# Chapter 27 Current Resistance

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11/4/2012
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```							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
 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
• 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

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