Coulomb's Law

Austin A.Winther               Paul Robeson High School  
                               6835 S. Normal
                               Chicago, IL 60621
                               1-312-723-1700, ext. 411

Objectives: 
     Observe qualitatively the relationship between electrical charge and force.  
     Determine experimentally the quantitative relationship between force and the 
center to center distance between charged objects.  
     Experience reinforcement of the above concepts through preferred perceptual 
modes (tactile, kinesthetic, auditory, or visual)  through appropriate activities.  

Materials:
     Van der Graff generator, ring stand, coffee can, styrofoam cups, string, 
rubber tipped dart, paper and plastic sheets (optional), overhead projector, 
file folders, graphite coated pith or styrofoam balls, nylon stranded string, 
masking tape, centimeter or other grid on overhead transparency, plastic sticks, 
wood blocks, oak tag board, markers, coat hangers, ruler, index cards, transparent 
tape, large (approximately 8m*.5m) sheet craft or butcher paper, rubber and glass 
rods, silk cloth, piece of fur, electrophorus. 
    
Procedure:
     1. Set up the Van der Graff, in line with a veriac if the generator does not  
have a variable control, with the coffee can inverted over the top of the ring 
stand in such a way that, when the generator is turned on, a clearly visible spark 
jumps from the generator to the coffee can. Have the students observe the spark 
and the noise produced and describe what they see. Draw the connection with 
lightning and be sure the students understand that the light and noise which they 
saw and heard were due to electrical charge. Run the generator at a MUCH SLOWER 
SPEED and have the students note the sensation the generator produces when they 
hold their arms close to it.  
     2. Attach the broad end of a rubber tipped dart to the top of the generator 
with a piece of string  tied around the opposite end of the dart. The other end of 
the string should be taped to the outside bottom of a styrofoam cup. The cup 
should then be inverted over the upper end of the dart attached to the generator. 
When the generator is turned on at high  speed the cup will be repelled from the 
end of the dart and will be held in mid-air by the charges on it and on the 
generator. Remind the students of Newton's first law. Since the cup was moved from 
a state of rest by the charge produced by the generator clearly a force was 
produced by the charge. More examples can be given of the same phenomenon using 
pieces of paper, plastic, fur or other material. The students should now be able 
to make a qualitative statement that charged objects produce a force on one 
another.  
     3. The quantitative aspects of this activity are available in several 
published sources. Two which are useful are "Coulomb's Law,"Laboratory Physics, 
James T.Murphy and Judith L.Doyle,Charles E.  Merrill,1986.,and "Coulomb's Law on 
the Overhead Projector," J.B.  Johnston,The Physics Teacher(January,1979).  
     4.Several activities can be devised to reinforce the main concepts of this 
lesson and others related to them. Such activities  are by no means limited to the 
following suggestions.  
     Kinesthetic activity: Winther's Linear Accelerator: Using a 7.5m *.5m sheet 
of craft paper, butcher paper or plastic divide the sheet into fifteen equal 
squares. Label the center square "start" and number out in both directions to 
seven. Pick five students who are kinesthetic learners. One student is the 
operator. The other four are divided into two teams of two. Each team has either 
2
positive or negative charges. The operator gives each player in turn a card which 
tells that player how to move. The team which is the first to have a player to 
exit the accelerator is the winner. The instruction cards read,  "move one space 
left," "move one space right," "move one space closer to an opposite charge," and 
"move one space further away from a like charge." This will reinforce for these 
kinesthetic learners the concept that like charges repel and unlike charges 
attract . The number and content of the cards may be varied depending on the group 
using them.  
     Tactile/Visual Activity: Learning Circle: A learning circle can be 
constructed out of oak tag board. Cut two circles from  the board of the same size 
approximately 18" in diameter. Divide each circle into eight equal wedge shaped 
sections. Attach the two circles to each other with a coat hanger sandwiched 
between them in such a way that they can be hung up.  The faces of the circles 
which have been divided into sections should be aligned with each other, facing 
outward.  In each section of one circle put a a statement from the chapter summary 
with a key word or phrase omitted.  Attach each of the omitted words or phrases to 
a cloths pin so that the students can attach the word or phrase to the appropriate 
section of the circle to make a completed statement. The  back of the cloths pins 
and the sections  on the back of the learning circle may be coded to facilitate 
checking the answers of the students after they have matched the words and phrases 
with the appropriate statements.  As either in-class or homework activities 
students can construct their own learning circles using one the teacher or other 
students made as a model.  This activity serves as reinforcement for 
tactile/visual students. Puzzles or electro-boards serve the same purpose.