Given the following question,

Two batteries are connected in parallel. The emf and internal resistance of one battery are 120 V and 10 Ω respectively and the corresponding values for the other are 150 V and 20 Ω. A resistor of 50 Ω is connected across the battery terminals. Draw the circuit and calculate the current through the 50 Ω resistor.
Would both of the circuits below satisfy the problem (the difference is in the polarity of the voltage sources)?

 

Hi,

I hate to say this but the answer to this question depends highly on what you have learned in the past in your course. I say this because the definition of parallel varies a little when we are dealing with signed polarized components.
Some will say that for two batteries to be in parallel their positive terminals must be connected together and the same for the negative terminals, and connecting with one in reverse is either called "antiparallel" or just in series. That's probably the most reasonable, however two elements connected with both terminals connected either way might be called in parallel too (second diagram).
I would probably go with the first idea that the second diagram shows batteries in antiparallel or in series and only the first diagram shows them in parallel, but if you have learned about series and parallel circuits you should show what you learned so far about parallel and series circuits. That would help decide for sure.

 

Hi Mr Ai,

Thank you for your answer. I think your explanation is quite sufficient. I was considering the fact of current division in the from either source through the load.

Cheers.

 

My guess is there is an unstated assumption the batteries are connected as you draw on the left figure, but you are correct there are other ways, in fact there are four different ways to connect two batteries. That is why battery holders are so well marked.

 

Either interpretation is arguably correct from a purely strict, legalistic viewpoint. But human beings are very good at properly inferring intent from incomplete descriptions and, as a result, we tend to be sloppy and frequently do not provide bullet-proof descriptions because we assume that the reader is a human being with a shred of common sense. This can cause problems, either because the reader does NOT have a shred of common sense, or because the description is too incomplete, often in some subtle way.

In this case, common sense argues strongly that when "batteries are connected in parallel" that the positive terminal are connected and the negative terminals are connected. But it's good that you are seeing the potential for multiple interpretations -- that should serve you well down the road.

 

Thank you Ernie and Bahn. your contributions are very useful.