Purpose
To illustrate the electric field about
a charged object.
Description
Standing on an insulated stand with
one hand on a Van de Graff generator, long hair can be observed to fly
radially outward.
(See Demo: 5A50.30
Van de Graff Generators for demo preparation and Van de Graff operation)
Beforehand:
-On the day before / week prior to
your classroom demonstration, make sure Van de Graff control box is plugged
in and heater is turned on. This will reduce moisture within the Van de
Graff.
-Check the base's desiccant stores.
If desiccant is pink instead of blue, it needs to be replaced (or dried
out in a 400 oven for 12 hours.)
- If during trial run no discharge
is produced (and all connections, etc. are in order), securely attach a
grounding lead from dome to base (with duct tape) and run belt motor for
~30 minutes. (No HV!) This will help remove excess humidity from inside
dome.
In Class:
1. Locate Van de Graff in classroom
5 feet away from any student/ student desk/ classroom electronics/ etc.
(Van de Graff has been seen to fry a computer's muse board. That's right,
Fried Mice in class!)
2. Make sure grounding rod and control
box connections with Van de Graff base are secure.
3. Choose a pace-maker free student
with oil-free, relatively well groomed, med./ long hair. Have student remove
any metal objects/ jewelry, especially from the side of their body that
will be closest to Van de graff. (Small earrings are ok; Van de Graff has
been known to discharge to students' metal shoe buckles.)
4. Have student stand on insulating
stand and place one or both hands firmly (and flatly) of Van de Graff dome.
(Inform student that if they remove their hand from the dome at any time
durin the demonstration, they should not put it back on the dome.)
5. While holding grounding rod pole
in one hand, flip on belt motor at control box with the other.
6. Flip on HV (high voltage) at control
box. Observe hair to stand on end, illustrating the electric field about
the student's head.
7. To turn off Van de Graff, turn
off High Voltage first and then the belt motor. As student's hair falls
back down, have student remove hand. Once hair is fairly "back in Place",
student may step off of insulating stand. By rubbing shoulders with another,
most excess charge will be removed.
8. Before moving Van de Graff aside,
touch grounding rod to dome one last time to insure that dome is completely
discharged.
NOTE:
- Depending on the day's humidity,
discharge will be small or large.
- Internal belt and brushes remove
electrons from Van de Graff dome, leaving dome positively charged. When
brought in close contact, grounding rod replaces these negative charges;
belt immediately strips them off again.
- Dome's potential will reach ~20,000V
with respect to grounding rod.
PEDAGOGY: Remind students that excess charge distributes itself all over dome and student alike. Each strand of hair will have this same charge (positive). Thus, the strands of hair repel each other
Equipment
Van de Graff generator, control box
and grounding rod; Insulating Stand, Student w/ long, oil-free hair