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.
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 cell (small battery) laying on table, the +ve terminal of the cell (battery) 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 we are trying to make here are..
- Current needs path or a medium to flow.
- 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..).
- Bad conductors are also called insulators.
Now, suppose we connect a pipe between these two beakers, and the water starts flowing..
But now the question is, how long will the water flow ?
Right, the water from full beaker to empty beaker will flow till the water levels in both the beakers become equal. Or water level difference between two beakers become zero.
Similarly when a wire is connected between two terminals of cell in order to provide a 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 difference between 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 so that they can be used again.