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Electricity and Magnetism

Investigation 2 – Concept Day

 

 

 

 

 

 

 

Electricity and Magnetism: Investigation 2

Concept Day

SLIDE ELECT-2-1

In this Investigation, we continue to explore the interaction between voltage, current, and resistance in an electrical circuit.

We will begin by taking time to look at the common D cell battery and how it works. This wonder of ingenuity melds chemistry and electronics into one of the most useful portable sources of energy we have today.

We will also focus on some of the practical uses of resistance in circuits including the incandescent light bulb and other heat- and light-generating devices.

Finally, we will look at how resistors are represented in schematic drawings and review some of the circuits that you will use in Investigation 2 lab.

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SLIDE ELECT-2-2

A battery is a source of chemical potential energy. It contains chemicals that, when the battery is introduced into a circuit, cause a flow of electrons (e). In a completed circuit, the chemical potential energy stored in a battery changes into the kinetic energy of moving electrons of electricity. Therefore, chemical potential energy is transformed into electric kinetic energy.

Also at the top of the slide, reduction and oxidation reactions are briefly defined. In the simplest of terms, a reduction reaction accepts electrons (e-) and an oxidation reaction gives up or donates, electrons. As we will see, a battery works by simply controlling the movement of electrons from the oxidation of one chemical (zinc in this example) to another chemical (carbon in this example), which accepts them. In other types of modern batteries, oxidation and reduction reactions involve other metals and elements. Lithium batteries, for example, are currently being used in a wide variety of electronic devices, ranging from the smallest hearing aid and watch batteries to electric cars and trucks. In lithium batteries, lithium metal is oxidized with the release of electrons to produce the electric current.

As can be seen, the outer zinc case is where the oxidation reaction occurs. The carbon rod is where the reduction reaction occurs. Zinc wants to give up its electrons and carbon wants to accept them, but they are not able to do so in the battery because an electrolyte paste separates them. The metal bottom of the battery is attached to the zinc case. The metal cap at the other end of the battery is attached to the carbon rod.

Unless the battery is connected into a complete circuit, the situation described above remains in place and there is no electron flow between the zinc and carbon components of the battery. However, once the battery is inserted into a circuit, things change. Electrons from the zinc leave the battery through the bottom metal plate, called the anode, which has a negative sign (-) indicating that it loses electrons. These electrons with flow through the completed circuit toward the other end of the battery.

The electrons flow through the wire(s) of the circuit, where they eventually reach their destination, the cathode of the battery, which is attached to the carbon rod. The carbon accepts the electrons given up by the zinc. The cathode is labeled with a positive sign (+) because it accepts electrons.

NOTE: While chemical oxidation and reduction reactions are extremely common and important, we do not wish to overemphasize them here. If you simply know that a reduction reaction accepts electrons (carbon in this case) and an oxidation reaction gives up electrons (zinc in this case), you will at least have a frame of reference when these types of reactions are examined further in high school chemistry.

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SLIDE ELECT-2-3

This slide is similar to one seen in Investigation 1. It is used here to show a completed circuit following the discussion of how a battery works on the previous slide.

Electrons leave the anode (negative terminal) of the battery and move through the copper wire in the direction of the cathode (positive terminal) of the battery. In the image, we see individual electrons moving along the battery from the anode to the cathode.

However, the current is considered to move in the opposite direction as the electrons, as noted on this slide. As discussed previously, the directionality of the current was established prior to knowledge of the movement of electrons in a circuit.

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SLIDE ELECT-2-4

This slide introduces resistance. As noted, if a material in a circuit has a high resistance, it means that it is difficult for electrons to get through it (it resists electron flow). When such materials are encountered in a circuit, heat and light are often given off. Sometimes we think of heat as wasted energy. For example, you might notice that your computer can become fairly warm after it runs for a while. Since the function of your computer has nothing to do with heat generation and there are more efficient ways to heat your office, we may think of this loss to heat as a waste of energy.

On the other hand, resistance can be controlled and exploited to our advantage in a whole range of electrical devices. Electric ovens, irons, toasters, and driers all use resistance to electron flow to produce useful heat. Resistance can also be used to produce light, as we will see in further detail on the next slide.

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SLIDE ELECT-2-5

An incandescent light bulb uses electrical resistance to produce light. In general, thinner wires are more resistant to the flow of electrons than thicker wires. This is one reason power cables conducting electricity from power plants are enormous while those inside a battery-operated stopwatch are quite thin. All wires that are used to conduct electricity are rated for the amount of current that they can safely handle without overheating.

In an incandescent light bulb, a current is forced to pass through the very narrow tungsten wire of the filament. In addition to the narrowness and extended length (about two meters) of the filament, the element tungsten itself has a lower conductivity than copper wire, so it has added resistance for this reason as well.

As a result of the extreme resistance of the thin tungsten filament, it quickly heats to a temperature of over 2,000oC when it is switched on. This causes the filament to glow and give off usable light in the form of photons.

The reason we use incandescent light bulbs is for the light they produce. Therefore, the heat they generate in the process is essentially wasted energy. In fact, about 90% of the electrical energy used by incandescent light bulbs is lost as heat; only 10% goes to producing photons and light. Although the incandescent light bulb has served us well for over a hundred years, it is obviously a very inefficient and energy-wasting device. Fortunately, new technology such as LED lights is much more efficient because they provide light while producing much less heat. LED technology is very much different than that used in incandescent light bulbs and we will not go into further detail concerning LED lights here.

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SLIDE ELECT-2-6

This is simply an introductory slide for resistors. While we have referred to resistance in devices such as hair driers and the filament of incandescent light bulbs, the term “resistor” is also used to refer to a specific type of electrical component. In this sense, a resistor is a component introduced into electrical circuits to resist electron flow and change the current.

In the lab, you have already used such resistors in your circuits. In Investigation 2 lab, you will study resistance and resistors in greater detail.

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SLIDE ELECT-2-7

This slide shows the laboratory setup and schematic drawings of various resistor configurations. The second and third setups introduce two resistors into the circuit, the first pair of resistors in series and the second in parallel.

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SLIDE ELECT-2-8

The final two slides show the laboratory setup and schematic drawings of a circuit that you will use in Investigation 2 lab.  In this slide, you are shown how to measure current in a circuit while different resistor configurations are tested.

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SLIDE ELECT-2-9

In this final slide, you are shown how to measure voltage in a circuit while different resistor configurations are tested.

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