Current carriers :
The charged particles which constitute an electric current in solids , liquids or gases are knowns as current carriers.
a) Solids :
In conductors (e.g. metal like Copper , Silver , Aluminium Etc ) , Free Electrons constitute an electric current.
Electrons in the outermost orbits of the atoms ( Valence electrons) of conductors are loosely bound. These electrons can move about in the whole conductor are known as Free Electrons.
Under the effect of external electric field , these free electron move in a direction opposite to the direction of the external field and constitute electric current.
Free electrons or valance electrons are the current carriers in conductors.
In insulators , all the electrons are tightly bound to their parent atoms . Hence they do not have free electrons and as such there is practically no current carrier in an insulator.
In semi conductor, the current carriers are free electrons and holes.
b) Liquids :
Some liquids which allow electric current to pass through them and consequently dissociate into ions are called electrolytes . Ex - CuSO4 solution , AgNo3 , NaCl solution, H2S04 etc.
These electrolytes dissociate into positive and negative ions ( Cu+2 , SO4-2 , Ag+ , No3- , Na+ , Cl- ). Under the influence of external electric field , the positive and negative ions of the electrolyte move in definite directions to constitute electric current . Thus , positive and negative ions are the current carriers in the liquids.
C) Gases :
Generally , gases behave as insulators of electricity but thet get ionized and become Conductor at low pressure when high potential difference is applied across them.
Ionized gas contains positive ions and electrons . These positive ions and electrons are the current carrier in gases.