Students will have a hands-on experience with electric charge.  At the end of the lesson, they will have experienced that:

(1) A mysterious "electric" force can be generated by rubbing some kinds of materials together

(2) That this force is sometimes attractive and sometimes repulsive

(3) That the ability to impart this force can be transferred through certain materials (conductors), and cannot be transferred through other materials (insulators)

(4) That materials which receive their electric power from the same source all repel each other (likes repel), and that conductors which receive their power from opposite sources attract each other (opposites attract)

(5) That whatever causes this power seems to move around to the outer extremities of conductor materials.  Students may develop a "fluid" or "charge" model to explain how this influence moves.

(6) This lesson establishes a concrete experience of electrical force to refer to when the students begin to work with electrical circuits later in the course. We also establish an experimental mindset for dealing with what is essentially a brand new phenomenon for them.

General

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

Physics: Standards 5.1,5.4. Movement of electric charges on insulators and conductors, generation of charge imbalances, and Coulomb's law

Balloons, acrylic and other plastic rods, scraps of fur, silk, and wool.

Pivot so electrostatic forces can be seen easily

Wimshurt machine, or other electrostatic generator (a hand crank motor will also work)

Electroscope, the one shown can be placed on an overhead. A simple electroscope can be made by hanging 2 ballons with fishing wire so they are separated by about 2-3 inches. They can be charged by rubbing them against each other or by rubbing a charged rod against them.

Procedure

Opener

Discrepant Event:  Show students a simple AM/FM radio, discuss their prior conceptions about how the radio works.  Settle onto "signals travel through the air to get to the radio."  Turn radio on, tune it to a station with clear reception.  Ask students what we might do to prevent the signals from getting to the radio.  They will probably suggest thick materials, so have some thick insulators on hand to try.  Show that reception does not seem to be affected much by insulating materials (don't use any conductors yet).  Now take a piece of aluminum foil, and wrap it around the radio.  The reception will go out completely.  Ask them what happened.  See if they can use the phenomenon to determine where the radio waves are coming from -- place foil just on top of the radio (not wrapped around) -- the reception remains clear.  Place foil underneath the radio, reception remains clear.  Show that radio must be completely covered by foil before reception goes out.  Ask them why.

Note:  This discrepant event is not entirely "clean", in the sense that radio waves are not strictly speaking a phenomenon of charge, and we may give them the impression that it is charge that is traveling through the air to get to the radio.   The idea we want them to get by the end today is that the conducting foil allows any moving charge to move to the _outside_ surface of the foil, so whatever electric "influence" is perpetuated by radio waves and is required by the radio can't get to the radio when the foil is _closed_ around it. This demo proved to very compelling for many of the students.  

Development

1.  Have students play with static electricity -- fur, silk, ballons, acrylic and other plastic rods, rotating pivots.  Try to find which materials produce attractive forces and which repulsive forces.   Take notes, write down observations.

2.  Discuss how we seem to be able to generate static by rubbing things together.  Ask, How does that make this "electric" phenomenon?  (Explain how electric means "amber" in greek)  Write in small groups a short theory on what rubbing does to generate the phenomenon.

3.  Use balloon electroscope to visually show attraction and repulsion to whole class

4.   Introduce electroscope as a means of measuring the presence of this force.  Allow them to see the scopes and write down how they work, in terms of the theories they produced previously.

5.  Attempt to discover what materials can transmit the force -- allow them to connect paper, wires, etc. to the electroscope.

6. Use the Wimshurst machine to generate larger static charges -- convince students that one end of the machine generates one kind of charge, and the other end generates the other kind. Show that the charge on one end seems to be attracted to the charges that are generated on the rods when we rub them with fur, silk, etc., while the charge on the other end of the machine seems to be attracted to the charge on the fur, silk, etc.

7. Show that the electroscope needle can be made to move farther up, or move down, depending on the charge added to it.

Continue to make Wimshurt available for several days for experimentation.

Questions:

  1.  Can you think of an explanation for the phenomena we have seen?

  2.  How come some materials can transfer the "influence", and others can't?

  3.  Is there more than one kind of this "influence"?

  4.  How does all of this relate to the radio demo that we started with?

1. Explore simple circuits to convince students that something is moving inside the wires -- connect this to electrostatics by connecting a wire to the electroscope.

2. Build a substance model for electricity -- have a large supply of pennies to represent electric "charges". Put them in a big pile, and ask students what the pennies would do if they were allowed to move around. Ans: The charges would move as far apart as possible.

3. Can we tell what kind of charge is actually moving in a circuit? Could it be all "positive", all "negative" or both?

4. Is the charge in a circuit all coming from the battery? Use a hula-hoop model to show that the charge could be in the circuit already, and that the battery (the teacher rotating the hoop) acts to "push" the charge around.

1. Most students will have some ideas about how electricity works, and some of them will even be right.  But many will have misconceptions that are difficult to unravel. Try to refrain from using standard terminology until it has been "earned" through a concrete experience, try to emphasize to the students the idea of understanding a mystery through observation.  Encourage them to form working hypotheses about the electric phenomena that can be justified in terms of the phenomena themselves instead of some remembered vocabulary.  Introduce vocabulary to help describe phenomena and hypotheses as their conceptions begin to gel in the "right" direction.

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