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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


ELECTROSTATICS

Figure I. Figure II. Figure III.


Figure IV. Figure V. Figure VI.


Figure VII. Figure VIII. Figure IX.


Figure X. Figure XI.

Description

Equipment: Rotor -- A small cylindrical piece of plastic with a groove on top is set on top of a pin which is mounted on another cylindrical piece of plastic. (See Figure VII.)

Custom-made charge meter -- A bent piece of brass is mounted on a plastic base. An aluminum tube is placed the appropriate slot in the brass. See Figure III. for an image of a small one and Figure VI. for an image of a large one. When charge is near, the aluminum tube bends towards the charged object.

Large balloon
Teflon sticks
Silk handkerchief
Wooden stick
Large 2x4
Coke can

Note: There are many different little demonstrations that can be done using electrostatics. This page will discuss many of these, but it is unlikely that all would ever be shown at a single show, nor is this intended to be an exhaustive list.

Also, electrostatic experiments are best performed when there is very little relative humidity. In fact, when the relative humidity exceeds 50%, this demonstration is usually not performed.

Step 1: A face is drawn on the balloon with a magic marker. This will be used to identify where the charge is on the balloon. The balloon is rubbed up and down on the demonstrator's head. The balloon is then put against the wall and it sticks! (See Figure I.)

Step 2: A large balloon is rubbed up and down on the demonstrator's head. A Coke can is placed on its side in the middle of the table. The balloon is brought near the Coke can and the Coke can rolls towards the balloon! (See Figure XI.)

Step 3: The balloon is brought near the small charge meter and the aluminum tube on the charge meter reacts by tilting. (See Figure II.) When the balloon is taken away, the charge meter returns to its normal state (untilted). The balloon is then brought to touch the top of the charge meter (on the brass). The aluminum tube on the charge meter reacts by tilting, but this time, when the balloon is taken away, the charge meter remains tilted. (See Figure III.) A volunteer from the audience is chosen. He/She touches the top of the charge meter with his/her finger and the aluminum tube returns to its normal state (untilted).

Step 4: The same experiments are then performed using the Teflon stick and silk handkerchief. This time, however, the large charge meter is used. The Teflon stick is rubbed against the silk handkerchief. (See Figure IV.) The Teflon stick is brought near the large charge meter and the aluminum tube on the charge meter reacts by tilting. When the Teflon stick is taken away, the charge meter returns to its normal state (untilted). The Teflon stick is then brought to touch the top of the charge meter (on the brass). (See Figure V.) The aluminum tube on the charge meter reacts by tilting, but this time, when the balloon is taken away, the charge meter remains tilted. (See Figure VI.) A volunteer from the audience is chosen. He/She touches the top of the charge meter with his/her finger and the aluminum tube returns to its normal state (untilted).

Step 5: The small plastic rotor is introduced to the audience and its properties are discussed. (See Figure VII.) One Teflon stick is rubbed against the silk handkerchief and placed in the rotor's groove. Another Teflon stick is rubbed against the silk handkerchief and brought near the first Teflon stick. The Teflon stick on the rotor is repelled and swings away from the second Teflon stick. The demonstrator uses the second Teflon stick to guide the Teflon stick on the rotor in a circle. (See Figure VIII.)

Step 6: The ordinary wooden stick is put on top of the rotor instead of the Teflon stick. A Teflon stick is rubbed against the silk handkerchief and then brought near the wooden stick. The wooden stick is attracted to the Teflon stick and swings towards it. The demonstrator uses the Teflon stick to guide the wooden stick on the rotor in a circle. (See Figure IX.)

Step 7: A large 2x4 is balanced on a piece of plastic in the middle of a table. A large Teflon stick is rubbed many times against the silk handkerchief. The large Teflon stick is brought near the 2x4 and the 2x4 is attracted to the Teflon stick. The demonstrator uses the Teflon stick to guide the 2x4 in a circle. (See Figure X.)

Description

Basic Ideas: There are two types of charges: positive and negative. Like charges repel and opposite charges attract.

In general, a material is either a conductor or an insulator. A conductor allows electric charge to travel through it easily; an insulator does not.

A person's body acts as a conductor.

When certain types of materials are rubbed against other certain types, charge may be transferred from one to the other.

When an uncharged object is placed near a charged object its charges rearrange themselves. Those charges attracted to the charged object move towards the charged object and those charges repelled move away. This effect is known as polarization.

Step 1: When the balloon is rubbed against the demonstrator's head, charge is transferred. The balloon material and human hair are such that when they are rubbed together, charge moves from one to the other. So the balloon is now a charged object.

When the balloon is brought near the wall, the wall is polarized. The wall's negative charges move away from the balloon and the wall's positive charges move towards the balloon. So although the net charge on the wall is still zero, the wall will behave as if it is positively charged. Since the balloon is negatively charged and the wall is acting as if it is positively charged, the balloon is attracted to the wall. The force of attraction is so strong, in fact, that the balloon 'sticks' to the wall!

Step 2: As in Step 1, the balloon becomes negatively charged by being rubbed against the demonstrator's head. When the balloon is brought near the Coke can, the Coke can is polarized, just as the wall was in Step 1. Since the balloon is negatively charged and the Coke can is acting as if it is positively charged, the Coke can is attracted to the balloon. The force of attraction is so strong, in fact, that the Coke can begins to roll towards the balloon!

