Which way do I measure the potential difference

[JessicaHelena]

Homework Statement

All the bulbs in the circuits are identical, as are all the batteries. Rank the magnitude of the potential difference between points M and N.

Homework Equations

V=IR

The Attempt at a Solution

My most fundamental question is actually which way do I know to take the potential difference of? For instance, in A, should I only take the potential difference of the batter, or the other way round (where the bulbs are)? In fact, scenario C seems even more ambiguous to me... But once I get the direction of the potential difference right, I think I can tackle the problem.

 

[CWatters]

The battery terminals are marked + and -.

 

[cnh1995]

For instance, in A, should I only take the potential difference of the batter, or the other way round (where the bulbs are)?

The potential difference between any two points in the circuit is the same regardless of the path you choose (exception exists but that's not relevant here).
What does that tell you about V_MN in A?

 

[JessicaHelena]

@cnh1995 — that it's equal to the battery's voltage? But if it's regardless of the path chosen, I don't quite get C because it seems one way you can get V/2 volts (if V is the battery's voltage) and the other way you can get V + V/2 = 3V/2 volts?

@CWatters — I'm sorry if this should be obvious, but how do the + and - help?

 

[cnh1995]

the other way you can get V + V/2 = 3V/2 volts?

No, you must get V/2 in this case too. BTW, what is your "other way"?
Check the polarity of the voltages you use in this path (Refer the battery polarity (+ and -)).

 

[CWatters]

@CWatters — I'm sorry if this should be obvious, but how do the + and - help?

Gives you the "direction" of the potential difference. Eg it tells you if M is positive or negative with respect to N.

Perhaps I missunderstand what you mean by "direction".

 

[CWatters]

. the other way you can get V + V/2 = 3V/2 volts?

No that other way you get V - V/2 = V/2

Starting at N you go "up" V through the battery then "down" V/2 through the top bulb to get to M.