Current

Current is always in relation with voltage, one can think voltage as a cause and current as its effect.

The relationship between current and voltage can be understood analogically by water flow mechanism.
 

beaker_images

For example consider the above figure showing two beakers. Beaker 1 is filled with water but beaker 2 is empty.

Now, Can you tell at what rate water will flow from beaker 1 to beaker 2 ?

Obviously, there won´t be any flow because there is no path (i.e pipe ) available for water to flow between two beakers.

The above situation is very much similar to a pencil cell laying on table, the +ve terminal of the cell has high potential and -ve terminal has low(zero potential) , but here also there won't be any flow of current between terminals because there is no path.

The points I am trying to make here are..

  1. Current requires path to flow and wires are easy way to provide these paths.
  2. All materials do not allow current to flow through them. The materials which allow current flow are called good conductors-of-current (e.g metals, graphite..), and others which do now allow are called bad conductors-of-current (e.g plastic, wood, fibers etc..).
  3. Bad conductor-of-current are also called insulators.

Now, suppose we connect a pipe between these two beakers, and the water starts flowing..

equal_water_beaker_images

But now the question is, how long will the water flow ?

simple, till the water levels in both the beakers become equal.

Similarly when a wire is connected between two terminals of cell in order to provide path for current to flow how long the current will flow ?

Correct! Till the potential on both the terminals become equal or technically the potential difference across terminal becomes zero.

This is what generally happens when cells die out and we throw them out. Actually the cell is not damaged, but since the potential on both the terminals have become zero, it cannot be further used to maintain current flow.

Rechargeable cells provide facility where potential of one terminal can be raised by re-charging.