Basic Requirements for Current
Video Lectures created by Tim Fiegenbaum at North Seattle Community College.
Lecture Transcript: In this chapter we are going to begin to apply many of the principles we have looked at in chapters one, two and three. Introduction, there are only four basic ways to interconnect the various components in an electrical circuit and the subject matter of chapter four is going to be those four basic ways. You must be able to analyse circuits to determine the expected values of the circuit. You must be able to apply the principles of basis electronics to account for differences between expected values and measured values in a circuit. This chapter begins with 4.1: basic requirements for current. Before we look at the circuits we are going to be looking at some of the basic requirements for current. The term “current flow” refers to, and notice this term, sustained current flow. Momentary or transient currents will be considered later in this text. There are two basic requirements for current flow. Number one: there must be a source of EMF. We talked about that in previous chapters and that is electromotive force. Number two: there must be a complete path for current. EMF is the difference of potential that does not decay as charges are transferred and here we have a battery and an electronic circuit and the battery is our source of EMF. Now many comparisons are made between hydraulic systems and EMF and there is… they do compare. Here is a hydraulic system and here this would be the pump in the hydraulic system and hydraulic systems… the pump is comparable to EMF because it provides the pressure in the system and the battery is the difference of potential which provides the pressure that actually induces current to flow it through the circuit. In the hydraulic system it flows through the system and the fluid actually does the work, in the case of electronics the current does the work. In the hydraulic system if there was a break in the pipes then the system simply will not work. It is the same in the case of electrical systems, if there is a break in the circuit of the wires break then the current will cease to flow and the work will cease to be done. EMF is a potential difference that does not decay as changes, as charges are transferred. Electromotive force may be supplied either by and usually a couple of different sources, either a battery or power supply. Ohms law describes current flow of a circuit as “current equals voltage divided by resistance”. If there is no EMF in a circuit the current will be zero so if this is zero then the current will also be zero so there must be EMF for there to be current flow. The second requirement for current flow is a closed loop between two terminals of the EMF. Here we have this battery representing our EMF and here is terminal one… two terminals and there must be a closed circuit between these two terminals. For most purposes an open circuit is considered to have an infinite resistance, if you have an open circuit you are looking at infinite resistance and current will not flow well through that. An open in any circuit will result in zero current flow. Does this circuit represent a closed loop and quite obviously it does not. In this particular situation there would be no current flow. This is like the hydraulic system with the pipes cut. We could make this a complete circuit quite easily if we go in here and draw a connection and now we can have electron flow through this circuit, we can have a complete path and we can do work. Direction of current: we are going to be looking at electron flow. Electron current flows from negative to positive since electrons are negatively charged particles and so the way we are going to depict this is we are going to start out in item A here where we see a common line which we can understand to be ground and here we have three voltage supplies in this case minus five volts, positive five volts and positive ten volts. If we were to connect a resistor in this circuit and let us connect to the ground and we will go to the positive five volt line. Now, in this scenario remember current electron flow, current flows from the negative to the positive terminal so common we consider that to be zero volts, so from zero to positive five is going to be more positive so the current flow will flow through the resistor in this direction and this will be the direction of current flow. In this scenario down here we are still connected to the common line but we are connected to minus five volts and so now this represents the more negative of the two connection points because it is negative five volts, it is below zero. The direction of our current flow is going to be in the direction of the arrow like this. In item D here we are not using the ground at all. What we have is two voltages and we are going to be looking at what is more negative… this is going to be in a relative sense. In this case positive five volts in more negative than positive ten. Again, current flows from the negative to the more positive so in this case current flow through this component will be in this direction. In each of these cases, if this resistor is the same value we are going to have the same amount of current flowing through each component and we would have the same power dissipated but the voltage is slightly… here it is positive five and here it is a negative five and here it is a positive five, but at any rate it is five volts and through a given component it will give the same current and it will yield the same power. This is the first part of chapter four and the two main things we looked at were the requirements for current flow in a circuit and they are an EMF and a complete path for current.