Basic electrostatics -- that like charges repel and unlike charges attract.

Experience building a few simple circuits using batteries and a lightbulb

Students will discuss and observe some evidence that something is moving in electrical circuits.  At the end of the lesson, they will have experienced that:

(1) Electrical current can produce magnetic effects

(2) The current in a series circuit is constant throughout the circuit (the beginning of Kirchoff's current law.

(3) The direction of compass deflection gives some evidence about the direction of current flow

(4) An ammeter works similarly to a compass, and can be used to measure the amount of current moving in a circuit.

General

From the Massachusetts Frameworks for Science and Technology/Engineering (2006)

Physics: Standards 5.1,5.2,5.45.6 . Movement of electric charges on insulators and conductors, Coulomb's law, relationship between electric and magnetic forces, measurement of current.

Simple compass, transparent for use on overhead projector

Basic circuit elements -- batteries, connectors/wires, and lightbulbs. We used the SnapCircuits kits from the Engineering the Future curriculum, developed by the Boston Museum of Science

Analog multimeters. Digital multimeters are of course safer to use as they self-correct for overloads, but the connection between the compass and meter is less clear, and a digital does not force them to connect the meter with the right polarity. Digitals are much more expensive as well, but several analog meters WILL get burned out in the first day of use -- Teacher be warned.

Procedure

Review

1.  What are the basic elements we need in order to light a light bulb?

        Ans:  Battery, light bulb, connections

2.  Electricity seems to flow, right?  What are the basic things we need in order to make it flow?
    Ans:  Source of charge (?), conducting materials (metals and the like)

    Ans:  A complete circuit.  Because the charge has to have someplace to go -- if it builds up anywhere, it will stop itself.

4.  What does any of this have to do with the aluminum foil/radio trick?

Ans: This is a conceptual question that they still do not quite get. The idea is that because they have seen that electric charges try to move away from each other in a circuit, they also move away from each other on the surface of the aluminum foil, somehow preventing the radio waves from reaching the radio.

Development

5. Discrepant event: Show them a simple compass, have them discuss how it works (aligns with earth's magnetic field). Ensure that none of them think that a compass is electrical in nature, so they expect that it won't be affected by electrical charges.

Now place it underneath a wire in a simple circuit with a battery and lightbulb on an overheard projector, with the compas needle aligned with the wire. Turn on the current, and the needle will deflect between 10-45 degrees. Make a big deal out of this!

Ask what will happen if we move the compass around in the circuit. They will expect that the deflection will be different "downstream" and "upstream" of the lightbulb. Show that this is not the case -- make sure to leave the compass in place and rotate the entire circuit into position to test different locations in the circuit (it is easy to confuse the issue to make it appear as though current is flowing in different directions in different parts of the circuit if you rotate the compass).

6. Ask what will happen if we reverse the leads to the battery?

Ans: The compass needle will deflect in the opposite direction, suggesting that the direction of current flow depends on how the battery is connected. You can use this to review, introduce, or solidify the idea of convential current flow from the positive terminal of the battery, through the circuit, and to the negative terminal.

7. What will happen if we put more light bulbs in the circuit?

Ans: They have already seen that adding extra bulbs makes them all light less brightly. With the compass, they will be able to see a smaller deflection of the compass needle -- suggesting that less current is flowing through the circuit. They should have some intuitive idea that more bulbs implies more "resistance", and thus less current, though they don't really have a solid concept of resistance yet.

8. Ask, how could we use this phenomenon to measure the amount of current in a circuit?

Ans: They will suggest, with some prodding, that the angle of deflection could be used to measure the amount of current.

9. Now have them break out their circuit kits and give them multimeters to measure current in various positions in various circuits. Handout here is a possible guide. It is important that they always connect the meter IN SERIES with the rest of the circuit, so the current is forced to travel THROUGH the meter. They should NEVER connect the ammeter directly to the batteries, as this will cause an overload current through the meter.

Questions:

1. What is the relationship between the current at different points in a series circuit? In a parallel circuit?

2. Explain conceptually the connection between a compass and your analog multimeter.

It is worth emphasizing again that several analog multimeters WILL get burned out in the first day of use, so some teachers may find it is more cost effective to provide digital meters. We found that the analog meters were much more effective pedagogically, as the students could "see" how they worked -- the connection to the compass is very clear to them. If you use analog meters, insist on checking their circuits before they are allowed to touch the meters, and make sure they see how to place the meter in series with the circuit. It may also help to show them how many different places in the circuit are equivalent to connecting directly across the battery -- connecting to the battery, even a 1.5V AA, is the fastest way to fry a meter.

Erica Wilson, The Engineering School, 12th grade Physics and Engineering teacher