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Phys 132L Spring 2009
Laboratory 2: Electric Dipoles and Polarization
It is a frequent observation that charged objects exert attractive forces on electri- cally neutral objects. This apparently inconsistent phenomenon can be explained, via electrostatics, by considering the distribution of charges within the neutral object. This laboratory explores this and the properties of materials associated with it. Many of the exercises in this laboratory are adapted from those that appear in Chabay and Sherwood, Electric and Magnetic Interactions, Wiley, 1995.
1 Electric Dipoles The arrangement of electrons in some molecules (water is an example) is not sym- metrical. Although the total charge in the molecule is zero, the asymmetry in the ar- rangement of electrons implies that one side of the molecule is predominantly negatively charged while the opposite side is predominantly positively charged. Schematically this is illustrated as follows.
Such a molecule is said to be polarized and is called an electric dipole. From the point of view of electrostatics, an electric dipole is equivalent to two rigidly attached pointlike particles which are separated and oppositely charged.
− + equivalent to
a) Suppose that an electric dipole is placed near to a charged particle in the configu- ration as illustrated.
Draw the force vectors for the forces exerted on the charged particle by each end of the dipole. Describe what happens to the charged particle if it is initially at rest.
b) Draw the force vectors for the forces exerted on the two ends of the electric dipole by the charged particle and describe what happens to the electric dipole if it is initially at rest.
c) The previous example featured a dipole and a charged particle in a linear arrange- ment. Now consider the following configuration.
Draw the force vectors for the forces exerted on the two ends of the electric dipole by the charged particle and describe what happens to the electric dipole if it is initially at rest.
d) It is possible to construct an “large scale electrostatic dipole” from two pieces of oppositely charged tape. However, it is first necessary to show that tape does not conduct electrical charged. Prepare a U-type piece of tape (see page 8) and suspend it from the underside of the desk. Verify that the tape is charged by placing your hand near to it. Neutralize the lower half of the tape by rubbing it with your fingers several times. Do not rub the upper half. Now hold your finger near to the lower half. Is the tape attracted? Hold your finger near to the upper half.
the effect of one tape dipole on the other.
g) Suppose that two dipoles are free to rotate about their centers, whose locations are held fixed. They are initially held fixed as illustrated and then released. Sketch the final equilibrium orientation of the dipoles and explain how you arrived at your result.
2 Polarization of Neutral Objects Many neutral objects contain a vast collection of electric dipoles (e.g. water molecules) and are thus able to interact electrostatically with charged particles.
a) Your finger contains electric dipoles in the form of water (and other) molecules. Suppose that your finger is far from any charged objects but near to a neutral sheet of paper. Sketch several dipoles in the finger in the schematic diagram. Do the dipoles point in the same direction? If so, describe what effect your finger would have on a neutral piece of paper held near to it. Is this consistent with what you observe. If not, modify your sketch appropriately.
Paper
b) Prepare a U-type piece of tape (see page 8) and suspend it from the underside of the desk. Recall that this tape is positively charged. Hold a finger near to the tape. What observable effect does the finger have on the tape?
c) Your finger is electrically neutral (or uncharged) and it would appear that it should not be able to exert an electrostatic force on the piece of tape. How is it possible that this does actually occur? Sketch several of the dipoles inside the finger while the positively charged tape is held near to the finger. Comment on the difference
c) Now check whether the foil is charged by holding your finger near to but not touching the foil. Determine the type of charge by holding the rubbed plastic near to the foil.
d) Explain, using diagrams how the foil became charged.
4 Preparing the Tape There are two types of tape that need to be produced. The methods of producing both types of tape utilize a base layer of tape. In order to make the base layer proceed as follows.
a) Stick an entire 20 cm long strip of tape to the surface of the desk. Gently rub the tape several times with your finger. The effects that this accomplishes are accentuated by first breathing on your hand (this is not magic - there is a good reason for this).
In order to make an “upper” or U-type piece of tape proceed as follows:
a) Gently rub the base layer with your finger several times. b) Remove a 20 cm strip of tape off the roll and bend one end over to form a handle. Stick this tape down on top of the base layer and gently rub it with your finger several times. Label it “U.” c) Quickly pull the U tape off the base layer. Minimize contact between the tape and any surrounding objects. Store the tape by suspending it from one end from the underside to the desk for later use.
In order to make a “lower” or L-type piece of tape proceed as follows:
a) Gently rub the base layer with your finger several times. b) Pull a 20 cm strip of tape off the roll and bend one end over to form a handle. Stick this tape down on top of the base tape and gently rub it with your finger several times. This will form an “lower” type tape. Label it “L.” c) Pull a third 20 cm strip of tape off the roll and bend one end over to form a handle. Stick this tape down on top of the L tape and gently rub it with your finger several times. This will form an “upper” type tape. Label it “U.” d) Slowly pull the L tape up (the U tape will be lifted along with it). Without separating the U and L tapes, hang them from the desk and gently rub both with your fingers. e) Rapidly separate the U and L tapes. Minimize contact between the tape and any surrounding objects. Each can be suspended by one end from the underside to the desk for later use.
