Van de Graaff generator (Arevalo, Mark Rothenay)

Standards

NGSS:

3-PS2-3 Ask questions to determine cause and effect relationships of electric or magnetic interactions between two objects not in contact with each other.

CA Science Standards | 4th grade:

1.e. Students know electrically charged objects attract or repel each other.

Materials needed

• Van de Graaff generator

• step stool with rubber treads on bottom (or Rubbermaid container top) for insulation

• power strip with on/off switch

• * triboelectric series to further explain movement of charges*

• Optional - Fluorescent tube light

Procedure

1. Read all safety instructions for the generator.

2. The Van de Graaff should be plugged into a power strip with an on/off switch with a long enough cord so as not to "zap" the switch-operator.

3. Instruct the volunteer to stand on the step stool, with one hand down by his/her side, and the other hand GENTLY placed on top of the dome.

4. Instruct volunteer to keep hand remaining on dome until the generator has been turned off.

5. Volunteer may like to gently shake his/her head to increase "hair-rising" effect.

6. To discharge, volunteer may either "shock" another volunteer or step down and touch a neutral surface, like a table top.

7. Optional: Repeat step 3 and have another student step on another plastic lid then hold hands with the student touching the generator.

8. Have another student come next to the second student who is on the plastic lid and give a high five. Notice how the electrons have flowed from the two students and reached the third student.

Explanation

When the Van de Graaff generator starts charging, it transfers the charge on the sphere to the person who is touching it. Since the person's hair follicles are getting charged to the same potential, they try to repel each other since like charges do not attract. This is why the hair actually stands up. It would not make a difference if the polarity of the Van de Graaff generator were reversed. As long as the person is insulated or on an insulated surface such as when they are on top of a plastic lid, the charge will build up and the hair will appear to stand-on-end (assuming, of course, that the hair is clean and dry). If the students work as a series in which they are all standing on an insulated surface, the charge will transfer from student to student and eventually exit into the ground through the student that is not insulated.

Questions

1. Why does the hair stand up?

   • Each strand of hair is the same charge, and like charged objects repel.

2. How is this similar to two balloons repelling each other?

   • When two balloons are rubbed on one's hair, they receive the negative charges from the hair and become negatively charged. When two objects have like charges, they repel.

3. What is the zap?

   • The "zap" is the discharge of excess charges. That is, the objects way of going back to neutral.

Everyday examples of the principles illustrated

1. the zap one inflicts on another after dragging "socked" feet across the carpet

2. the ballooning out of a laundry bag after emptying it of its content

3. lightning

4. plastic wrap (a.k.a. cling wrap)

5. the static build up after sliding down a play structure

6. Insulation in the form of rubber and plastic that coats electrical wiring.

Photos