Kirchhoff’s current Law.

Gustav Kirchhoff - Prussian. 1824 – 1887

This really is an extension of the Ohm’s law we use to find out the relationship between current, resistance and voltage. When there are too many connections this is not sufficient. The  two laws of Kirchhoff come to rescue.

First Law (Current Law)

The sum of current entering into a junction is equal to the sum of the current leaving the junction.

Giving a positive value for the currents entering and a negative value to currents leaving and add them up the answer should be zero.

In the diagram here arrows indicate the direction of the current.  Ampere readings are also given except in one. The diagram shows how you can use the Kirchhoff ‘s law to get the direction and value of the Blue wire.

Fig.1 Current at a junction.

Kirchhoff’s Voltage Law.

In a circuit the voltages of the cells in a closed loop add up to zero.

We can follow this well if we use a sign convention.

Fig.2 How many circuits or loops are in this diagram?

You may be right. There are three loops shown in three colours.

1.       Green loop. Keep your finger at the point marked as the start. Then move it ina clock-wise direction along the circuit adding the voltages algebraically. That means if you meet a + you have to add , if you meet a minus subtract it.

Let us note the values:

                      + 6v  - 4.5v – 1.5v = 0

2.        Lets consider the red loop.

              +1.5v+2.5v -4v = 0

3.        Outer most blue loop

                + 6v – 4.5v +2.5v  - 4v =0