Kirchoffs Circuit Laws

Kirchoffs Circuit Laws

We saw in the Resistors tutorial that a single equivalent resistance, ( R_T ) can be found when two or more resistors are connected together in either series, parallel or combinations of both, and that these circuits obey Ohm's Law. However, sometimes in complex circuits such as bridge or T networks, we can not simply use Ohm's Law alone to find the voltages or currents circulating within a circuit. For these types of calculations we need certain rules which allow us to obtain the circuit equations and for this we can use Kirchoffs Circuit Laws.

In 1845, a German physicist, Gustav Kirchoff developed a pair of rules or laws which deal with the conservation of current and energy in electrical circuits, one which deals with current flow, Kirchoffs Current Law, (KCL) and one which deals with voltage, Kirchoffs Voltage Law, (KVL).

Kirchoff's First Law - The Current Law, (KCL)

Kirchoff´s current law states that the "total current or charge entering a junction or node is exactly equal to the charge leaving the node as it has no other place to go except to leave, as no charge is lost within the node". In other words the algebraic sum of ALL the currents entering and leaving a node must be equal to zero, I_(exiting) + I_(entering) = 0. This idea is known as the "Conservation of Charge".

Kirchoff's Current Law

Here, the 3 currents entering the node, I₁, I₂, I₃ are all positive in value and the 2 currents leaving the node, I₄ and I₅ are negative in value. Then this means we can also rewrite the equation as;

I₁ + I₂ + I₃ - I₄  - I₅ = 0

The term Node in an electrical circuit generally refers to a connection or junction of two or more current carrying paths or conductors such as cables and components. Also for current to flow either in or out of a node a closed circuit path must exist.

Kirchoff's Second Law - The Voltage Law, (KVL)

Kirchoff´s voltage or loop law states that "in any closed loop network, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop" which is also equal to zero. In other words the algebraic sum of all voltages within the loop must be equal to zero. This idea is known as the "Conservation of Energy".

Kirchoff's Voltage Law

Starting at any point in the loop continue in the same direction noting the direction of all the voltage drops, either positive or negative, and returning back to the same starting point. It is important to maintain the same direction either clockwise or anti-clockwise or the final voltage sum will not be equal to zero.

Example No1

Find the current flowing in the 40Ω Resistor, R₃

The circuit has 3 branches, 2 nodes (A and B) and 2 independent loops.

Using Kirchoffs Current Law, KCL the equations are given as;

At node A :    I₁ + I₂ = I₃

At node B :    I₃ = I₁ + I₂

Using Kirchoffs Voltage Law, KVL the equations are given as;

Loop 1 is given as :    10 = R₁ x I₁ + R₃ x I₃ = 10I₁ + 40I₃

Loop 2 is given as :    20 = R₂ x I₂ + R₃ x I₃ = 20I₂ + 40I₃

Loop 3 is given as :    10 - 20 = 10I₁ - 20I₂

As I₃ is the sum of I₁ + I₂ we can rewrite the equations as;

Eq. No 1 :    10 = 10I₁ + 40(I₁ + I₂)  =  50I₁ + 40I₂

Eq. No 2 :    20 = 20I₁ + 40(I₁ + I₂)  =  40I₁ + 60I₂

We now have two "Simultaneous Equations" that can be reduced to give us the value of both I₁ and I₂ 

Substitution of I₁ in terms of I₂ gives us the value of I₁ as -0.143 Amps

Substitution of I₂ in terms of I₁ gives us the value of I₂ as +0.429 Amps

As :    I₃ = I₁ + I₂

The current flowing in resistor R₃ is given as :     -0.143 + 0.429 = 0.286 Amps

and the voltage across the resistor R₃ is given as :     0.286 x 40 = 11.44 volts

The negative sign for I₁ means that the direction of current flow initially chosen was wrong, but never the less still valid. In fact, the 20v battery is charging the 10v battery.

Application of Kirchoffs Circuit Laws

These two laws enable the Currents and Voltages in a circuit to be found, ie, the circuit is said to be "Analysed", and the basic procedure for using Kirchoffs Circuit Laws is as follows:

• 1. Assume all voltage sources and resistances are given. (If not label them V1, V2 ..., R1, R2 etc)
• 2. Label each branch with a branch current. (I1, I2, I3 etc)
• 3. Find Kirchoffs first law equations for each node.
• 4. Find Kirchoffs second law equations for each of the independent loops of the circuit.
• 5. Use Linear simultaneous equations as required to find the unknown currents.