CURRENT

•   When the potential difference between two charges forces a third charge to move, the charge in motion is called current.

•   To produce current, therefore, charge must be moved by a potential difference.

•   In solid materials, such as copper wire, the free electrons are charges that can be forced to move with relative ease by a potential difference, they are required a little work to be moved. As illustrated in fig. if a potential difference is connected across two ends of a copper wire the applied voltage forces the free electron to move.

•   This current is drift of electrons, from the point of negative charge at one end, and returning to the positive charge at the other end.

•   Each electron in the middle row is numbered, corresponding to a copper to which this electron belongs.

•   Considering the case of only one electron moving, note that the electron returning to the positive side of the voltage source is not electron S which left negative side.

•   All electrons are same. Therefore, the drift free electrons resulted in the charge of one electron moving through the wire.

•   This charge in motion is current.

•   Current is the constant flow of electrons.

•   Only the electrons move, not the potential difference.

•   The current must be the same at all points of the wire at all times.

Another definition of current can be made as

•   Let q(t) be the total charge that has passed a reference point since an arbitrary time t=0, moving in the defined direction. A contribution to this total charge will be negative if the negative charge is flowing in the reference direction. 

•   As the figure shows a history of total charge q(t) that has passed a given reference point in a wire.

•   The current at a specific point and flowing in a specified direction as the instantaneous rate at which net positive charge is moving past the point in the specified direction.

•   Current is symbolized as I or i .

•       Mathematically it can be given as

                                                                                I = dq/dt

GRAPHICAL SYMBOLS FOR CURRENT

POTENTIAL DIFFERENCE IS NECESSARY FOR CURRENT

•   The number of free electrons that can be forced to drift through the wire to produce the moving charge depends upon the amount of potential difference across the wire, with more applied voltage, the forces of attraction and repulsion can make more free electrons drift, producing more current.

•   With zero potential difference across the wire, there will be no current.

•   As another case, connecting the same potential across the terminals of a wire will result in no current flow.

THE AMPERE OF CURRENT

•   Sine current is the movement of charge, the unit for stating the amount of currant is defined as the rate of flow charge.

•   When charge moves at the rate of 6.25x1018 electrons flowing past a given point per second, the value of current is one Ampere.

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