The resistance of a material is the opposition to any current that passes through it. All materials conduct electric current, but some of them conduct well - they have a low resistance - and some conduct badly - they have a high resistance. All metals are good conductors and have a low resistance. . Metals have lots of electrons that are free to move within the metals and so carry the current.
Many materials have very few free electrons. A high voltage can be applied across any object, high enough to tear electrons away from atoms and enable them to carry current.
Ohm's Law states that the resistance  
  of a conductor is equal to the voltage  
  divided by the current through the conductor  
  and we can write  
. The units of resistance are Ohms (symbol  
), and the ohm is defined from the equation above as the resistance of a conductor requiring a potential difference of 1 volt to produce a current of 1 Amp.
More fundamental than resistance is the concept of resistivity.

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