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Jumping Grains (Electrostatics) Physics Concept

Different materials respond in different ways to electric fields depending on their ability to hold a charge. An uncharged grain of salt becomes polarized when another charged object is held near it. That is, the grain builds a positive charge on one side and negative on the other. Some materials become polarized more easily than others. Also some materials can pick up or lose charge easily. These properties can be used to separate different materials. Charged objects can join be attracted to each other by having positive charge on one end and negative charge on the other.

Materials An aluminum pie pan, dry unflavored gelatin powder, grits (or cornmeal), black pepper, and salt. A piece of Styrofoam to charge by rubbing in your hair.

Method In the pie pan put separate teaspoonful-sized mounds of the gelatin, grits (cornmeal), pepper, and salt. Hold the charged Styrofoam above each mound and carefully observe what happens. The grits and pepper should jump up to the Styrofoam and stick to it. The salt grains may jump up and then jump back to the pan when they pick up charge from the Styrofoam. The gelatin grains will form tiny chains or fingers as they climb toward the Styrofoam. Each particle of gelatin becomes polarized and develops a plus and minus charge on opposite ends. These ends link up to form the chains you see.

Questions Which of the materials loses its charge the most easily? If you mix the salt and pepper can you separate them using charge?