Current Electricity Unit Overview

The Current Electricity Unit is intended to engage students in myriad experiences with hands-on and computer-based materials that will help them modify their existing ideas and construct new ideas about current electricity. The unit begins with an introduction to electrical devices, to help students realize that electrical devices are not “magical”, but they are simply loops of conductors and non-conductors. From this basis, Cycle II starts to develop ideas of flow of charge and push on charge, and how series and parallel circuits are different. Cycle III introduces the capacitor and the origin of charges as everywhere in conductors. Cycle IV develops a full model of series circuits, and a more sophisticated idea of resistance and flow. Students are also introduced to the idea of electrical pressure differences. In Cycle V, students pull together all the ideas to explain parallel circuits and series-parallel circuits.

The Unit consists of five cycles:

Cycle I: What Are the Conditions Necessary to Light the Bulb?

Cycle II: Beginning To Build A Conceptual Model (effect of bulbs and

batteries in a circuit)

Cycle III: What is the Location of the Charges that Flow in a Circuit?

What Causes the Charge to Move?

Cycle IV: What Effect Do Bulbs and Batteries Have on the Movement of

Charge in Series Circuits?

Cycle V: How Does Charge Flow In Parallel Circuits?

Target Ideas for Unit

The Current Electricity Unit was designed to provide the opportunities for students to construct ideas, which are closely aligned with the ones listed below. At the end of each activity in the Development Phase, students are asked to add or modify an idea in their Idea Journal, based on evidence gathered within that activity. We have found this semi-structured approach for development of a common set of ideas to work well with high school students, prospective and in service elementary teachers. Naturally, as part of their consensus discussion for each cycle, students will probably develop these ideas in their own words. However, the conceptual content of their own ideas should be similar to these. The Teacher Guide for each cycle provides examples of the kinds of statements students actually develop in the class. After the class agrees on a set of ideas the teacher should introduce appropriate terminology and conventions so that the students' are more closely aligned with the corresponding ideas they would find in textbooks or when they talk with other students.

Target Ideas for Cycle I

1. Conductor/nonconductor Idea: Things made of metal material seem to conduct. Most other materials seem to insulate or not conduct well.

2. Two-endedness Idea: Each electrical device seems to have two conducting ends or connection points (terminals). Each connection point must be involved for the device to function in lighting the bulb.

3. Path of Conductors Idea: There needs to be a continuous loop of conducting material all the way from one end of the battery through each of the electrical devices to the other end of the battery for all the devices to function in lighting the bulb. The bulb will light when (1) the bulb filament is part of a continuous loop of conductors from one end of the battery to the other, and (2) there is no other loop of conductors from one end of the battery to the other that does not go through the filament.

4. Purpose of nonconductors Idea: The purpose of the nonconductors in the construction of bulbs, sockets, battery holders and switches is to prevent the possibility of a second path of conductors that does not go through the bulb filament (which would cause the bulb to go off).

5. Talking science: Mental models are the abstractions (ideas) we build to help us make sense of the phenomena we observe. Observations are statements or recorded measurements that represent something you can actually sense (see, hear, touch, taste, smell). Inferences are statements that represent something that requires mental modeling or a logical conclusion.

6. Operational Definitions: An operational definition is a set of criteria (a sort of recipe) for determining something. Operational definitions in this cycle include: A circuit is closed when there is a continuous loop of conductors from one end of the battery to the other end of the battery. A circuit is open when there is no continuous loop of conductors from one side of the battery to the other. A closed circuit is called a "short" circuit when there is a continuous loop of conductors from one end of the battery to the other that does not go through a bulb filament.

Target Ideas for Cycle II

1. Bulb as resistance Idea: The filaments of bulbs create a resistance to the flow of electricity. In a short circuit this flow seems to take the easiest path through the wire without a filament.

2. Bulb brightness Idea: As bulbs are added in series, the bulbs glow equally dimmer and dimmer. As bulbs are added in parallel, each bulb glows with the same brightness as a single bulb.

3. Battery and flow of electricity Idea: The greater the number of batteries connected (+) to (-) in a circuit, the greater the flow of electricity in the circuit.

4. One direction flow Idea: One kind of electricity seems to flow from one end of the battery to the other end in closed circuits.

5. Operational definitions: We call a closed circuit with only one bulb a "single-bulb" circuit. For multi-bulb circuits, we can define different kinds of closed circuits: A circuit is a series circuit when the bulbs are all in the same continuous path of conductors. A circuit is a parallel circuit when each bulb has its own conducting path. A series circuit is called a "shorted-bulb" circuit when there is a path of loop of conductors around one (or more) of the series bulbs that does not go through the bulb filament.

Target Ideas for Cycle III

1. Origin of Charges Idea: The charges that move in a circuit are located everywhere within the conductors of the circuit.

