Analysis: In Figure 1.3(a), the voltage and current at both ends of the element are related reference directions, and obviously assumed to be loads. A negative value of the current in the associated reference direction indicates that the actual direction of the current through the element is opposite to the reference direction, so the element is actually a power supply. (2) As shown in figure 1.3(b), if the current I =- 100A and the voltage across the element U= 10V are known, find the electric power p, which indicates whether the element absorbs power or emits power. Analysis: In figure 1.3(b), the voltage and current on the element are in the direction of irrelevant reference. In the direction of irrelevant reference, it is obviously assumed that the element is a power supply, so the power emitted by the element is W, which is actually the absorbed power. Therefore, the element in figure 1.3(b) is actually a load. (3) What are the similarities and differences of voltage, potential and electromotive force? Analysis: the expressions of voltage, potential and electromotive force are the same, so their units are the same, all of which are volts V; Voltage and potential are physical quantities reflecting the action of electric field force, and electromotive force is physical quantities reflecting the action of power supply force; Voltage and potential can exist outside and at both ends of the power supply, while electromotive force only exists inside the power supply; Voltage only depends on the difference between two points in the circuit, so it is an absolute quantity, and its direction points from a point with high potential to a point with low potential, so it is often called voltage drop. There are only high, low, positive and negative potentials. There is no direction. The high, low, positive and negative potentials are all relative to the reference point in the circuit, so the potentials are relative quantities. The direction of electromotive force points from the negative pole of power supply to the positive pole of power supply. (4) Electrical appliances with high electric power must also have high electric power. Is this statement correct? Why? Analysis: The electric power P marked on the nameplate of the electrical appliance reflects the energy conversion ability of the electrical appliance, which is determined from the factory; The magnitude of electric work W reflects the actual energy consumption of electrical appliances, because W=Pt, obviously the magnitude of electric work is related to the length of power consumption. If the electric appliance with the largest electric power is not connected to the power supply, that is, when t=0, the electric work W=Pt=0. Therefore, it is wrong to say that electrical appliances with high electric power must also have high electric power. (5) What is the purpose of introducing reference direction in circuit analysis? When applying the reference direction, you will encounter several pairs of words "positive, negative, addition and subtraction, same and opposite". Can you explain their differences? Analysis: The purpose of introducing reference direction in circuit analysis is to provide convenience and basis for analyzing and calculating circuits. "Positive and negative" encountered when applying a reference direction refers to the positive and negative signs in front of the values of voltage and current in the reference direction. If the next current in the reference direction is "-2A", it means that its actual direction is opposite to the reference direction, and the next voltage in the reference direction is "++20V", which means that the actual direction of the voltage is consistent with the reference direction. "Addition and subtraction" refers to the positive and negative symbols in front of each quantity when writing circuit equations in the following reference directions; "Same and opposite" refers to whether voltage and current are related reference directions, and "same" refers to that they are related reference directions, that is, the current inflow end is the high polarity end of voltage; "Opposite" means that voltage and current are irrelevant reference directions, that is, current flows in from the low polarity end of voltage. 1.3 Kirchhoff's Law 1, Ohm's Law (1) and Kirchhoff's Law Ohm's Law, Kirchhoff's Law of Current and Kirchhoff's Law of Voltage are collectively called the three basic laws of the circuit, which reflect two different constraints in the circuit. Ohm's law expounds and solves the constraint relationship between an element and the basic variables of the circuit (that is, the voltage at both ends of the element and the current through the element); Kirchhoff's two laws explain and solve the constraint relationship between the overall structure of the circuit and the basic variables of the circuit (voltage in the loop and current at the node) after the interconnection of circuit elements. We should distinguish these two different constraint relationships in learning. (2) When learning the basic laws of circuits, the application scope of lumped parameter circuits should be paid attention to: it is limited to the analysis of lumped parameter circuits. The so-called lumped parameter circuit means that the electromagnetic energy in the circuit is only stored and consumed in the components, and the components are connected by an ideal wire that is unobstructed and non-inductive, and the capacitance between the wire and each part of the circuit can also be ignored. In other words, as long as the size of the circuit is much smaller than the wavelength corresponding to the highest frequency in the circuit, it can be called lumped parameter circuit regardless of its connection