# Kirchhoff’s law

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This is probably too broad of a topic but what is Kirchhoff’s law? I tried reading up on it but even the wiki page went over my head.

In: Engineering

Kirchoffs Laws are two different formulas that describe how electrical current and voltage distribute in a network. Through analogies you can apply them to other forms of flow networks too.

They are quite simple:

The sum of currents going in and out of a node is always 0. That simply means electrons don’t gather somewhere, what flows in must come out

The sum of all voltages in a loop is always zero. This is easy to understand when you compare voltages to climbs in mountains. When you wander a circle and end up where you started all height differences you encountered must sum up to zero.

Note that this applies only when you don’t have electrical and magnetical fields messing with your network.

It basically is that current that flows into a node or junction must have the same current flowing out of it.
So if you have a few different currents flowing into it the amount of exits the current must be the same as went in.

So there are two kirchoff laws. The current law and the voltage law.

* Kirchoff’s Current Law, means that the sum of all currents at a node is zero. That includes current that’s “coming in” and “going out”
* Essentially, you could remember this as “What (current) goes in must come out.”
* Kirchoff’s Voltage Law, means that the sum of the voltages in a **closed** circuit is zero. Note the word closed.
* Essentially, you could remember this as “What (Voltage) goes up must come down”

As for applications: imagine I have one battery and two different lightbulbs that I place in parallel. Using Kirchoff’s laws I can work out the current flowing through each lightbulb, and also the current being pulled out of the battery. This is pretty useful. The same equations scale up to vastly complex circuits, power grids etc.

In very basic terms, the sum of all the voltage drops in a circuit will equal the source voltage. For example, if you have 3 equal size resistors in series with a 12 volt battery, you will have a 4 volt drop across each one. If the resistors all have different ratings, the voltage drops across each will divide proportionately with the resistance, but the will still all add up to 12 volts. It comes in very handy when trouble shooting a circuit.