what happens if i connect a 10v cell and a 4v cell in parallel and the combination is in series with a resistor of 10 ohms.
assume that the cells have no internal resitance.what is the potential across the parallel combination.is it 10 volts???
how is the answer different if you consider that the cells have internal
resistances.

ok...does some current go to the"weaker" cell and some go to the resistor...
is this practically possible/sensible...

 

what happens if i connect a 10v cell and a 4v cell in parallel and the combination is in series with a resistor of 10 ohms.
assume that the cells have no internal resitance.what is the potential across the parallel combination.is it 10 volts???.

You should not connect a 10V and a 4V batery in parallel. The 10V baterry will "charge" the 4V battery, with explosive results.

how is the answer different if you consider that the cells have internal
resistances.

Assuming that a battery has internal resistance (and real batteries always have), you have to expect a voltage drop to occur through that resistance. You can use the Ohm's law to determine that, if the current that the battery is supplying is known:
Vdrop = Rinternal x I

And so:
Vbattery = Videal - Vdrop = Videal - Rinternal x I

Notice that Vdrop will never be greater than Videal. In a short circuit situation (the most extreme situation you can have), all the voltage drops throught the internal resistance, resulting that Vbattery = 0 (no potential difference across the short circuit element).

 

Tis why you should always replace ALL of your batteries in your electronics!