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Theoretical foundations of electrical engineering

# Problems solving in electrical engineering - Physical meaning of the first and second Kirchhoff’s laws

Both Kirchhoff's law are formulated clearly enough and have a simple physical meaning. Kirchhoff's first law states that if we consider any node of the circuit (that is, the branching juncture, where meet three or more wires), the amount of electric currents flowing into the circuit will be equal to the amount of the leaving currents, which, in principle, is a consequence of the conservation law of the electric charge. For example, if you have a T-junction of the eclectic circuit and currents flow to it via two wires, while on the third wire current will flow in the opposite direction, and it will be equal to the sum of two entering currents. The physical principle of the law is simple: if it was followed, the node would continuously accumulate electric charge, and it does not happen.

Kirchhoff's second law is also easy to understand. If we have a complex, branched circuit, it can be represented as a simple closed circuit. The current in the circuit can be distributed differently on these contours, and the most difficult is to determine how will flow currents in the complex circuit. In each loops electrons can get additional energy (e.g., from battery), or lose it (for example, resistance or other element). Kirchhoff's second law states that the increase of the electron energy in any closed loop circuit is equal to zero. The law also has a simple physical interpretation. If it was not true, every time it passed through a closed circuit, the electrons get or lose energy, and the current would constantly increase or decrease. In the first case, you could get a perpetual motion machine, and it is forbidden by the laws of thermodynamics; in the second case - all the currents in the circuits would inevitably fade out, and it is not observed.

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