Adding voltage source in series/parallel. Effect on the circuit?

[Lostintime]

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I was wondering what would happen if you added an extra voltage source in parallel/series to the circuit.

I was told that the voltage source would have to be the same V as the existing emf in order to be added parallel? what about in series?

 

[Arctangent]

As I understand it, for parallel, as you said, the voltage must be the same, or you will be breaking Kirchoff's loop law.

For series, voltages basically sum. One point that can be tricky is that direction of voltage sources must be considered. If a battery is reversed in direction, its voltage will subtract rather than add.