Kirchoff's voltage law

Kirchoff's voltage law can be stated in words as the sum of all voltage drops and rises in a closed loop equals zero. As the image below demonstrates, loop 1 and loop 2 are both closed loops within the circuit. The sum of all voltage drops and rises around loop 1 equals zero, and the sum of all voltage drops and rises in loop 2 must also equal zero. A closed loop can be defined as any path in which the originating point in the loop is also the ending point for the loop. No matter how the loop is defined or drawn, the sum of the voltages in the loop must be zero.

The sum of all voltages or potential differences in an electrical circuit loop is 0.

 

KVL example

V_S = 12V, V_R1 = -4V, V_R2 = -3V

V_R3 = ?

Solution:

∑V_k = V_S + V_R1 + V_R2 + V_R3 = 0

V_R3 = -V_S  - V_R1 - V_R2 = -12V+4V+3V = -5V

The voltage sign (+/-) is the direction of the potential difference.