Textbook for Operation and Maintenance Engineer of Powe […]
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1.2.3 Kirchhoff's law
Ohm's law can only solve the relationship between the current, resistance and voltage of a single power source and a series parallel circuit. For a complex circuit with multiple power sources, it needs to rely on Kirchhoff's two laws to analyze and deal with it.
1.2.3.1 Kirchhoff's first law (nodal current law)
Each branch of a circuit that passes through the same current is called a branch. The connection points of three or more branches are called nodes. In Figure 1.2-3, there are three branches. The current of each branch is I1, I2 and I3, respectively. There are two nodes, C and D.
Kirchhoff's first law stipulates that the current flowing into the node is equal to the current flowing out of the node, that is, according to the principle of current continuity, there should be no charge accumulation at the node. In Figure 1.2-3, the inflow current of point C is I1, I2, and the outflow current is I3. Therefore, I1 + I2 = I3 can be rewritten as I1 + I2 - I3 = 0 or Σ I=0。
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(1) Power supply
Power supply is a device that continuously converts other forms of energy into electrical energy. For example, the battery is a device that converts chemical energy into electrical energy, and the generator is a device that converts mechanical energy into electrical energy.
The potential difference between the two poles generated by the chemical reaction inside the power supply is called the electromotive force, which is expressed by the symbol E, in volts, and expressed by the symbol V.
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For any node in the circuit, the algebraic sum of the current flowing into (or out of) the node is equal to zero. This is Kirchhoff's first law.
1.2.3.2 Kirchhoff's second law (loop voltage law)
A circuit composed of many branches is called a network, and any closed path in the network is called a loop. In a closed circuit, if it makes a detour in any direction, the algebra of all electromotive forces in the circuit is Σ E must be equal to all electricity in the circuit
Algebraic sum of resistive voltage drop Σ IR, the expression is:
Σ E= Σ IR or Σ E= Σ U The electromotive force and voltage drop that are the same as the bypass direction are specified as positive values, and - 5 the electromotive force and voltage drop that are opposite to the bypass direction are specified as negative values. As shown in Figure 1.2-4, E1 - E2 in loop 1=I1R1 - I2R2, while E1 in loop 2=I1R1 + I3R3.