • Zinc, lead dioxide, Cadmium
 Electrolyte—acid in which electrodes sit
Batteries
 Electric cell
  – One set of electrodes sitting in an electrolyte
    solution
  – AA, AAA, C, D, etc. batteries are technically
    electric cells
 Battery
  – Several electric cells connected together
Batteries
 Acidic reaction continues until saturation
  – Potential difference exists between electrodes
 When terminals (outside part of electrodes)
  are connected, electrons flow from cathode
  to anode
   – Acid reaction begins again
   – Potential difference is held constant
Electric Current
 Circuit—conducting path from positive
  terminal to negative terminal
  – Closed circuit—path is continuous and
    uninterrupted
  – Open circuit—path is broken and/or incomplete
 When circuit is closed, charge will move
  continuously
Electric Current
 Electric current—amount of charge flowing per
  second
   – I = current
                              Q
   – Q = charge
                       I
   – t = time
                              t
 Direction—from Positive to Negative
   – Defines as the flow of positive charge, even though the
     electrons are what’s actually moving
 Units of Amperes (A)
  – A = Coulomb per second (C/s)
Resistance
 Georg Simon Ohm (1787-1854)
 Found that Voltage and Current are directly
  related
   – Double the voltage, and current will be doubled
 Found that the ratio, V/I, depended on the wire
  through which current flowed
   – For one wire 1 Volt would cause 1 Ampere of current
   – For another wire, 1 Volt would cause 2 Amperes of
     current
   – Conclusion: Certain wires resisted current flow more
     than others
Resistance
 Definition of Resistance           V
                                  R
  – Applies to all materials         I
  – Units of Ohms ()
 Ohm’s Law
  – Not really a law, but don’t tell Ohm
  – Applies to most metal conductors
  – Assumes R is constant
  – Materials to which it does not apply are called
    “nonohmic”
                     V  IR
Resistance
 General rule: the more obstacles an electron must
  overcome in the wire, the higher the resistance to
  current flow
 For a given wire:              L
                          R
   – L = Length of wire          A
      • The longer the wire, the more atoms to bump into
   – A = cross sectional area of wire
      • The thicker the wire, the more space an electron has
        to move
   –  = “rho” = resistivity of wire
      • Depends on the material out of which the wire is
        made
      • Insulators (1012)
      • Conductors (10-8)
Resistance
 Conductivity ()—the inverse of resistivity
  – If resistivity is high, conductivity is low



                               1
                   
                              
                         0 (1  T )
Resistance
 Temperature dependence of resistivity
  – 0 = resistivity of material at a base
    temperature, usually 0°C or 20°C
  – T = temperature difference from reference
  –  = temperature coefficient of resistivity
 Temperature dependence of Resistance

            R  R0 (1  T )
Superconductivity
 Resistance increases with temperature
 For some materials, resistivity suddenly
  drops to zero at very low temperatures
  – Termed “superconductors”
 Most commonly occur between 23K-90K (-
  250C— -183C)
                       Energy used
               Power 
Electric Power            time
 Power—rate of energy usage
   – Units of Watts (W), Joules per second (J/s)



           P  IV 2
         V
      P
          R               PI R         2