please explain concept of emf in a cell and what kind of questions

 a device that maintains potential difference between two points in a circuit is said to develop electromotive force.(emf)

 the chemical action taking place in the cell removes elctrons frome anode)(example – cu plate) transfer them to cathode(example-zn).This transfer of electrons place through the agency of dil.solu(example H2So4).consequentle anode plate attains a positive charge and cathode plate attains a negative charge.The chemical action has done certain amount of work to do so.

If two plates are connected externally through wire.then electrons flow from cathode to anode.

thus current isflowing in the circuit. 

The chemical action of cell now transfer equal amount electrons frome anode to cathode internally through the cell to maintain potential difference between the rods.This process in continues as long as circuit is complete or so long as there is a chemical energy Thus potential difference causes the current flow while emf maintains potential difference.