DC Circuits

Kirchhoff's Current Law
This fundamental law results from the conservation of charge. It applies to a junction or node in a circuit -- a point in the circuit where charge has several possible paths to travel.

In Figure 1, we see that IA is the only current flowing into the node. However, there are three paths for current to leave the node, and these current are represented by IB, IC, and ID

Once charge has entered into the node, it has no place to go except to leave (this is known as conservation of charge). The total charge flowing into a node must be the same as the the total charge flowing out of the node. So,

    IB + IC + ID = IA
Bringing everything to the left side of the above equation, we get
    (IB + IC + ID) - IA = 0

Figure 1 Possible node (or junction) in a circuit
Then, the sum of all the currents is zero. This can be generalized as follows

Note the convention we have chosen here: current flowing into the node are taken to be negative, and currents flowing out of the node are positive. It should not really matter which you choose to be the positive or negative current, as long as you stay consistent. However, it may be a good idea to find out the convention used in your class. 
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