Kirchoff's First Law - The Current Law, (KCL)
Kirchoff's Current Law or KCL, 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 by Kirchoff is known as the Conservation of Charge.
Kirchoff's Current Law
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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 elements such as cables and components. Also for current to flow either in or out of a node a closed circuit path must exist. We can use Kirchoff's current law when analysing parallel circuits.
Application of Kirchoff's 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 Kirchoff's 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 Kirchoff's first law equations for each node.
- 4. Find Kirchoff's second law equations for each of the independent loops of the circuit.
- 5. Use Linear simultaneous equations as required to find the unknown currents.
As well as using Kirchoff's Circuit Law to calculate the voltages and currents circulating around a linear circuit, we can also use loop analysis to calculate the currents in each independent loop helping to reduce the amount of mathematics required using just Kirchoff's laws. In the next tutorial about DC Theory we will look at Mesh Current Analysis to do just that.