mode. (3) Kirchhoff's law Kirchhoff's first law, also known as the law of node current, solves the constraint relationship of current in each branch of a circuit node: for any node of the circuit, the algebraic sum of current flowing into the node is always equal to zero. The law holds when the positive and negative values of the current flowing into the node and the current flowing out of the node are different. Kirchhoff's second law, also known as the law of loop voltage, solves the mutual constraint of voltage drops on all components in a loop: for any loop of a circuit, the algebraic sum of voltage drops on all components is always equal to the voltage rise of the circuit. This law holds when the voltage drop or loop voltage rise is consistent with the bypass direction after the circuit bypass direction is calibrated, otherwise it is negative. 2. Analysis of test learning results (1) Can you explain what branches, loops, nodes and grids are from the perspective of understanding? Analysis: A branch refers to a bifurcated circuit connected between two points in the circuit. One or several elements in this bifurcated circuit may be connected in series, but the current flowing through each element in series is the same. A loop refers to any closed path in a circuit; The meeting point of three or more branches is called a node; Mesh is a closed path with no internal branches on the plane circuit diagram. (2) Can you explain the difference between Ohm's law and Kirchhoff's law in terms of circuit constraints? Analysis: ohm's law reflects the constraints of the characteristics of linear resistance elements on the voltage and current of the elements themselves; Kirchhoff's law reflects the constraints of the connection between components on voltage and current. Therefore, when using ohm's law, we only need to consider the characteristics of the components themselves, but not the relationship between them; When we use Kirchhoff's law, we consider the connection between components or the overall structure of the circuit, but we don't consider the characteristics of the components themselves. (3) When applying KCL law to solve problems, why should the reference direction of current entering and leaving the node be agreed first? What does it mean that the calculated current is negative? Analysis: When applying KCL law to solve problems, the reference directions of each branch current collected at the node are assumed and marked, and then the positive and negative signs before each current in the current equation can be determined according to these reference directions; If the calculated current is negative, it means that the current reference direction marked on the circuit diagram is opposite to the actual current direction. (4) When applying KCL and KVL law to solve problems, why should the reference direction of current be marked on the circuit diagram and the reference circuitous direction in the loop be given in advance? Analysis: the reference direction of current is marked on the circuit diagram in advance, and the reference circuitous direction in the loop is given in advance, in order to provide the positive and negative values of each term for the equation written in the column. (5) How do you understand and master the popularization and application of KCL and KVL? Analysis: The promotion of KCL should first grasp which parts of the circuit can be used as generalized nodes, and the promotion of KVL should grasp which parts of the circuit can be used as virtual circuits. The rest are omitted. 1.4 voltage source and current source 1, study guide (1) ideal voltage source The ideal voltage source is called voltage source for short, and it is also called constant voltage source because of its constant voltage value. Note that the current value through the constant voltage source is determined by it and the external circuit. In addition, constant voltage source belongs to infinite power supply, which does not exist in practice. (2) Ideal current source The ideal current source is simply called current source, and it is often called constant current source because of its constant current value. Note that the voltage across the constant current source is determined by it and the external circuit. The ideal current source is also an infinite power source. When studying, we should master the basic properties and characteristics of two ideal power supplies, and compare the two power supplies with the help of volt-ampere characteristics in analysis to deepen our understanding. (3) On the basis of understanding the ideal power supply, the two power supply models find out the differences and connections between the actual power supply and the ideal power supply. The actual voltage source always has internal resistance. We hope that the smaller the internal resistance of the voltage source, the better, so that the voltage value provided to the external circuit will be basically stable. When the internal resistance of the actual power source is equal to 0, it will become an ideal voltage source. The internal resistance of the actual current source is always limited. We hope that the greater the internal resistance of the actual current source, the better, so that the more stable the current it outputs. When the internal resistance of the actual current source is infinite, it will become an ideal current source. 