how electromotive force is calculated?

Answer 1

Electromotive force (emf) is defined as the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric field produced by that device.

it can be calculated as
E= -Ldi/dt(for electrostatics)
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Answer 2

Electromotive force = F = qE; where q = 1 coulomb and E is the electro magnetic force field. E is just lines of attractive or repulsive energy tugging or pushing on that point charge of 1 coulomb. E is relative to the source of the energy (e.g., another q) and varies inversely as the square of the distance between the source and the affected q particle.

We can write F = kqq'/r^2, where k is a constant of proportionality, q is one charged point and q' is the source of E, and r is the distance between the two points. Thus, E = kq'/r^2 in scalar terms. Notice how the F equation is very similar to F = G mM/R^2, which is Newton's gravitational force equation.

Answer 3

Using Faraday's laws of e.m.induction,
electromotive force(e.m.f)=-rate of change of magnetic flux linked to the circuit