Factors Which Control Resistance of a Conductor

The Material

The Length

The cross-sectional area

The temperature

• Positive temperature coefficient material

• Negative temperature coefficient material
  
  

The Material

• Different materials have different resistances.  Some, such as silver and copper, have a low resistance, which others, such as iron or nichrome have a higher resistance
  
  

The Length

• For a given material that has a constant cross-sectional area, the total resistance is proportional to the length
  
  

The Cross-Sectional Area

• Current flow can be compared to flow of matter in a pipe. We know that if we make a pipe bigger (an increase is cross-sectional area), more water will flow even though the pressure is the same.  There is a similar situation with respect to a conductor, in that the resistance decreases as the cross-section increases. If we double the cross-section of a material at constant length, the cross-section only half, the resistance will double.
  
  
• Resistance is INDIRECTLY proportional to the cross-sectional AREA
  
  
• Smaller cross-sectional area higher resistance
  
  

The Temperature

• Though temperature effects are generally small compared to the effects of material, length, and cross-sectional are, the can be important, particularly when we want to keep resistance at a fixed value and the temperature is not constant.  Generally, in metals, the resistance increases as the temperature increases.  This is basically caused by the fact that the heat energy makes the free electrons in the material bounce around readily, and it is more difficult to get these electrons to flow
  
  
• Positive temperature coefficient