Advanced Knowledge of Current as Flow of Charges

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Sign convention of current and voltage

The terminal voltage at higher potential is positive whereas the terminal at lower potential is negative.
If the current is flowing from higher potential to lower potential, then the current is a positive value. If the current instead is flowing from lower potential to higher potential, then the current is a negative value.

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Electric current

Rate of flow of charge through a given cross-section of a wire is called electric current. 
It is given by the formula, 

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Unit of current

The unit of current is or

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Bound electrons

The electrons closely bound to the nucleus are called 'bound electrons'. In a metal, these electrons are not free to move.

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Free electrons

Each atom has different shells. These shells contain electrons. The electrons in the outermost shell of a metal are loosely bound by the nucleus. These electrons are free to move. These free electrons are responsible for their conductivity.