Kirchhoff's First Law


The algebraic sum of the currents flowing through a junction is zero. Currents approaching the junction are + while currents going away from the junction are -.

Kirchhoff's first law when officially stated (see insert at the right) sounds more complicated than it actually is.  Generally speaking, it says, the total current entering a junction must equal the total current leaving the junction.  After all, no charges can simply disappear or get created, so current can't disappear or be created either.   A junction is any place in a circuit where more than two paths come together.

The diagrams below represent several possible circuit junctions and show the currents entering and leaving a junction.

1 current enters, 1 leaves a junction (3k) In this example you will notice that 8 Amps of current enter the junction and 3 and 5 Amps leave the junction. This makes a total of 8 Amps entering and 8 Amps leaving.
2 currents enter one leaves a junction (6k) In this example you will notice 8 Amps and 1 Amp entering the junction and 9 Amps leaving.  This makes a total of 9 Amps entering and 9 Amps leaving.
2 currents enter 2 leave (4k) In this example you will notice 8 Amps and 1 Amp entering the junction while 7 Amps and 2 Amps leave.  This makes a total of 9 Amps entering and 9 Amps leaving.

1999 Science Joy Wagon