Name(s) ELECTRIC CURRT The primary purpose of this activity is to decide upon a model for electric current. As is the case for all scientific models, your electricity model should be able to explain observed phenomena and to predict the outcome of experiments. Part 1 CREATING A MODEL OF ELECTRICITY 1) Obtain a light bulb, a battery, and as many wires as you wish. Determine how to make the light bulb turn on, and draw a picture of your circuit below. Arrange your circuit components in two different ways, similar to what you just did, only this time arrange the components so that the bulb does not turn on. 2) Draw a picture of each of your two new circuits which do not work below. Underneath each picture, explain why you think the bulb did not go on. I17
1 3 ELECTRIC CURRT FOUR MODELS FOR ELECTRIC CURRT Four different models for electricity flow in a circuit lighting a light bulb are presented below. These models represent the four most common models of electricity suggested by students. (See, for example, Learning in Science: The Implications of Children s Science, Osborne, R., and Freyberg, P., Heinemann, Aukland, New Zealand, 1985.) Analyze these four models, and decide which one seems to be the best. In order to help you decide, you should try various combinations of bulbs and batteries and test each of the four models. You may use multiple numbers of bulbs and batteries in any given circuit in order to carry out your tests. In the space provided below and on the next page, make arguments and drawings showing how you came to your conclusion. Model A: Charge comes out of one end of the battery (either the positive or negative side), goes into the bulb, and produces light. Model B: Positive charge comes from the positive side of the battery, and negative charge comes from the negative side of the battery. These two different charges meet in the bulb and produce light. Model C: Charge comes from one end of the battery (either positive or negative). Some of this charge gets used up in the bulb to produce light, and the rest goes back into the battery. Model D: Charge comes from one end of the battery (either positive or negative). All of this charge goes back into the other end of the battery. UNIT 1 SCITIFIC MODELS I18
1 3 ELECTRIC CURRT DO NOT PROCEED BEYOND THIS POINT UNTIL AFTER YOU HAVE DISCUSSED THE MODELS FOR ELECTRIC CURRT AS A CLASS. UNIT 1 SCITIFIC MODELS I19
1 3 ELECTRIC CURRT PART 2 APPLICATIONS OF THE MODEL In a sentence or two, describe a model for electric current in a circuit containing a source of power (a battery) and a light bulb. 1) Obtain a cardboard card with holes. Some of the holes in the card are connected to other holes by strips of aluminum foil that are hidden under the cardboard. Without disassembling the card, use a battery, a bulb, and perhaps some wire and determine which holes are connected. Show your answer on the diagram below. A B C 10 D E 10 F 2) Make a string of at least three identical light bulbs such that if one of the bulbs is unscrewed, then all of the bulbs will go out. Draw a diagram of your circuit below. (You may want to use more than one battery for this activity if your bulbs are too dim with just a single battery.) Note: These lights are said to be in series. What is an obvious disadvantage to this arrangement of lights? UNIT 1 SCITIFIC MODELS I20
1 3 ELECTRIC CURRT 3) Arrange three identical light bulbs with one battery such that even if any one of the bulbs are removed, the other two bulbs will not go out. Draw a diagram of your circuit below. Note: These lights are said to be in parallel. The advantage to this arrangement of lights is obvious. Compare the brightness of these bulbs to the brightness of the bulbs when they were in series. Can you imagine a possible disadvantage of putting too many lights in parallel? Experimenting with switches Several different kinds of switches will be needed to answer questions 4 through 6. 4) Find a way to put a switch in a circuit such that a bulb goes on when the switch in the circuit is "closed" and off when the switch is "open". Draw a diagram of this circuit that clearly shows the working of the switch. 5) Make a circuit using one battery and two bulbs such that one bulb is on and the other bulb off when a switch is in one position, and where the opposite bulb goes on and the first one goes off when the switch is in the opposite position. Draw a diagram of this circuit that clearly shows the working of the switch. UNIT 1 SCITIFIC MODELS I21
1 3 ELECTRIC CURRT 6) Make a circuit with one battery, one light bulb, and two switches such that both switches can turn the bulb on or off no matter what position the other switch is in. This is a common circuit found in homes. Draw a diagram of this circuit that clearly shows the workings and positions of the two switches. (This is a hard problem! If you don t solve it within about 15 minutes, you probably should go on to something else.) UNIT 1 SCITIFIC MODELS I22
