Explain the Factors on which the electromotive force depends?

Electromotive force (emf) is the external work expended
per unit of charge to produce an electric potential difference across two open-circuited
terminals. The electric potential difference is caused by the presence of an electric
field which separates positive and negative charges. The created electrical potential
difference can drive current flow if a circuit is attached to the source of emf.
Any method that creates an electric field, or separates charges creating an electric
field, can be a source of emf. The most common methods are chemical reactions,
thermodynamics, electromotive force, or radiation induced charge
separation.

A battery is an example of a chemical reaction
that stores separated charge. Molecules are held together by chemical bonds. The
molecule in isolation is a stable entity, but when different molecules are brought
together, some types of molecules are able to steal electrons from others, resulting in
charge separation.

Thermodynamics forces an equilibruim to
develop between any two materials brought into contact with each other. If the thermal
energy is sufficient, electrons can be stripped from one material and transfered to
another.

Electromotive force is a common technique for
creating emf, as it takes mechanical energy and converts it into voltage using
electromagnetic induction. Generators use this technique to produce
electricity.

All light possesses energy, and this energy
can be converted from one form to another, for example heat or voltage/current. Solar
cells use this principle to capture incoming radiation, at which point an electron is
forced loose from it's molecule. The electron is then captured by the solar cell, and
contributes to the emf of the cell.