What is the EMF of the battery if the sources are the same and the polarity of connecting one of them has been changed?

With a series connection of sources, the EMF of the battery is the sum of the EMF of individual sources

EMF = EMF1 + EMF2.

If one of the sources has reversed its polarity, then

EMF = EMF1 + (- EMF2).

Since EMF1 = EMF2, then the EMF of the battery

EMF = EMF1 – EMF1 = 0.

Answer: The EMF of the battery is 0 V.

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