A circuit with two batteries is shown below. The directions of the currents have been chosen (guessed) as shown.

Which is the correct current equation for this circuit?

 Pink: I₂ = I₁ + I₃                  

Yellow: I₁ = I₂ + I₃                             

Green: I₃ = I₁ + I₂

 Blue: None of these.

 Answer: Green, I₃ = I₁ + I₂. Consider the upper junction (marked with a dot in the diagram below). The current into the junction is I₃ (current arrow pointing towards the junction means current into the junction). The current out of the junction is I₁+I₂. Kirchhoff's 2nd law says total current in equals total current out. You get exactly the same answer if you consider the lower junction.