Properties of Electrostatic Charge: Interactive Lecture Demonstration

Students should develop a model of the nature of electrostatic charge from this activity. Students should understand the conservation of charge,and be able to sketch the electron distribution and movement for several situations:

1. Isolated positive and negative charges
2. Transfer of charge from a neutral or charged object by contact
3. Polarization of charge in a neutral conductor or insulator
4. Thransfer of charge from a neutral or charged object by induction

Educational level: Physical Science or Introductory Physics
Setting: Classroom
Time required: 40 minutes
Special equipment: Electrostatics materials
Pre-requisite knowledge: Properties of electrons, atomic structure

SETUP AND INTRODUCTION

Before class the teacher should set up the demonstration by placing a group of electrostatics materials on the demo table, including:

• PVC, Teflon and glass rods
• Fur, wool, silk
• Electroscope
• Balloons or pith balls, preferably on two separate stands.
• A soda can
• A watch glass and a meter stick
• Computer charge sensor (optional)

Although it is not necessary to always identify the type of charge for class understanding it is helpful to know:

• Rub PVC or Teflon with fur: The rod will be negative, the fur positive
• Rub Glass with silk: the rod will be positive, the silk negative

PROCEDURE

1. Charge objects
   • Concept:
     Electrostatic charge is defined as the absence or excess of electrons.
     Electrons are easily removed or added to an object by vigorously rubbing an object (rod) with another object (fur, silk, etc)
     There are two types of charge: positive, which is the absence of electrons and negative which is the excess of electrons
     Charge is always conserved
      
   • Procedure:
     Rub the PVC rod with fur or wool.
     Ask the students to record the charges on each item after the rod and fabric are separated
     Discuss the charge on each object and verify with a computer charge sensor if available.
     Repeat for glass rod with silk
2. Transfer charge by contact
   • Concept:
     When two objects touch the electrostatic electrons transfer from one object to another until equilibrium is reached
     Charge by contact results in both objects having the same type of charge
   • Procedure:
     Neutralize both objects by touching with a grounded object (your hand)
     Charge a rod by rubbing vigorously
     Touch the charged rod to an uncharged pith ball or balloon
     Ask students to sketch the transfer of charge
     Discuss the movement of charge and verify with a charge sensor
     Repeat for different charge
3. Polarization of charge
   • Concept:
     When a charged object is adjacent (but not touching) to an uncharged object the charges in the uncharged object redistribute
     There is no change in the net charge of the uncharged object
   • Procedure:
     Neutralize a rod and a soda can with a grounded object (your hand)
     Charge a rod by rubbing vigorously
     Bring the rod close to the can, which is free to roll along the table
     Slowly move the rod away from the can - the can will roll
     Ask students to sketch the charge distribution for this situation
     Discuss the charge distribution and ask student to correct any errors they made on the original sketch
     Repeat for a meter stick balanced on a watch glass (see attached files)
4. Charge by induction
   • Concept:
     An object charged by induction has the opposite charge as the charging object
     Initially the charge on the uncharged object polarizes and then a ground is provided to remove some of the charge
     The two objects never touch each other
   • Procedure:
     Ground both objects, charge a rod and use it to polarize the charge in an uncharged pith ball or balloon
     Very quickly, touch the side of the pith ball or balloon on the opposite side from the rod while simultaneously pulling the rod away
     Ask students to sketch the movement of charge for this demo
     Discuss and verify with an electroscope or computer charge sensor
     Repeat for a differently charged rod

Student Electrostatic ILD worksheet - Predictions (Microsoft Word 54kB Jul16 07)
Student Electrostatic ILD worksheet - Results (Microsoft Word 58kB Jul16 07)

 

Balloons hung on strings are best for demonstrating the movement of charge because of the visibility for the class. However, if the humidity is too high on the day of the demonstration it may be necessary to use Mylar??? balloons or small pith balls. The visibility of pith balls can be improved by placing them on an overhead projector or Elmo???. If the humidity is very high it may be necessary to dry the fabrics with a hairdryer and rub the hard materials with rubbing alcohol before class.

It might also be good to show the Balloons and Static Electricity simulation from the Phet site.

Teaching Notes and Tips (Microsoft Word 32kB Jul16 07)

The results sheet should be collected at the end of class to assess student understanding. A follow up question or problem should be given at the beginning of the next class.

Information about the demos which will be used can be found at many sites: here are a few:
http://www.physicsdemos.com/
'http://demoroom.physics.ncsu.edu/html/electricity_and_magnetism.html'