Sunday, January 4, 2015

Basic circuit concepts : four measurable circuit quantities, voltage, current, resistance and watts

Basic Circuit Concepts
4–1 FOUR MEASURABLE CIRCUIT QUANTITIES

In dealing with any useful quantity, whether vegetables, steel bars, or electrons, a system of measurement must be used to keep track of the production, transfer, and use of the commodity. So it is with electric circuits that are arranged to obtain practical use of electrical energy. There are four fundamental concepts that constitute the elements of an electric circuit, namely

Voltage, Current, Resistance, and Watts

Since these words represent abstractions, quantities that cannot be directly perceived by one’s senses, it is very important that you develop a correct mental image of the key concepts. The following sections are designed to help you gain a clear understanding of these terms.

4–2 VOLTAGE

Let us consider a simple source of DC, a common flashlight battery, which is technically known as a dry cell; see Figure 4–1. The little minus signs in the drawing represent a huge quantity of electrons. This should suggest that the shell, accessible at the bottom plate, has a vast surplus of electrons as compared with the number of electrons at the top cap of the cell.

Remember that the electrons have a negative charge, making the bottom plate with its surplus of electrons more negative than the top cap with its electron deficiency. It is this difference in potential (difference in the number of electrical charges) that is known as voltage. Frequently, you will find the term electromotive force (emf ) used to describe the same condition.

It is important to note that protons do not enter into our discussion when we de- fine the positive pole of the dry cell in Figure 4–1. (Remember, protons are the carriers of positive charges.) Both the positive and the negative poles can be defined in terms of their relative number of electrons. In this context the word positive simply means less negative, as compared to some other reference point.

Voltage is always measured between two points of different potential. The unit of measure used for this is called the volt (V), named after the Italian physicist Alessandro Volta.

image

Thus we can say that our dry cell has an emf of 1.5 volts. Compare this with the two terminals of a car battery. Between these two points we would expect to measure 12 volts. To measure such a voltage, we use an instrument known as a voltmeter.

Another common concept of voltage is electrical pressure. Voltage is the force that pushes the electrons through a circuit in much the same way that pressure pushes liquid through a pipe. The higher the voltage, the greater the force moving the electrons. Electric circuits are often compared to a water system because the concepts are similar. In the illustration shown in Figure 4–2, a pump is used to supply the pressure necessary to cause water to flow through the water system. In like manner, a battery supplies the electrical pressure (voltage) necessary to cause electrons to flow through the electric circuit. In the water sys- tem a flowmeter measures the amount of flow in gallons per minute. In the electric circuit an ammeter measures the flow of current. In the water system a pressure gauge measures the difference in pressure produced by the pump in pounds per square inch. In the electric circuit a voltmeter measures the voltage of the battery. A reducer is used in the water system to limit the flow of water. A resistive device is employed in the electric circuit to limit the flow of current. It is important to understand that neither pressure nor voltage flows. Pressure causes water to flow and voltage causes electrons to flow. Water can flow and current can flow, but pressure and voltage cannot.

image

4–3 CURRENT

For discussion purposes, let us pretend that we connect a heavy wire between the poles of our battery. In reality this must never be done because it constitutes an undesirable condition known as a short circuit. Figure 4–3 shows how such a wire provides a path by which the electron surplus can drain off toward the point of electron deficiency. It is this motion of electrons that we refer to as current. Notice that this definition of current considers the motion of free electrons from a point of electron surplus to a point of electron shortage. In other words, the electron theory defines current flow as a motion of free electrons from the negative pole of a source, through the outer circuit path, and back to the positive pole. This is the definition of electron current that we will use throughout this book.

However, there is yet another theory that leads to the conclusion that current flows from positive to negative. This is known as conventional current flow. The concept of conventional current can be very useful and will be reintroduced to you at a later date when you begin your study of electronics and semiconductors.

image

Current as a Rate of Flow

In measuring the rate of electron flow, we are concerned with a quantity rate rather than simple speed. In ordinary electrical devices, the number of electrons passing through the device each second is the important consideration, not their speed in miles per hour. Water pumps are rated in gallons per minute, ventilation fans in cubic feet per minute, and grain-handling equipment in bushels per hour. All of these are quantity rates.

To establish such a rate for electrons, we must first decide on a measure of quan- tity. We could use the number of electrons passing a point per second, but so many pass by a point in one second that the number is too great to use. Instead, we lump together 6,250,000,000,000,000,000 (6.25 3 1018) electrons and call this quantity a coulomb of elec- trons (in honor of Charles Coulomb, a French scientist). To show just how large a number 6,250,000,000,000,000,000 is, assume that this many flies are in New York State in the sum- mer. If all of the flies were killed, they would cover the land area of the state to a depth of 51⁄2 feet, so densely packed that 500 fly cadavers would be compressed into each cubic inch.

