Kirchoff's Voltage Law states that the sum of the differences in Voltage (sic) between different points in a closed electrical circuit is zero. Example:
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P                               |
With power source, P, and two resistors. The voltage of P + the change in voltage from A to B + the change in voltage from B to C = 0. You can set up an equation with the resistances ( R1 and R2 ), the current ( I ), and the voltage of the power supply ( V ):
V - I * R1 - I * R2 = 0
Kirchhoff's Voltage Law (KVL), also called Kirchhoff's Loop Rule, states that as one travels along any closed path in a circuit, the algebraic sum of the voltage drops across all of the components in that path must total zero.

In other words, this law guarantees that as you go around a mesh in circuit, you will come back to same voltage that you started at.

An analogy to gravity can be made:

You start at the 5th floor of a building, and take an elevator to the 40th floor. From there, you can travel various other elevators and stairs (or jump out a window) to take you both up and down, but eventually you come back to the 5th floor. If you add up all the floors you went up, total, and subtract all the floors you went down, total, you'll find that the total is zero.

In a circuit you might start at a potential of 5V. You travel across two nodes connecting a voltage supply increasing the potential to 40V. From there, you travel across various components that have voltage rises or voltage drops, but, travelling along the closed path, when you return to the node you started at its voltage is still 5V. If you add up all the voltage rises, and subtract all the voltage drops, you'll find that the total is zero.

KVL, along with Ohm's Law is the principle behind mesh analysis, and is, in general, one of the most basic tools of circuit analysis.

See also Kirchhoff, Gustav Robert, Kirchhoff's Current Law, voltage.

Also very commonly misspelt Kirchoff's Voltage Law. Please see Kirchhoff's Current Law.

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