CHAPTER 6


ARE ELECTRICITY AND MAGNETISM RELATED?

The grand aim of all science is to cover
the greatest number of empirical facts by
logical deduction from the smallest
number of hypothesis....
--Albert Einstein
1950

James Clerk Maxwell, a nineteenth century English scientist, discovered that a simple ratio existed between the strength of the electric force between electric charges and the strength of the magnetic force between electric charges when in motion. This ratio that Maxwell discovered turned out to be the square of the speed of light! This monumental discovery of a simple pattern among electric, magnetic and light phenomena constituted as a gigantic step forward in science as Newton’s discovery of the Laws of Motion and Gravitation two hundred years earlier.

What scientific progress since Newton and Benjamin Franklin led Maxwell to his discovery? We will begin our study of electricity with an experiment which will lead us to understand what does and does not constitute and electric circuit.


Personal Lab 7: Are Force and Extension of Your Slinky Related?
[Chapter6]

Personal Lab 8: What Kinds of Materials Conduct Electricity?
[Chapter6]

CIRCUIT COMPONENTS




Below is a key to the symbols for various circuit components in electric circuit diagrams, and a brief description of each component.

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Resistor: Has stripes of different colors, each coded to indicate the numerical value of the "resistance". Resistances are measured in units of Ohms (W) and resistors, as the name implies, impede the flow of electric current in a circuit. Examine the 220 W resistor used in your circuit tester. It has two red bands and a brown band painted around the circumference of the resistor. In Appendix B the codes for the different colors in the bands of resistors are given. The first two bands indicate the first two numerical values of the resistance and the third band, brown, indicates the number you multiply times the first two. In this case, brown corresponds to 100, so 100 x 22 is 220 W.

The nichrome (nickel plus chromium) wire has a very large resistance compared to the normal copper wires. The length of the nichrome wire connected into the circuit will affect the bulb brightness. Why? The temperature rises in the a circuit component with a high resistance to electric current. How would you design a filament of a light bulb? Why do you think it is coiled?
Resistors are used to decrease the electric current in a circuit. Why do you think a 220 W resistor was included as part of the electric circuit of your circuit tester?

Capacitor: Come in a variety of forms as indicated in the photograph. A capacitor stores electric charge, just as a thunder cloud does during a thunder and lightning storm. A basic capacitor is simply two metal plates that can store a net positive or negative electric charge. When sufficient charge of opposite types has accumulated on the two plates of a capacitor, and an electric circuit is connected, the capacitor will discharge across the gap between the two plates permitting the charge that constitutes the current to flow in the circuit until the stored charge on the two capacitor plates is balanced again, and there is no net charge. Your flash bulb on your camera operates on the principle of a capacitor, storing charge between flashes, and completing the circuit for a moment as the capacitor discharges. The flash shuts off when the capacitor has been discharged, usually in fractions of a second. Can you think of other uses of capacitors in electric circuits? What did you observe that the capacitor did to the bulb brightness in your circuits? Capacitance is a measure of a capacitor’s ability to store electric charge, and is measured in units of Farads.

Diode: The diode has a stripe at one end to indicate the negative end. Be sure to connect this end to the negative terminal of you battery, or you could damage the diode. Electric current flows in only one direction through a diode, as your students should discover in their explorations. We refer to this property of diodes as "polarization". Is a battery polarized? The reason for this characteristic is related to electric current being associated with electrons, which are negatively charged or positive ions, which are positively charged atoms. In copper wires it is the electrons that move. In the human body it is the positive ions, such as ionized sodium, Na+, or ionized potassium, K+, which move, and constituent the electric current when we receive a shock.

Light Emitting Diode (LED):
This circuit component was used as the testing light in your circuit tester. It is a diode that emits light when the current flows in the correct direction. Note that one of the two wires extending from the diode is shorter than the other, and also that one side of the resin encasement of the LED is flat. The flat side and the short wire both indicate the negative side of the LED, and should always be connected to the negative side of the battery. Check that this is so for your circuit tester. The 220 W resistor was added to the circuit of your circuit tester to reduce the current through the LED. With no resistor in the circuit, the 9 volt battery would produce a current that could destroy the LED. So the 220 W resistor was introduced to protect the LED, by reducing the current flow in the circuit of the circuit tester. LED’s, diodes and other electronic components of circuits, such as transistors, require very little current for their operation. This has led to the miniaturization of electric circuits, and to the reduction of power required to operate appliances and instruments of all types. It is instructive to make a list of as many electronic circuits that you can, and to try to imagine electric circuits before when diodes were called "vacuum tubes" and were as large as small cans of tomato paste!

Light Bulb: You should at some time carefully break the bulb on one of the small light bulbs sometime, and examine the bulb filament with a magnifying lens. You should also figure out the path of the electric current as it enters, flows through the filament, and exits the bulb. Current is conventionally thought of as flowing from the positive terminal of a battery through a circuit to the negative terminal of the battery. (The electrons in a copper wire actually flow the opposite way, but we rarely need to consider the actual electron flow when dealing with electric circuits. Also remember that there can be circuits in which it is the positive ions that flow through a conductor.)

Why is the filament of a bulb encased in glass? Connect your broken bulb in a simple circuit to see a graphic demonstration of why. The bulb has no air (vacuum), so the filament does not oxidize (burn) when it is resisting the flow of electric current, and thus being heated to very high temperatures. Caution: Do not try this experiment with 120 volt power and a normal bulb, as it is quite dangerous to do so.

Switch: The switch in your simple circuits were devised very simply by using a 1/4S steel bolt to complete the connection. Would an aluminum bolt have worked? The simplest of all switches are referred to as "single pole, single throw". Can you design a circuit in which a "double pole, double throw" switch might be useful? The point of using the switch is to make the battery last longer, since current flows only when the circuit is "closed" (switch is in "on" position). Any time that the circuit is "open" (switch is in "off" position), the electric energy stored in the battery is not being expended, and no current is flowing in the electric circuit.

Battery: You have experimented with 1.5 and 9 volt dry cell direct current batteries. They are called dry cell, because the acid material inside the zinc case is dry not wet. The "buttonlike" knob at the positive end of a battery is the end of a copper post which extends into the acid material of the battery innards. The combination of the zinc case and the copper electrode protruding into the acid environment causes zinc and copper atoms to dissolve from the two kinds of metals. Chemical reactions of these two different kinds of metal atoms with the acid gives rise to metal ions which respond to the net static electric charges of the copper electrode and the inner zinc walls of the battery case. One of the metals has a greater propensity of migrating to the wall or the electrode than the other, giving rise to an electric charge polarity inside the battery. When the battery is introduced to a closed electric circuit, the unequal net charges on the battery wall and the central electrode tend to neutralize by sending a current of electric charge through the circuit to the other electrode of the battery. Since there is only a finite amount of materials inside the battery, any battery eventually becomes inactive, or dead.

Coil: To complete our far from complete inventory of electric components, we mention a coil, which in its simplest form is simply a coil of wire which can be introduced to the circuit. When we study connections between electricity and magnetism, it may become clear why and how coils might be useful in electric circuits.


Personal Lab 9: What Kinds of Materials Are Magnetic?
[Chapter6]


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