Answer:
The EMF of each battery is 6 volts and the EMF of two batteries connected in series is 12 volts.
Explanation:
According to the Kirchhoff's Voltage Law, net voltage of a closed circuit must be zero. That is:
(1)
Where
is the voltage of the i-th component of the closed circuit, measured in volts. Please notice that voltage for a power generating element (i.e. EMF) is less than zero and for a power consuming element is greater than zero (i.e. Batteries being charged by the EMF). In addition, if n-elements are connected in series, then the equivalent voltage is:
(2)
Let consider that closed system is formed by two batteries connected in series and an EMF, by (1) and (2) we have that:

(3)
If we know that
, then the EMF of the battery is:

The EMF of each battery is 6 volts and the EMF of two batteries connected in series is 12 volts.
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