2. Analysis of test and learning results (1) What are the characteristics of ideal voltage source and ideal current source? What is the main difference between them and the actual power supply? Analysis: The actual voltage source always has internal resistance. In circuit analysis, the actual voltage source is characterized by the series combination of ideal voltage source and resistance element. Therefore, the greater the internal resistance of the power supply, the more the divided voltage, and the smaller the voltage provided to the outside. We always hope that the internal resistance of the actual voltage source is as small as possible. When the internal resistance is zero, it will become an ideal voltage source. Because the ideal voltage source does not have the problem of internal resistance and voltage division, the output voltage value is constant, but the current passing through the ideal voltage source is determined by it and the external circuit. Actual current sources always have internal resistance. The actual current source generally adopts an ideal current source and a resistance element in parallel as its circuit model, and the parallel resistance can shunt, so the smaller the internal resistance of the power supply, the more shunt, and the smaller the current provided to the outside world. We hope that the greater the internal resistance of the actual current source, the better. When the internal resistance of the actual current source is infinite, it will become an ideal current source. Because the internal resistance of the ideal current source is infinite, there is no shunt problem, so the output current value is constant, but the voltage at both ends of the ideal current source is determined by it and external circuits. (2) The resistance of carbon microphone changes with the sound intensity. When the resistance changes from 300 Ω to 200 Ω, how much does the current change if it is supplied by an ideal voltage source of 3V? Analysis: The stronger the sound sent into the carbon microphone, the smaller the resistance and the greater the current. When the resistance is 300 Ω and 200 Ω, the current is a and a respectively. The calculation results show that the current changes from 0.0 1A to 0.0 15A under the ideal voltage source of 3V. Figure 1. 13 Two circuit models of actual power supply (a) voltage source model RI+US-RUIS (b) current source model (3) circuit model of actual power supply is shown in figure 1. 13(a). It is known that US=20V and load resistance RL = 50Ω. What can the calculation results show when power is supplied? Analysis: When Ru ′ = 0.2 Ω, a; When ru "= 30, a. According to the calculation results, the smaller the internal resistance of the actual voltage source, the better. When the internal resistance is too large, the internal resistance partial pressure of the power supply is too large, resulting in low external power supply voltage and insufficient power utilization. (4) What effect does the internal resistance of the current source have on the circuit? Analysis: The internal resistance of the current source and the load are connected in parallel, which can shunt. Therefore, when the internal resistance of the current source is small, the current it allocates to the internal resistance will be large, which will lead to the corresponding small current allocated to the external circuit load, which will not only make the utilization rate of the power supply too low, but also cause the internal resistance to overheat, which is not conducive to power supply. 1.5 equivalent transformation of circuit 1, learning guidance (1) The chapter of resistance equivalence touches on the problem of circuit equivalence, which is a main line throughout the whole process of circuit analysis. When learning, we should deeply understand the concept of "equivalence" of circuits: equivalence refers to the same effect on circuit parts except equivalent transformation, but generally different effects on circuit parts of equivalent transformation. The key of resistance equivalence is to find the node correctly and determine the series-parallel relationship or Y or δ relationship between resistors. (2) Equivalent transformation between power supplies There is no equivalence between two ideal power supplies because they are infinite power supplies. The two actual models can be equivalently interchanged. In the process of equivalent exchange, it must be noted that the position of the terminal button connected with the power supply model cannot be moved. When the voltage source model connected by the terminal buttons of two circuits is transformed into the current source model (or the current source model is transformed into the voltage source model), the internal resistance of the power supply remains unchanged, but the value of the current source is equal to the value of the voltage source divided by its internal resistance (or the value of the voltage source is equal to the value of the current source multiplied by its internal resistance). 2. Analysis of test learning results (1) In the circuit shown in figure 1. 18(a), let US 1=2V, US2=4V, ru1= ru2 = r = 2ω. Find the power emitted by the ideal current source of the circuit in Figure (c) and the ideal voltage source in Figure (d), and then find the power absorbed by the load R in the two equivalent circuits respectively. What conclusion can you draw from the calculation results? Analysis: Firstly, two voltage source models in the circuit in Figure (a) are transformed into two current source models in Figure (b), a, Ari 1 = Ri2 = Ru 1 = 2ω. Therefore, the current source model in Figure (c) and the voltage source model in Figure (d) are IS = IS1+IS2 =1+2 = 3a. Ri = ri1∨ ri2 = 2 ∨ 2 =1ω us = is× ri = 3x1= 3vru = ri =1ω Find the terminal voltage UAB in figure (c) and figure (d) =2V A Therefore, the power emitted by the ideal current source in the circuit in figure (c) is PI =IS×UAB=3×2=6W, and the power absorbed by the resistor R is w, and the power emitted by the ideal voltage source in figure (d) is pu = i× us =1× 3 = 3w.