Two battery system voltage question

[drinfinity]

Daughter asked me about this on Alevel Physics sample paper. Not a real paper, so could have some dodgy information in it.

Setup is a bit like charging a leisure battery from the starter battery, hence I’m asking for expert thoughts here.

Two batteries in parallel, with different emfs.
In part 2, the mark scheme suggests the voltage across the 30ohm resistor when variable resistor is set to 0 is 10V.

I don’t understand how the voltage across one battery is independent of the other battery. If that was the case, a split charge unit would never work?

 

[Sim60]

The two batteries are not connected directly together. - There is R2 in between. One end is at 12V. With R1= 0, then the other end is directly connected to V1. So voltage across R2 = 12-10 = 2V.
Current through R2 is 2V/10Ohm = 0.2A

 

[FWhite]

V1 and V2 represent IDEAL voltage generators=>once R1 is removed (0 Ohm) the voltage across R3 is BY DEFINITION 10V.
This also brings that the current I2 is simply (V2-V1)/R2=(12-10)/10=0.2 A

Real life accumulators are NOT ideal. They always have a a series resistance (that can be modeled in the same position as R1)

 

[drinfinity]

ok, thanks, got it. The ideal V generator is the part I was missing here.