We begin with an exploration, Flying Hydra, in which a piece of shredded wrapping tape is suspended against gravity using electrostatic repulsion from a PVC rod rubbed with wool.
Then we try Flying Tinsel, In which tinsel is tied into a loop with two little legs sticking out. The PVC rod is rubbed with wool and the tinsel is not. At first the rod attracts the tinsel then after a "snap" sound the tinsel jumps away from the rod and flies.
The last activity is Jumping Spices (a new version of the snack Jumping Electrical Fleas), A piece of acrylic is placed above a piece of paper. Oregano or dill is sprinkled on the paper. The acrylic is rubbed with wool and the spice particles jump up to the plastic then suddenly jump down.
Teacher Observations and Questions and PD's comments:
At first the plastic hydra lies limp and shredded.
PD This is one of the most important observations,
unrubbed objects do nothing.
They neither attract nor repel other objects.
Rub the hydra with wool. Notice that its hairlike fibers repel each other and attract your hand and everything else they come near.
PD The plastic of the hydra can repel some things and attract others.
When you rub something and it then attracts or repels other objects this is a sign that electrostatic forces are involved.
PD When you rub one object with another both can become electrically charged.
PD Physics texts will tell you that there are:
2 kinds of charge, plus and minus.
Like charges repel. Plus repel plus and minus repel minus.
Unlike charges attract. Plus and minus attract.
History: These two types of charge were named by Benjamin Franklin.
PD What texts don't usually tell you is that charges both plus and minus attract uncharged things. This attraction is due to electrical polarization and will be discussed later. The thing about electrical polarization is that it is strong at short distances and gets weaker quickly. So two objects can attract when they are close and repel when they are further away.
Rub the hydra and then rub the PVC rod both with wool. Throw the hydra into the air and hold the PVC rod under it. Notice that you can "fly" the hydra using the rod.
PD When rubbed with wool both the plastic of the hydra and of the PVC rod get the same charge. They repel each other. When the electric force up is equal to the gravity force down then the hydra will balance at that height.
Notice that the height of the hydra decreases with time.
PD The height of the hydra is determined by the amount of charge on the hydra and the amount of charge on the rod. The more charge the higher the hydra. The charge leaks off of the rod and hydra with time.
The more humid the day he faster the charge leaks
The more hand oils on the rod and the faster the charge leaks off.
PD Hand oils contain sweat, salt water, which is a conductor. The PVC rod itself is an insulator. When the rod becomes coated with sweat or with watery particulates from humid air, charge can leak off the rod.
A human must hold the rod to keep the hydra in the air.
PD Active feedback using eye, brain and hand, is a necessary part of flying the hydra.
Two people can pass the hydra back and forth.
History. The shredded plastic is easy to fly because its fibers not only hold more charge, they have more surface area to touch the wool, they also give it air resistance so that it moves slowly. Small oil drops in air also move slowly. Professor Robert Millikan flew tiny oil drops above an electrically charged plate. He noticed hat the oil drops would suddenly jump up or drop down. The sudden motion was caused by the loss or gain of one smallest amount of electric charge. The charge was always lost in units. These units were equal to the charge on one electron or one proton. Your hydra has too many charges coming and going to notice the sudden jumps.
Hold the tinsel loop near the rod.
At first the tinsel is attracted to the rod. The tinsel loop is not circular.
Release the tinsel.
It moves toward the rod, then after a snap it springs out into a ring and jumps away.
PD At first the rod is charged and the tinsel is not.
Charged things attract uncharged things so the tinsel is attracted.
After the tinsel is released from your fingers and moves toward the rod, some of the charge can leap from the PVC to the tinsel loop. When the loop is charged, it springs out into a circular shape and is repelled from the rod.
The process works better if you hold the tinsel so that its two little legs are near the rod.
PD Electric charge can jump more easily to points. This is the principle of the lightning rod.
Sometimes the tinsel wraps itself around the rod.
Sometimes it can be shaken loose other times it just stays stuck.
PD The charged rod attracts the uncharged tinsel.
However if the rod is charged enough the charge can leap across the air to the tinsel.
When the charge hits the conducting metal of the tinsel it flows around the tinsel equally repelling the tinsel into the shape of a circle. If the rod has too little charge it cannot make a spark in air and the tinsel remains attracted to the rod.
If the tinsel loop is flying and comes near a wall or person it will be attracted to the wall or person.
PD The charged tinsel attracts the uncharged wall or person.
The tinsel loop can oscillate back and forth between a metal panel in the ceiling and the rod.
PD at first the loop of tinsel is repelled toward the ceiling by the rod, then as t gets close it is attracted to the ceiling. When it touches the metal of the ceiling electric charge flows off the tinsel into the metal and the tinsel loop collapses and falls back down under gravity.
Dancing Electrical Spices
When you rub the acrylic the spices dance up and down.
PD The rubbed acrylic becomes charged. Charged things attract uncharged things. So the spices are attracted to the plastic.
The spices gain the same electric charge as the plastic then they are repelled back down.
Move your hand over the surface of the plastic and the spices rearrange themselves under the plastic. You are writing a pattern in the spices.
History: Chester Carlson took this same idea and used it to invent xerography, dry writing. He charged a selenium plate by rubbing it with rabbit fur. Then made an image of a piece of paper on the selenium. The white places made the selenium a conductor so the charge left. The dark places were insulators. The charge remained. Then oppositely charged plastic spheres of ink were sprinkled Over the surface. they stuck where the surface was charged, where it was dark. The plastic was heated, the ink and plastic melted into the paper, and the paper became a copy of the original.
Scientific Explorations with Paul Doherty
25 July 2000