# Kirchoff’s Laws

**Kirchofff’s First Law** states that the sum of the currents entering and leaving a node is equal to zero. Let us evaluate a simple circuit comprised of two resistors R1 and R2 connected in parallel with a +12 vdc battery. In the following circuit there are 3 currents associated with node (point A).

Let’s evaluate node (pointA) in the above figure. The current entering node A is labeled I (Total).

The current leaving node A and passing through R1 is labeled I (1).

The current leaving node A and passing through R2 is labeled I (2).

Let’s also define currents entering the node as positive and those leaving the node as negative.

Applying Kirchoff’s First Law we can write: I (Total) – I(1)-I(2) = zero or I (Total) = I (1) + I (2)

**Kirchoff’s Second Law** states that the sum of the voltages around a loop is equal to zero. Let us evaluate a simple circuit comprised of two resistors R1 and R2 connected in resistors with a battery. In the following circuit there are 3 voltages associated with the loop which starts and ends at the battery.

Let’s evaluate the voltage loop in the above figure.

The voltage at the battery is labeled E.

The voltage across R1 is labeled V1.

The voltage across R2 is labeled V2.

The voltage polarities reflect current flowing from + to -. We can then write the following equation:

E – V1 -V2 = zero or E = V1 +V2

Observe that if I apply Ohm’s Law (V = IR) we can also write E = IR1 + IR2

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Radio Shack (Local Store or on-line at www.radioshack.com)

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DeMar Electronics at www.demarelectronics.com/