How do I know what the internal resistance of a battery is in a diagram?

[chiddler]

Homework Statement

I am trying to find terminal voltage of the batteries in the circuit shown.

Homework Equations

V = IR

The Attempt at a Solution

The solution manual says that it's the battery voltage, v, minus the voltage drawn from the resistance v = I * R. I understand this.

I do not understand how I'm supposed to know that the 1 ohm resistor adjacent to each battery corresponds to the battery's internal resistance. I would very much appreciate help with this. Thank you!

 

[ideasrule]

It's labeled with "r". There's no rhyme or reason to this; it's just convention that whenever you see a resistor labeled "r" beside a battery, you should assume that it's internal resistance.