Electrostatics Lab

Purpose

The goal of this exercise is for you to observe the two different types of charge and the various means by which these can be produced and detected. Also you will gain firsthand experience with materials (conductors and insulators), devices (proof planes and electroscopes), and processes (conduction, induction, triboelectrification).

Materials

Ring stand

Red clamp

Nonmetallic pen or pencil (you supply)

Scotch tape

Electrostatics set (PVC pipe, Acrylic pipe, glass rod, fur, felt, silk, and two rotating stands)

Electroscope set (includes charge sampler, two proof planes and insulated handle)

Two electrical cables (one of each: double banana plug and banana plug/alligator clip)

Plastic pipe (CPVC) (This pipe is thicker wall than the one in the set and is labeled.)

Wool pad

Balloon

Procedures – Producing/Observing Charged Objects

Charged objects will be produced by triboelectrification – charging due to contact between two materials. In order to observe the charge on a piece of tape, it is handy to have a horizontal rod on which to hang it. Attach a pen or pencil (preferably wood or plastic) to a ring stand in order to make a horizontal support to which the tape can be attached and hung. For rods or pipes use the rotating stands. See the diagrams below.

Triboelectrification Methods:

1. Single Piece of Tape: Remove a piece of tape about 10 cm long (4 in). Fold over about 1 cm at one end of the tape to form a non-sticky “handle”. Press the tape firmly on the surface of the table and then use the handle to remove it. Hang the tape on the stand as shown in the diagram.

2. Tape “pair”: Make a piece of tape as you did in step #1 and tape it to the table. Write the letter “B” (for bottom) on the handle. Attach an identical piece of tape directly on top of it and label it “T” (for top). You now have two pieces of tape stuck together, one on top of the other, with sticky sides down – sort of like two pieces of bread in a sandwich. Grasping both handles, remove the pair as one unit and then separate the two pieces and hang on the support side by side.

3. Other charges: Try rubbing various combinations of materials together: glass/silk, fur/PVC, wool/acrylic, hair/balloon, etc. These combinations should work but feel free to experiment with other combinations.

Note: electric charge will dissipate over time (more so in humid conditions). So, you may have to “recharge” objects after a certain period of time.

Procedures – Using the Electroscope

The electroscope is an apparatus designed to detect and (qualitatively) measure electric charge. Electroscopes come in a variety of configurations, but always involve metallic surfaces that repel one another when a charge is introduced. The electroscope you will be using consists of a metal “needle” that can swing very easily around an axis but which normally will stay in a vertical position because of a very slight imbalance in its weight distribution. The needle’s axle is held by an “S-shaped” metallic support structure. If the needle and support structure have the same type of electric charge the needle will deflect from vertical. Greater amount of charge will deflect the needle further. The support structure connects to a “charge sampler” on top of the electroscope. Charged objects are brought near the sampler and/or touched to it.

Connect the base of the electroscope to ground using a cable as directed by your instructor. Connect a second cable to the same point on the base. The second cable may be used to discharge the electroscope by simply touching the free end to the metal support or charge sampler.

Important note: the charge sampler, support structure, and needle are all metallic conductors that are interconnected such that charge can move throughout these three objects. However, these surfaces are insulated from the rest of the electroscope.

Note: the electroscope diagrams are meant to be only a general guide. The steps that follow do not exactly match what is shown. Read carefully!

Electroscope methods:

1. Charge by conduction (above left): Use the spherical “charge sampler”. Simply touch the sampler with the object and then remove it to a relatively great distance. If the object is charged it will transfer to the needle and cause a certain amount of deflection.

2. Charge by induction (above right): Again use the spherical charge sampler – not the proof plane as shown. Bring an object close to the sampler but do not allow it to touch or spark. The needle will deflect a certain amount but only while the object is held at a position near the sampler.

3. Discharge the electroscope: Touch the free end of the second cable to the metal “S” support or the charge sampler. The needle should return to a vertical orientation. You may also find that you can discharge simply by touching with your finger. Or you can try touching the sampler with an object with an opposite charge.

Part A – Triboelectric Tendencies

1. Using the methods described above, experiment with the various materials and determine which type of charge each object is most likely to attain. The CPVC pipe will become strongly negative when rubbed by the wool pad. Take this as a starting point.

2. Be careful to note that a neutral object can be attracted to either type of charge due to the effects of induction. Only when objects repel one another can it be determined conclusively that both objects are charged. However, attraction of two objects can be regarded as “evidence” of two oppositely charged objects (if not absolute proof).

3. Complete the table by placing each material in one of the three columns: reliably negative, reliably positive, or inconclusive. Test each of the following: PVC, acrylic, glass, copper, single piece of tape, tape-pair bottom, tape-pair top, balloon, fur, felt, silk, wool pad.

Part B – Insulator Properties

1. Experiment with the materials and find ways to demonstrate that charge on an insulator is “localized” and “locked in place” (such that it cannot travel freely through the material). Record observations that are consistent with this property.

2. Things to try: charge one end of a pipe but not the other. Charge one side of a balloon. Try charging the electroscope by conduction through an insulator. Touch an uncharged electroscope with one point on a charged PVC pipe. Let the needle settle down and then touch other points on the same pipe.

Part C – Conductor Properties

1. Experiment with the copper wire and/or other metal objects and find ways to demonstrate that charge can move freely through a conductor. Also a conductor should be able to easily assume either type of charge. Record observations that are consistent with this property.

2. Things to try: Remove the charge sampler sphere and connect it to an insulating handle. Charge it by contact with a known charge and then demonstrate that the sphere is charged. Try charging the electroscope by conduction through the copper wire.

Part D – Conservation of Charge

1. Because charge is a conserved quantity the net amount produced in any process should be zero. Experiment with the materials and find ways to demonstrate this property. Record observations that are consistent with this law of physics. Note: because you are not measuring amounts of charge quantitatively there will not be a way to prove this.

2. Things to try: Compare charges on the top and bottom pieces of a tape-pair. Attempt to observe the charge on the “rubbing materials” – the wool, felt, fur, and silk, in contrast to the object that was rubbed. Try using an opposite charge to neutralize a charged object or a charged electroscope.

Part E – Permanent Charge by Process of Induction

1. Start with the electroscope completely discharged. Is it possible to produce a “permanent” net positive charge on the electroscope by using a negatively charged object? Read on!

2. Rub the plastic pipe with the wool. Bring the pipe near the charge sampler without touching (but close enough to make the needle deflect) and keep it there, being careful not to actually transfer charge. With the charged pipe in this position make contact between the bottom end of the needle’s support structure and the grounding cable.

3. Remove the grounding cable first and then remove the pipe from the vicinity of the charge sampler. If successful, the needle will remain in a deflected position indicating a net charge on the electroscope. Test the response of the needle to the presence of the plastic and/or the glass rod in order to determine the type of charge on it. You should find that the electroscope now has a net positive charge. You may have to try this a few times to get the sequence of events correct and obtain a successful outcome. Record observations.

4. Can you repeat this process ending with a negative charge on the electroscope? Try it and record observations detailing your success or failure.

Observations

Part A – Triboelectric Tendencies

Reliably Positive Materials

Reliably Negative Materials

Inconclusive Materials

Part B – Insulator Properties

Observations/demonstrations consistent with expected properties of an insulator: (Be specific!)

Part C – Conductor Properties

Observations/demonstrations consistent with expected properties of a conductor: (Be specific!)

Part D – Conservation of Charge

Observations/demonstrations consistent with conservation of charge: (Be specific!)

Part E – Permanent Charge by Process of Induction

Observations, including evidence of success for each type of charge: (Be specific!)