L2 2

Well, the second question is how to look upon a circuit. First, we can divide circuits into different groups according to their loads. If the load of the circuit consists of resistors entirely, You can see it consists of resistors entirely. then we call it a resistive circuit. If the circuit contains capacitors (and/or inductors), then it's a dynamic circuit. For dynamic circuits, we also can divide them according to the time we're interested. If we're interested in the change of voltages and currents in the changing process of the dyanmic circuits, we can it transient analysis. For example, in a circuit like this. If we know the source is a dc source, and by learning the following lectures, we can know this is the changing trend of the voltage across the capacitor. the waveform's changing process, from this point to this point, is the transient analysis. Besides this, we may also be interested in the steady-state operation point. In other words, we're interested in the value before and after the change process. It's called steady-state analysis. We just divide according to the load. We also can divide according to the source. The circuit may only contain dc sources, i.e. the flashing straight line on the screen. It is a dc voltage source. The courses can also contain the ac sources. The ac sources can be a sinusoidal ac as the waveform shown on the screen or a periodical source more generally. In the "Principles of Electric Circuits", we will introduce you the methods to analyse all kinds of the circuits, including the resistive circuit and the dynamic circuit with dc, sinusoidal, and periodical source. The second question we're going to discuss is the circuit model. Let's start by the ideal circuit elements. These ideal circuit elements describing the simple relationship between voltage and current are all abstracted from real circuit elements. The two-terminal ideal circuit elements discussed in this course include the resistor. It's a block in Chinese standard and it's a such symbol in western textbooks. We don't differ them in the course. The two-terminal ideal circuit elements also include the inductor. Here's its symbol. The capacitor's symbol is here. And the source. If a source is a dc one, it's often drawn like this. More generally, we may draw it like this. In the second aspect of the question, let's discuss the how to model. In other words, how to model the ideal electric circuit elements from the real ones. For the lighting circuit of an electric torch you've just seen, using the abstraction, we can get the model of battery switch 63 00:03:31,700 --> 00:03:34,294 bulb and transmission line. One of the simple models is like this. We use a resistor and a voltage source connecting in series to represent a battery. We use an ideal switch to represent a real switch. We use a linear resistor to represent a bulb. We use ideal lines to represent real connecting lines. Actually, the model is very simple. Why? The value of the resistor is constant in the model. But in fact, the resistance of the bulb will be changing with the lighting time. The inner resistance of the battery is constant in the model. But in fact, it will be changing with the power supply time. The whole resistance of the line is ignored in the model. And the line is thought to be an ideal one. But in fact, it has the resistance in real. And there's also the resistance in the connection of lines and elements. So here's a problem: You must think why not use more accurate models to model them? In the following lectures, we will discuss your problem in detail. Next, we will introduce the classification of circuits. We mainly discuss two classifications in this lesson. First is the linear and nonlinear circuit. We take the resistors for example. We're all familiar with them. If I mark the direction of voltage and current like this, then there must obey the ohm's law. Multiply R by I, we get U . Then we often use stimulation and response to describe the relationship of an element in the circuit. For example, if we think the stimulation is the voltage and the response is the current, then the relationship between stimulation and response is U=RI. It's a linear relationship. And the so called linear element is the element that the relationship between its stimulation and response is linear. Now we discuss why U is R multiplied by I is a linear relationship. It needs to be discussed in two aspects. Firstly, if the stimulation changes K times, then from the equation, U is R multiplied by I, then the response must be changed to K times U divided by R. It equals to U divided by R times K. So the response become K times of the former one. Secondly, if the stimulation become the sum of two stimulations, then the response must become the sum of two stimulations divided by R. 108 00:06:20,278 --> 00:06:26,564 They're respectively U1 divided by R plus U2 divided by R. 109 00:06:26,661 --> 00:06:31,115 It's the sum of two responses under independent stimulations respectively. If any relationship between stimulation and response meet such a relationship, we call it homogeneity. If it meet such a relationship, we call it additivity. The relationship satisfying homogeneity and additivity is a linear relationship. If all the loads in a circuit are linear elements, the circuit is a linear circuit. A linear circuit is described by a linear equation. You will comprehend this in the future. So what's the nonlinear circuit? As long as one or more loads have nonlinear relationship, then it's a nonlinear circuit. and described by a nonlinear equation. The second discussion is about planar circuit and non-planar circuit. The so called planar circuit means that it can be drawn on a plane thus enables all the elements don't intersect with each other. We take such a circuit for example. All the elements don't intersect with each other. Now let's look at such a circuit. It seems that this element intersects with this one. In fact, we can redraw this circuit in this way to make it look like this. You should pay attention to what I draw here. You see, this circuit and this one is totally same in topology. For the right circuit, there're no intersections between any two elements. So it's still a planar circuit. There must be non-planar circuits respect to planar circuits. In other words, no matter how you redraw it, the circuit still has intersections between the elements. Take such a circuit for example. No matter how you redraw the circuit, the element will intersect with it, or with it, or with it, or with it. So such a circuit is a non-planar circuit. It's relatively easy to analyze planar circuits. Non-planar circuits need some special considerations. In this course, the circuits we discussed are all planar circuits. OK, this is the end of the lecture 2.