July 9, 2008 Educational Radio Net, PSRG 7th session
Much like any radio talk show I will "set up" the topic and then allow time at the end for questions or comments. In reality most fundamental ideas in electronics and radio are best described mathematically but, given that we do not have a "white" board for graphic illustration, I will attempt to convey fundamental ideas verbally.
This session is the 6th in the impedance series. Given that impedance is the combination of reactance and resistance and, further, that reactance is an alternating current phenomenon it is clear that we must have some elemental definitions under our belts to fully appreciate the subject. This multi-part narrative series is an attempt to elevate participants to an intuitive level of electrical understanding without using any serious mathematics as well as provide some review for those of us who have not spent a lot of time on fundamentals lately.
Where are we going with these discussions you might ask? Once we have the notions of electrical current, voltage, and power well in hand I will introduce the physical property of materials called resistance and then merge the voltage, current, and resistance trio into the workhorse notion of Ohm's Law. Subsequent parts of the series will introduce AC, or alternating current, and DC, or direct current, followed by capacitance and inductance, then reactance, and, finally, I will introduce impedance as the combination of resistance and reactance. All discussion material will be reviewed continually and be available on the blog.
Part 1 developed the idea of electrical current consisting of moving charge and defined the ampere as 1 coulomb of charge moving past a fixed point in 1 second. One coulomb was defined as a collection of charge numbering 6.24 x 10^18 electrons.
Part 2 developed the notion of mechanical "work" and considered objects at different "potential" levels in a gravitational field. The concept of "voltage", also known as electrical potential difference, and the relationship of voltage to current follows closely with the idea of a mechanical weight being moved between different levels. In both cases work is being done and energy is being manipulated in various ways.
Part 3 capitalized on Bob's lightning series to review electrical current in the context of a charged cloud redistributing charge in the form of lightning where modest amounts of charge make a large impression if moved rapidly.
Part 4 developed the notion of potential difference and ended with a definition of voltage. If you move 1 coulomb of charge from point A to point B in an electric field such that 1 joule of work is done then the potential difference between points A and B is defined as 1 volt. Another way to state this is that 1 joule of energy is required to push 1 coulomb through a potential difference of 1 volt.
Part 5 developed the notion of power by using a mechanical analogy. Power is the relationship between energy and time. Specifically power is the change in energy, known as delta P, divided by the change in time, known as delta t. Conversely, energy is power multiplied by time.
Part 6, tonight's edition, will deal with the notion of resistance.
Ok, on with the notion of electrical resistance.
Let's design a scheme to study the behavior of moving current. Suppose we use a standard D battery, a simple on/off switch, a fuse, and a pair of terminals to fabricate our "circuit" and also include some means of measuring current such as an ammeter. Let's wire the components in series which means that the various components are daisy chained in a ring. So, the positive terminal of the battery goes to one terminal of the fuse and the other fuse terminal goes to one side of the ammeter. The second terminal of the ammeter goes to one side of the switch and the other side of the switch goes to one of the DUT terminals. DUT is short for "device under test". The second DUT terminal returns to the battery negative terminal. Now we have a series circuit such that we can connect "something" to the DUT terminals and measure moving charge, or electrical current, with a meter. The fuse is the safety valve should something go horribly wrong.
The first interesting thing to note is that without anything connected to the DUT terminals the ammeter shows nothing regardless of on/off switch position. So, an "open" series circuit will pass no current. The second interesting thing of note is that connecting a heavy metal object, for example a spoon, across the DUT terminals and closing the switch will blow the fuse. So, a "short" in this circuit will pass so much current that the protective device is activated. We are interested in connecting objects to the DUT terminals which are not open or short from the viewpoint of the series circuit.
Let's assemble some candidates to study with our series circuit. First we have a piece of plastic that will fit across the DUT terminals. Next we have a piece of very fine wire about a foot long. Finally, we have a piece of what appears to be charcoal that will also fit across the DUT terminals.
With the circuit in front of us let's place the piece of plastic across the DUT terminals and throw the switch to on. The ammeter shows no deflection so we conclude that no current moved through the plastic. Let's try the charcoal next. Repeating the above procedure we find that some current does flow since the ammeter moves a bit upscale. Finally using the wire across the DUT terminals and repeating the procedure we see the ammeter needle move much higher upscale than the previous two readings. Clearly there is a difference in the amount of current that each substance will pass when each is substituted in the identical circuit. We can say that the plastic "conducts" nothing, that the wire conducts very well, and that the charcoal lies someplace in between the two.
We have just demonstrated relative conductivity in a very simple fashion. Relative to charcoal the plastic has low to no conductivity. Relative to charcoal the wire has high conductivity. Relative to the wire both charcoal and plastic have low conductivity.
In the case of the wire with high conductivity we can say that the "resistance" to electrical current flow is low. In the case of the plastic with low conductivity we can say that the resistance to electrical current flow is high. So high, in fact, that it is in a class of material called insulators. Clearly various materials differ in their conductivity hence resistance. Metals tend to be very good conductors with silver being the best followed closely by gold. Carbon is a popular resistance material and most inexpensive resistors are made of carbon. Deposited metal film resistors are more expensive than carbon and tend to be less affected by temperature variations.
Resistance is a physical property of materials. Resistance does not magically pop up in the presence of an electric field rather it is related to the population of free electrons in the material or the ease of producing free electrons if subjected to an electric field. Low conductance is analogous to high resistance and vice versa. In fact, conductance and resistance are reciprocals and their product equals one. Resistance is measured in ohms whereas conductance is measured in mho's or ohm's spelled backwards.
We now have enough information under our belts to move into the relationship of voltage, current, and resistance known as Ohm's Law and we will pursue that next session.
This concludes the set up discussion of electrical resistance. Are there any questions related to the concept of resistance?
This is N7KC for the Wednesday night Educational Radio Net.
Tuesday, July 8, 2008
IMPEDANCE SERIES PART 6, Lee week 7
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Labels: conductance, insulator, mho's, ohm's law, resistance
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