in a electrochemical cell , when external potential is applied which is less then the 1.1v that current flows from right to left and when it is higher then the 1.1v then the direction of current is reversed ,why is it so? explain

The conditions mentioned in the question is for Daniel cell only when concentration of Zn2+ and Cu2+ ions is 1 M then cell emf is 1.10 V.
The cell reaction occurring is-
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)

When the external opposite potential is increased slowly then the reaction continues to take place till the opposing voltage reaches the value of 1.1 V.
As long as the external opposite is less than 1.1 V, the electron continue to flow from Zn rod to Cu rod and hence current flows from Cu to Zn. Zinc dissolves in anode and copper deposits at cathode. 
When the opposing voltage reaches the value 1.10 V, the cell reaction stops altogether and no current flows through the cell. This is because there is no potential difference across the cell.
Further increase in the external potential now again starts the reaction but in the opposite direction. The cell now starts functioning as an electrolytic cell. In this case, electrical energy is used to carry out non-spontaneous reaction. The electrons flows from Cu to Zn rod and current flows Zn to Cu because direction of current is opposite to flow of electrons, conventionally.

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