Resistance (electricity) | Wikipedia audio article
Channel: wikipedia tts
Date: 28/06/2019

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance


00:02:41 1 Introduction
00:04:59 2 Conductors and resistors
00:05:38 3 Ohm's law
00:06:47 4 Relation to resistivity and conductivity
00:10:08 5 Measuring resistance
00:10:34 6 Typical resistances
00:10:44 7 Static and differential resistance
00:13:26 8 AC circuits
00:13:36 8.1 Impedance and admittance
00:17:25 8.2 Frequency dependence of resistance
00:18:01 9 Energy dissipation and Joule heating
00:19:31 10 Dependence of resistance on other conditions
00:19:42 10.1 Temperature dependence
00:23:39 10.2 Strain dependence
00:24:27 10.3 Light illumination dependence
00:24:54 11 Superconductivity
00:25:50 12 See also



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SUMMARY
=======
The electrical resistance of an object is a measure of its opposition to the flow of electric current. The inverse quantity is electrical conductance, and is the ease with which an electric current passes.  Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).
The resistance of an object depends in large part on the material it is made of—objects made of electrical insulators like rubber tend to have very high resistance and low conductivity, while objects made of electrical conductors like metals tend to have very low resistance and high conductivity. This material dependence is quantified by resistivity or conductivity. However, resistance and conductance are extensive rather than bulk properties, meaning that they also depend on the size and shape of an object. For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. All objects show some resistance, except for superconductors, which have a resistance of zero.
The resistance (R) of an object is defined as the ratio of voltage across it (V) to current through it (I), while the conductance (G) is the inverse:

  
    
      
        R
        =
        
          
            V
            I
          
        
        ,
        
        G
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            I
            V
          
        
        =
        
          
            1
            R
          
        
      
    
    {\displaystyle R={V \over I},\qquad G={I \over V}={\frac {1}{R}}}
  For a wide variety of materials and conditions, V and I are directly proportional to each other, and therefore R and G are constants (although they will depend on the size and shape of the object, the material it is made of, and other factors like temperature or strain). This proportionality is called Ohm's law, and materials that satisfy it are called ohmic materials.
In other cases, such as a transformer, diode or battery, V and I are not directly proportional. The ratio V/I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance", since it corresponds to the inverse slope of a chord between the origin and an I–V curve. In other situations, the derivative 
  
    
      
        
          
            
              d
              V
            
            
              d
              I
            
          
        
        
        
      
    
    {\displaystyle {\frac {dV}{dI}}\,\!}
   may be most useful; this is called the "differential resistance".