The difference in potential between two electrodes which cause electric current to flow is called

Electromotive force

Explanation:

The  electromotive force or EMF of a cell is the driving force for the current in a circuit.  It is the potential difference between the two electrodes when no current is flowing.

It is this electromotive force that allows and fosters the movement of current  in a circuit.

what is the question?

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.