Similar and Dissimilar Charges

Purpose: To compare the responses of objects with similar and dissimilar charges.

Summary: Students use a hand-crank Van de Graaff Generator to charge different objects to determine whether they attract or repel each other.

Question: Do objects with a similar charge attract or repel each other?

Method/Materials: Hand-crank Van de Graaff Generator, piece of Styrofoam packaging material, string, and glass rod.

Students should start by making sure the Van de Graaff generator is totally discharged by using the grounded discharge wand provided. Tie one end of the string around the Styrofoam ball (or packaging "peanut") and the other around the glass rod. Have each student determine what will happen to the ball when it comes in contact with the generator dome that has been charged. While one student turns the wheel to charge the dome of the generator, another student should hold the glass rod so that the ball is touching the dome.

After students have made their observations of what happens to the ball that comes into contact with the dome, allow them to make predictions of what will happen when the student holding the glass rod puts his hand near the ball. Will it be attracted or repulsed by the student's hand? Encourage students to discuss their predictions with their classmates and provide rationale for their prediction.

Students will note that the peanut will be repelled by the dome after a few seconds. Discuss what happened to the original charge on the ball and the subsequent charge from the dome. Why did they repel each other? Why was the ball attracted to the student's hand? Explain how electrostatics occurs when there is an exchange of charge on two or more different objects.

Required Equipment
Hand-crank Van de Graaff Generator, piece of Styrofoam packaging material, string, and glass rod.


Required Equipment


Paul Hewitt"When showing electrostatics demonstrations, I quip with my students and ask who the Van de Graaff generator was named after. Then I whimsically answer 'Robert Generator,' which adds a bit of humor in class. I advise teachers do the same in my instructor manuals, which was with permission of Robert Van de Graff! One day a student asked if the device should have been named after "Robert Separator," for that's what it does. A Van de Graaff device is a separator; it separates negative charges from positive ones—what all charging devices do. So we keep learning, which adds to the delight of physics."

Paul Hewitt, former boxer, uranium prospector, sign painter, and cartoonist began college at the age of 28 and fell in love with physics. His name is synonymous with Conceptual Physics to physics educators everywhere. His textbook, the leading physics textbook for nonscientists since 1971, has changed the way physics is taught to both non-science and science majors as well.


Collin Wassilak

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