Electrical resistance and conductance

The electrical resistance of an object is a measure of its opposition to the flow of <a href="/facts/Electric_current/Dc1xdnUX">electric current</a>. Its <a href="/facts/Multiplicative_inverse/yczdYyCw">reciprocal</a> quantity is electrical conductance, measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with mechanical <a href="/facts/Friction/d0108Rpu">friction</a>. The <a href="/facts/International_System_of_Units/S7YLglNz">SI</a> unit of electrical resistance is the <a href="/facts/Ohm/QNVrlGjw">ohm</a> (<a href="/facts/Omega/1CrSPtKM">Ω</a>), while electrical conductance is measured in <a href="/facts/Siemens_(unit)/kLUHhcCj">siemens</a> (S) (formerly called the 'mho' and then represented by ℧).
The resistance of an object depends in large part on the material it is made of. Objects made of <a href="/facts/Electrical_insulator/SfSXkrh5">electrical insulators</a> like <a href="/facts/Rubber/CbsjJxhY">rubber</a> tend to have very high resistance and low conductance, while objects made of <a href="/facts/Electrical_conductor/BSuyupLP">electrical conductors</a> like metals tend to have very low resistance and high conductance. This relationship is quantified by <a href="/facts/Electrical_resistivity_and_conductivity/akyCKvMy">resistivity or conductivity</a>. The nature of a material is not the only factor in resistance and conductance, however; it also depends on the size and shape of an object because these properties are <a href="/facts/Intensive_and_extensive_properties/UcgSh8EG">extensive rather than intensive</a>. For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. All objects resist electrical current, except for <a href="/facts/Superconductor/pqQAlSzJ">superconductors</a>, which have a resistance of zero.
The resistance R of an object is defined as the ratio of <a href="/facts/Voltage/EN2po6N2">voltage</a> V across it to <a href="/facts/Electric_current/Dc1xdnUX">current</a> I through it, while the conductance G is the reciprocal:

R
        =
        
          
            V
            I
          
        
        ,
        
        G
        =
        
          
            I
            V
          
        
        =
        
          
            1
            R
          
        
        .
      
    
    {\displaystyle R={\frac {V}{I}},\qquad G={\frac {I}{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 <a href="/facts/Constant_(mathematics)/SxlxTWFT">constants</a> (although they will depend on the size and shape of the object, the material it is made of, and other factors like temperature or <a href="/facts/Deformation_(mechanics)/xK7R59oI">strain</a>). This proportionality is called <a href="/facts/Ohm%27s_law/PjfEyTgI">Ohm's law</a>, and materials that satisfy it are called ohmic materials.
In other cases, such as a <a href="/facts/Transformer/EYYfXWrF">transformer</a>, <a href="/facts/Diode/JyaoyGnk">diode</a> or <a href="/facts/Battery_(electricity)/bqEpm4ue">battery</a>, 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 <a href="/facts/Chord_(geometry)/aAyoEdLd">chord</a> between the origin and an <a href="/facts/Current%E2%80%93voltage_characteristic/wft425cq">I–V curve</a>. In other situations, the <a href="/facts/Derivative/7Ito70Dh">derivative</a> 
 
 
 
 
 
 
 
 d
 
 V
 
 
 
 d
 
 I
 
 
 
 
 
 {\textstyle {\frac {\mathrm {d} V}{\mathrm {d} I}}}
 
 may be most useful; this is called the differential resistance.

Electrical resistance and conductance open-in-new

Electrical resistance and conductance