We can measure the rate of electron flow in coulombs per second. This is comparable to measuring the flow of traffic in cars per hour or measuring air current in cubic feet per sec- ond. It can also be compared to measuring water flow in gallons per second; see Figure 4–4.

To express the rate of electron flow, the phrase coulombs per second is seldom used. Rather, we use one word that means coulombs per second. This word is ampere (A), again named in honor of a French scientist, André Ampère (1775–1836).

One ampere is a flow rate of 1 coulomb/second.

To measure the current flow in an electric circuit, we use an amperemeter, or ammeter.

Current Speed

People sometimes get into discussions of what is meant by the speed of electricity. A more exact term than speed of electricity is needed to distinguish between (1) the aver- age speed of individual electrons as they drift through the wire and (2) the speed of the impulse. We realize that when we turn on a light, the light is on immediately. In a house

image

wired with #12-gauge wire, calculations show that there are so many electrons in the wire that the average speed of individual electrons is only about 3 inches per hour when the cur- rent is 1 ampere. Three inches per hour is the speed of the electron drift through the copper wire. However, keep in mind that since the wire is full of electrons to begin with, they start moving everywhere at once when the switch is turned on. The actual speed of this impulse depends on the arrangement of the wires and may be anything from a few thousand miles per second to the speed of light, 186,000 miles per second, as a theoretical top limit.

4–4 RESISTANCE

A single stroke of the oars will not keep a rowboat moving indefinitely at the same rate of speed. Neither will voltage keep electrons moving indefinitely at the same rate. Friction slows the movement. This internal friction, which retards the flow of current (electrons) through a material, is called electrical resistance.

Electrons slide through a copper wire easily, like a boat through water. Electrons also move through iron and some metal alloys fairly easily, although not as easily as they do through copper. But there are many materials through which electrons can move hardly at all, even if a lot of pressure (high voltage) is applied. Trying to move electrons through sulfur, glass, plastic, or porcelain, for example, is about as effective as trying to row a boat on a concrete road or on plowed ground.

The list in Figure 4–5 compares the resistance of common materials. Those of highest resistance (so high it is difficult even to measure) are the best insulators. Those of lowest resistance are the best conductors. In the range between the two extremes are the materials that are poor conductors yet do not have quite enough resistance to be called insulators.

image

Recall from Chapter 1 that elements whose atoms have only one, two, or three electrons in the outer shell (or orbit) are conductors, because these electrons are free to move. Elements whose atoms have only five, six, or seven electrons in the outer shell are insula- tors, because there are no free electrons.

This unit of measure of resistance is the ohm, which is denoted by the Greek let- ter omega (Ω), named after a German scientist, Georg Ohm. Electrical resistance can be measured with an ohmmeter. Many multimeters (multipurpose instruments), such as a VOM, have an ohmmeter function built in.

The thickness of a conductor also affects the resistance of a circuit. Assume that the heavy wire we placed across the dry cell in Figure 4–3 has a short section of very thin wire inserted, as shown in Figure 4–6. This thin section represents a resistance to the flow of current in the same manner that the bottleneck of a one-lane detour represents a reduction in the flow of traffic on a broad freeway.

It is important to note that heat is being developed whenever an electrical current is forced through a resistance. It is conceivable that a resistance wire, like the one shown in Figure 4–6, becomes so hot that it begins to glow intensely and gives off light. This is called incandescence. Ordinary lightbulbs, called incandescent lamps, operate on this principle.

Most of the heat-generating appliances that you know contain an electrical resistance wire. Think, for instance, of the cigarette lighter in the dashboard of your car. It, too, has a built-in resistance wire that glows from heat whenever a voltage forces current through the resistance.

image

4–5 WATTS

Electrical power is measured in watts. Electricity is a form of pure energy and in accord with basic physical laws can be neither created nor destroyed. The form of energy can be changed, however. The watt is a measure of the amount of electrical energy that is changed to some other form. As we have discussed, when current flows through a resistor, the resistor becomes hot; this is an example of electrical energy being converted to thermal energy. When electricity is used to power a motor, electrical energy is converted to kinetic (moving) energy. The quantity watts is often called power and is generally represented by the letter P in electrical formulas. Although the letter P is used to represent power, the letter W denotes the quantity watts. Power in an electric circuit is measured with a wattmeter.