Lesson Plan --- Electric Charges*

Objectives:

1. Students understand that charging is the separation, not the creation, of electric charges. 2. Students will know how an electroscope works and how to construct a simple electroscope. 3. Students will understand how electric charges are distributed on charged conductors and how electric discharging related to a conductor’s shape (actually curvature). 4. Students will know some common applications of utilizing and avoiding static electricity.

Materials:

For demonstration: a Van de Graaff generator, a Tesla coil, a variable DC power supply, an electroscope, two cap nuts, an open plastic and an open metal container, some popcorn or packing peanuts, and a pair of metal objects. The two objects have the same shape and each of them has an insulating support. They can be put in close contact and can also be separated completely.

For each group of students: a transparent plastic cup with a small hole on the bottom, a paper clip, a wooden clip, a piece of aluminum foil (6” x 6”), an aluminum strip about half inch wide, a balloon, and a piece of wool cloth.

Activities:

1. Demonstrate that charging can only separate but cannot create electric charges.  Charge a pair of contacted metal objects by induction so that each object bears same amount but different type of electric charges after moving them apart.  Show students with an electroscope that each object bears same amount of electric charges.  Combine the two objects then measure the electric charges on them using the electroscope. There should be no charges at all. 2. Ask students in each group to construct their own electroscope then do the same experiment described above. (Students might have to share the use of the metal objects described above since there might not be enough such devices.) Tell students that conservation of electric charges is another important law of nature. 3. Use a Van de Graaff generator and popcorns or packing peanuts to demonstrate that electric charges on conductors are spread over on their outside surface only. In this demonstration, put the popcorns first in a plastic container then place the container on top of the generator. The popcorns will fly away from the container soon after the generator is turned on. Ask students to explain this phenomenon. Repeat this demonstration but this time the popcorns are put into a metal container. Not a single piece of the popcorn will move, let alone fly, no matter how long the generator is turned on. Ask students to think about why the popcorns behave so differently and guide them to reach the right conclusion. 4. Use a Tesla coil to demonstrate that: a) the distribution of electric charges on a conductor has something to do with the shape of the conductor; b) discharge will occur at the sharpest point on the conductor. In this demonstration, first use the initial electrodes to show the discharge. Students will see blue sparks between the electrodes. Ask students why the discharge occurs only between the two ends of the electrodes while each whole electrode is charged. Tell them it is because the density of electric charges is much higher at the ends (sharper points) than anywhere else. Ask students if a nearby charged object will affect the distribution. Repeat the demonstration but using the cap nut to cover the sharp ends of the electrodes. This time, no discharge occurs. Ask students to figure out the reason. 5. Tell students that properties of static electricity have been utilized in some commonly used appliances such as laser printer, photocopier, static electric precipitator, and electrostatic spray painting. Yet in many circumstances, possible damage caused by static electricity must be avoided. 6. Show a simulation about how lightning is generated. Tell students a huge amount of energy will be released in a lightning. Ask them to discuss possible ways to save this wasted energy. First, they need to identify two most difficult problems associated with the task if they are required to design a device to save the energy. Second, they are encouraged to propose a solution to solve the problems using wild imagination. Third, they are required to search the Internet to find related research about this topic, choose a most promising approach, and describe to the class the chosen approach.

Assessment:

Students will take a quiz about static electricity.

*How to generate electric charges and Coulomb’s Law are not covered in this lesson plan as they have been taught earlier.