Electricity Content Overview The very simplest electrical circuit is composed of three elements: 1. A source which causes electrical charges to move (This might be a battery or an electrical power plant, for example.) 2. A conductor or carrier of the electric current (This element is the pathway through which the electrical charges flow. It is usually a metal wire.) 3. A user something which turns the electrical energy into some other form of energy (such as a light bulb, an electric motor, etc.) [The concept of energy is covered more fully in Units 3 and 4.] In the activities preceding this discussion, the batteries caused the charges to move. The charges moved through the conductors (wires), which caused the light bulbs to turn on. Shorthand Notations There are standard ways of drawing electric circuits which help everyone identify the components making up the circuit. These schematic representations can be used to save time. 10 or a source of electrical energy = = wire = on / off switch = 10 is the same as UNIT 1 SCITIFIC MODELS I23
ELECTRICITY CONTT OVERVIEW The term electrical current refers to the electric charges flowing through the conductors in a circuit. According to the best model for electrical current, the charges moving in an electrical circuit move in a circuitous path (i.e., they eventually return to where they started), they do not get used up in the circuit. A diagram depicting this motion is shown below. There is just as much current (moving charges) at any location within this circuit as there is at any other point in the circuit. Charges moving away from energy source Source of energy to make charges flow element turning electrical energy into light Charges moving toward energy source Charges moving away from energy source 1current splits here 10 0 If there is a branch in a circuit, then the electrical current splits : some of the charges go one way, and the rest go the other way, but eventually all the charges come back together again. In the circuit shown at the right, half of the current would go through the bulb on the left, and half of the current would go through the bulb on the right. At all times there are just as many charges flowing away from the energy source as there are charges flowing into the energy source. Charges moving toward the energy source 10 current recombines here All materials contain both positive and negative charges. Therefore, it is not the job of the battery (or any other source of electrical energy in a circuit) to provide the charges for a circuit. The battery simply causes the charges already present to start moving. In almost all electrical circuits, including all the ones used in this course, it is the negative charges which flow to make the electrical current. However, it is very difficult to prove that this is the case. In fact, mostly due to historical reasons, most books talk about electrical circuits as if it were the positive charges that are moving. In order to be consistent with this convention, all the diagrams presented here have also been drawn as if the positive charges were flowing through the UNIT 1 SCITIFIC MODELS I24
ELECTRICITY CONTT OVERVIEW circuits. Remember, as far as most applications are concerned, it is impossible to tell whether or not it is the positive or the negative charges that are flowing. In fact, you may wish to reconsider your model for electricity in order to show that your model will work equally well no matter which kind of charge you assume is moving. There are several analogies that can be thought up to help understand electrical circuits. Some people find them helpful, others find them confusing. One of the more creative ones is to picture the electrical charges as little peasants carrying backpacks. A peasant on the road (wire) from battery to bulb has a backpack containing electrical energy. The peasant goes through the bulb and delivers this energy to the light bulb, and then trundles back toward the other end of the battery for a refill. Peasants are not consumed at the bulb, they just empty their packs at the bulb. Similarly, the battery does not create the peasants, but simply fills their packs. If two bulbs are connected in series, each peasant has to go through both bulbs, and gives up only part of its energy at each. If two bulbs are connected in parallel, then some peasants go through one bulb (giving up all their energy), while others go through the other bulb. Of course, this analogy isn t perfect (no analogy is), but some find it useful. UNIT 1 SCITIFIC MODELS I25
ELECTRICITY CONTT OVERVIEW UNIT 1 SCITIFIC MODELS I26