Step 3: Again as in Step 1, the balloon becomes negatively charged by being rubbed against the demonstrator's head. When the balloon is brought near the charge meter, the aluminum tube on the charge meter is polarized, just as the wall was in Step 1 and the Coke can was in Step 2. Since the balloon is negatively charged and the aluminum tube is acting as if it is positively charged, the aluminum tube is attracted to the balloon. The force of attraction is so strong, in fact, that the aluminum tube tilts towards the balloon! When the balloon is moved away from the charge meter, the aluminum tube becomes unpolarized, and it no longer tilts.

When the balloon is brought to touch the brass on the charge meter, some of the negative charge which is stored on the balloon travels through the brass and onto the aluminum tube. This occurs because both brass and aluminum are conductors. Both the aluminum tube and the brass are now negatively charged and therefore repel each other. When the balloon is moved away from the charge meter, the aluminum tube remains tilted because it is repelled by the brass.

When the volunteer from the audience touches the brass of the charge meter, the negative charges all run up the brass and onto the volunteer. This occurs because a person's body is a conductor. All of the excess charge is therefore drained off of the charge meter, and the aluminum tube no longer tilts.

Step 4: During this step, the exact same demonstrations are performed as in Step 3. The only difference is that a Teflon stick, silk handkerchief, and large charge meter are used instead of a balloon, the demonstrator's hair, and the small charge meter. The explanations are identical to those of Step 3.

Step 5: When the Teflon stick is rubbed against the silk handkerchief, charge is transferred, exactly as it was when the balloon was rubbed against the demonstrator's head. When both sticks have been rubbed against the silk, they are both negatively charged and they therefore repel. So when the second stick is brought near the first stick, the first stick is repelled and causes the rotor to swivel away from the second stick.

Step 6: As in the previous step, the Teflon stick becomes negatively charged when it is rubbed against the silk. When the Teflon stick is brought near the wooden stick, the wooden stick becomes polarized. Since the Teflon stick is negatively charged and the wooden stick is acting as if it is positively charged, they attract each other. So when the Teflon stick is brought near the wooden stick, the wooden stick is attracted to it and causes the rotor to swivel towards the Teflon stick.

Step 7: During this step, the exact same demonstrations are performed as in Step 6. The only difference is that a large Teflon stick and 2x4 are used instead of a small Teflon stick and small wooden stick. Although this step is much more dramatic, the explanations are identical to those of Step 6.

Description

Basic Ideas: F = kq1q2/r2, where F = electrostatic force, k = Coloumb's constant, q1 = first charge, q2 = second charge, and r = the distance between the two charges.

Each type of material allows electric charge to flow through it to a certain extent. The measure of how receptive a material is to electric current is called the conductivity. Those materials with a high conductivity are called conductors. Those with a low cuductivity are called insulators.

The human body acts as a conductor.

When certain types of materials are rubbed against other certain types, charge may be transferred from one to the other. The amount and type of charge is determined by the Triboelectric Series.

An uncharged object placed near a charged object becomes polarized.

Step 1: When the balloon is rubbed against the demonstrator's head, charge is transferred. The balloon's material is located below human hair on the Triboelectric Series. So the balloon is now negatively charged. When the negatively charged balloon is brought near the wall, the wall is polarized, and the balloon is attracted to the wall. The force of attraction is so strong, in fact, that the balloon 'sticks' to the wall!

Step 2: As in Step 1, the balloon becomes negatively charged by being rubbed against the demonstrator's head. When the balloon is brought near the Coke can, the Coke can is polarized, and the Coke can is attracted to the balloon. The force of attraction is so strong, in fact, that the Coke can begins to roll towards the balloon!

Step 3: Again as in Step 1, the balloon becomes negatively charged by being rubbed against the demonstrator's head. When the balloon is brought near the charge meter, the aluminum tube on the charge meter is polarized, and the aluminum tube is attracted to the balloon. The force of attraction is so strong, in fact, that the aluminum tube tilts towards the balloon! When the balloon is moved away from the charge meter, the aluminum tube becomes unpolarized, and it no longer tilts.

When the balloon is brought to touch the brass on the charge meter, some of the negative charge which is stored on the balloon travels through the brass and onto the aluminum tube. This occurs because both brass and aluminum are conductors. Both the aluminum tube and the brass are now negatively charged and therefore repel each other. When the balloon is moved away from the charge meter, the aluminum tube remains tilted because it is repelled by the brass.

When the volunteer from the audience touches the brass of the charge meter, the negative charges all run up the brass and onto the volunteer. This occurs because a person's body is a conductor. All of the excess charge is therefore drained off of the charge meter, and the aluminum tube no longer tilts.

Step 4: During this step, the exact same demonstrations are performed as in Step 3. The only difference is that a Teflon stick, silk handkerchief, and large charge meter are used instead of a balloon, the demonstrator's hair, and the small charge meter. The explanations are identical to those of Step 3.

Step 5: When the Teflon stick is rubbed against the silk handkerchief, charge is transferred, exactly as it was when the balloon was rubbed against the demonstrator's head. When both sticks have been rubbed against the silk, they are both negatively charged and they therefore repel. So when the second stick is brought near the first stick, the first stick is repelled and causes the rotor to swivel away from the second stick.

Step 6: As in the previous step, the Teflon stick becomes negatively charged when it is rubbed against the silk. When the Teflon stick is brought near the wooden stick, the wooden stick becomes polarized, and is attracted to the Teflon stick. So when the Teflon stick is brought near the wooden stick, the wooden stick is attracted to it and causes the rotor to swivel towards the Teflon stick.

Step 7: During this step, the exact same demonstrations are performed as in Step 6. The only difference is that a large Teflon stick and 2x4 are used instead of a small Teflon stick and small wooden stick. Although this step is much more dramatic, the explanations are identical to those of Step 6.

Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia