PURPOSE: To demonstrate that non-uniform electric fields produce a force on polar molecules.

DESCRIPTION: An electrophorus is used to charge an aluminum plate. A stream of carbon tetrachloride (CCl4), a non-polar molecule, is sprayed in front of the charged plate; the stream of carbon tetrachloride is unaffected by the electric field of the plate. A stream of water (H2O) sprayed in front of the charged aluminum plate deflects strongly, indicating that the centroids of the positive and the negative charges are not the same. The non-uniform electric field rotates the molecules and exerts a force on the dipole electric dipole of a non-polar molecule.

Models of carbon tetrachloride and water are available to illustrate the polar nature of certain molecules.

SUGGESTIONS: SAFETY NOTE: If you are hesitant to squirt around a volatile, carcinogenic liquid, a non-toxic alternative is light white paraffin oil, a squirt bottle of which is also provided.


EQUIPMENT: Electrophorus with insulated aluminum plate, water and carbon tetrachloride molecular models, squirt bottles of water, carbon tetrachloride and light paraffin oil, point source for shadow projection.

SETUP TIME: 5 min.

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