Charges/Current at a junction How is current distributed into two or more wires joined to an original wire at a junction? Is there a rule that allows us to find out the current in a single wire if the currents in the rest of the wires joined at the junction are known?
 A: This specific case is treated by Kirchhoff's circuit laws or more specifically Kirchhoff's junction law. It states that the algebraic sum of the currents flowing into a junction equals the sum of the currents flowing out.
$\sum{I_{in}} = \sum{I_{out}}$
I suggest you read https://www.boundless.com/physics/textbooks/boundless-physics-textbook/circuits-and-direct-currents-20/kirchhoff-s-rules-152/the-junction-rule-539-6331/ to understand the principles and how to apply them to practical problems.
A: Current describes the movement of charge per unit time. At a given junction, the total amount of charge arriving and leaving per unit time must be the same - otherwise, the junction would end up charging.
Compare this to a traffic junction. The total number of cars arriving at the junction per unit time must be the same as the number of cars leaving - if fewer cars leave, you end up with a pile-up of cars and that is not sustainable.
Mathematically, we can write $\sum I_i = 0$ which is known as Kirchhoff's current law (KCL). If, for example, you have two wires carrying 1 A into a junction, then the remaining (third) wire must carry 2 A away from the junction.
