RedCrab Math Tutorial
 
 

 



Resistor and Conductance
Ohm's Law
Specific Resistivity and Conductance
Resistors in Series
Resistors in Parallel
Wire Voltage Loss
Electrical Power
 


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Specific Resistance and Conductance

  Specific Resistance
  Specific resistance refers to the resistance of a wire made of a specific material 1 meter long and 1 mm2 in cross-section.
 
  • Wires of equal length and same cross-section made of different materials have different resistances

  • wires of equal length and same cross section of the same material have the same resistance at the same temperature

  • The specific resistance has the symbol ρ (rho)
   
  Electrical Conductance
  The reciprocal of the resistivity is the electrical conductivity or the conductance.
 
  • The specific conductance indicates which length of a wire, with a cross section of 1 mm2, gives the resistance of 1 ohm
  The symbol of the conductance is σ (sigma).
 
  The resistivity is the reciprocal of the specific conductance.
 
   
  Calculate Resistance
  The resistance of a wire is proportional to its length. A triple wire length also gives a triple resistance.
 
  The resistance of a wire is inversely proportional to its cross section. A double cross section gives half the resistance.
 
  Using the resistivity you get the formula:
 
  Using the specific conductance, you get the formula:
 
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  Overview of the specific resistance and the specific conductance of various conductive materials
 
Material Resistance ρ Conductance σ  

 
Silver 0.016 62.5  
Copper 0.018 56  
Gold 0.023 44  
Aluminium 0.029 35  
Magnesium 0.045 22  
Tungsten 0.055 18  
Zinc 0.063 16  
Nickel 0.08 ... 0.011 13 ... 9  
Iron 0.10 ... 0.15 10 ... 7  
Tin 0.11 9  
Platinum 0.11 ... 0.14 9 .. 7  
Lead 0.21 4.8  
Mercury 0.96 1.04  
Bismuth 1.2 0.83  
Carbon 100 0.01  
       
   
  Overview of the resistance of various insulating materials
 
Material Resistance ρ  

 
Amber 1018  
Glass 1014  
Mica 1014 ... 1017  
Rubber 1015  
Hard rubber 1012 ... 1018  
Ceramics 1012  
Resin 108 ... 1014  
Marble 109 ... 1011  
Paraffin 1016  
Polystyrene 1018  
Pressboard 1010  
Porcelain 1014  
Shellac 1016  
     
 
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