2. Push of Charges Idea: Charges push on each other, even over a distance. We assume, by convention, that the positive charges are mobile. As a battery (or other power source) pushes charge from the (+) terminal into a wire, this charge pushes on the next (mobile) charge in the wire, and so on through all the conductors in the circuit. The charge in the wire next to the (-) terminal of the battery is pushed back into the battery. In a circuit with a continuous path of conductors (i.e., not including a capacitor), the continuous movement of charge through the bulb will cause the bulb to light continuously.

3. Capacitor Idea: If there is a capacitor in the circuit, the charge does not flow through or jump over the nonconductor between the plates. Instead, the charge builds up on one plate of the capacitor. This compressed (built-up) charge pushes on the mobile charges on the other capacitor plate (over the distance between the plates), causing the movement of charge around the circuit and back into the battery. The charge that builds up on one plate also pushes back (over a distance) on all the charges in the wire from the (+) battery terminal. When the push of the battery on the mobile charges in this wire is the same size (but opposite in direction) from the push from the capacitor plate, the charge flow stops.

4. Comparison of "Power" Sources Idea: A battery stores charge and can push charge continually around a circuit (like a pump) because it provides a constant pressure difference. The energy to keep pushing is stored in the high-energy compounds inside the battery which are not present in ordinary conductors. A capacitor can store charge (like a battery), but does not have the chemical energy to keep pushing charge around a circuit; it cannot provide a constant pressure difference. A Genecon neither stores charge nor has the energy to push charge — the charge comes from the conductors inside the Genecon and the energy comes from the muscles of the person turning the crank. A Genecon cannot provide a constant pressure difference (although extremely smooth cranking can produce nearly constant pressure difference for a short while).

	Store Charge?	Provide Constant Pressure Difference?	Energy to Push Charge?
battery	yes	yes	yes — chemical
capacitor	yes	no	no
Genecon	no	no	no — energy from muscles

Target Ideas for Cycle IV

1. Rate of charge flow Idea: Bulbs control the rate at which charge flows in the entire circuit (not the total amount of charge that flows). The rate of charge flow (called "current") is the amount of charge that flows by a point in a unit of time. This is not the same as the speed of the moving charges. A compass deflection is an indicator of the rate of charge flow.

2. Resistance of wires and bulbs Idea: Different kinds of conductors (e.g., copper, nichrome) have different "resistance" to the flow of charge. The longer the wire, the greater the resistance and the smaller the rate of charge flow. The thicker the wire, the smaller the resistance and the greater the rate of charge flow.
3. Different bulbs have different filaments with different resistance (e.g., a long bulb has a higher resistance than a round bulb).

4. Rate of charge flow in series circuits Idea: The rate of charge flow is the same everywhere in a series circuit. That is, bulbs do not "use-up" current. (Conservation of Current)

5. Resistance of series circuits Idea: The more bulbs in a series, the larger the resistance, and the smaller the rate of flow of charge through the circuit. (This can be understood by considering that the total length of filament wire increases as each bulb filament is added to a series circuit.)

6. Rate of charge flow and battery voltage Idea: As more batteries are added in a circuit (greater battery voltage) there is a greater total push on the (mobile) charges in the circuit conductors, and the rate of charge flow (current) increases.

7. Bulb brightness and collisions Idea: The heating of a bulb filament arises from collisions between the moving charges and the atoms of the material. The amount of heating (and thus the bulb's brightness) depends both on the charge flow rate and the resistance of the bulb's filament. The greater the charge flow rate through a filament, the hotter the filament becomes and the brighter it will glow. If the charge flow rate through two or more bulbs is the same, the relative brightness of the bulbs will depend on their relative resistance: If the bulbs are identical (same resistance), they will glow equally; If the bulbs are not identical, the bulb with the higher resistance will glow more brightly.

Target Ideas for Cycle V

1. Independence of Loops Idea: The battery pushes charge at the same rate through each loop of a parallel circuit as if each loop were the only one in the circuit.

2. Conservation of current in parallel circuits Idea: When charge flows through a junction, it divides. The rate of charge flow (current) through each path depends on the total resistance of that path — the larger the resistance of the path, the smaller the rate of charge flow through that path. However, the sum of the flow rates through each path is equal to the total rate of charge flow before the junction.

3. Resistance of parallel circuits Idea: As branches are added parallel to the circuit, the total resistance of the circuit decreases. So the total rate of charge flow through the battery increases.

4. Resistance of series/parallel circuits Idea: When a branch is added parallel to a series bulb, the parallel combination has less resistance to the flow of charge than the least resistant bulb in the combination. So the total rate of charge flow through the remaining series bulbs in the circuit increases.

5. Rate of Charge Flow and Battery Run Down Idea: A battery transfers a fixed amount of energy to a circuit in its lifetime. How that energy is transferred depends on the circuit. The higher the rate of charge flow through the circuit, the faster the battery will run down.