Electrostatics

Tape Electroscope

or short version Tape Electroscope

Two 10 cm long pieces of Scotch magic tape when stuck to a table and pulled up repel each other and attract your hands.

The same thing done to each tape causes them to have the same charge. Like charges repel.

Charged tapes attract uncharged hands.

Stick one piece of Scotch magic tape on top of another, with the sticky side of the top tape against the smooth side of the bottom tape. Mark the tapes T for top and B for bottom. Pull the tapes apart. The tapes attract each other. Both tapes attract your hands.

In this case the tapes attract: they could have opposite charges or one could be charged and the other not charged. However, since both tapes attract your hand they are both charged and so they must have opposite charges.

Hold your bottom tape against your neighbors bottom tape. Notice it repels.
Hold your top tape against your neighbors top tape. Notice they repel
Hold your top ape against your neighbors bottom tape notice they attract.

Like charges repel, unlike charges attract.

Rub a comb with wool. The comb repels the bottom tape and attracts the top.

Benjamin Franklin arbitrarily chose the charge obtained by a rubber comb rubbed with wool to be minus. Thus the bottom tape is negatively charged. Attraction proves little but the attraction of the comb for the top tape indicates that it is positively charged.

You can use the tapes to find the sign of the charge on an unknown object.

PVC rubbed with wool repels the bottom negative tape and is negative.
Balloons rubbed with hair repel the negative tape.
Glass rubbed with silk repels the top positive tape and is positive.

Use your tape electroscope to find the sign of the electric charge on the front of your television. Mine is positive!

Electrostatic motor, can roller, paper roller

Notice that if you balance a 2x4 beam 4 feet long on a round topped salt shaker you can hold the charged PVC near it, and cause it to rotate. It is spectacular to see such a large object rotate.

An empty aluminum can can be attracted to a charged PVC rod and made to roll across the table.

A 1.5 inch wide 8.5 inch long piece of paper can be taped into a circle and made to roll by attracting it with a charged PVC rod.

All of these explorations work by inducing a charge separation in the wood, can or paper and then attracting the charge. In an insulator like the wood or the paper electron clouds are deformed. making the atom or molecule have a positive end and a negative end. In a conductor the moveable electrons move to make a charge separation in the aluminum can.

Charge and Carry

Charge a piece of blue household insulating Styrofoam about 1 foot square by rubbing it with Wool. Notice the feeling like moving through cobwebs when you move it near your skin. A feeling called formication.

Tape a Styrofoam cup handle to the middle of an aluminum pie plate. Place the aluminum pie plate on the charged Styrofoam. Use the insulating handle to hold the pan. Notice that when you approach the pan with a finger you feel a spark, hear a snap, and in dim light see a flash.

When you rub the Styrofoam you can se the tape to find it is negatively charged.
Place the neutral aluminum pan on it and bring a finger close.
The negatives on the pan are driven across the air to your finger as a spark.
The pan is now positively charged. Lift it away from the Styrofoam and test it with the tape.
This is called charging by induction.

Bring your finger near the positive plate when it s held above the Styrofoam and another spark jumps. This time negatives move from your finger to the pan making it neutral again.

Bring a neon glow tube near the pie pan and see it glow long before it touches the pan.

It takes 80 volts and 10-12 amps to make a glow tube glow. At these voltages and currents air is a conductor.

Actually to make a spark in air takes 3000 volts per centimeter.

Leyden Jar

Take a film can. Wrap it in aluminum foil. fill it with water. Stick a nail through the cover and into the water and you have a leyden jar. Currently called a capacitor. Charge the pie pan. Hold the leyden jar b its aluminum foil outside. Touch the pan to the center nail electrode 10 times.

While holding the aluminum foil outside of the leyden jar in one hand. Touch the center electrode with the other.

You will feel a much larger spark than with the pie pan alone.

The leyden jar plastic walls are insulators between two conductors. When you place charge on the inside. It attracts the opposite charge to the outside. (If the outside is attached to a large source of charge, called a ground, such as you.) This allows it to store a large amount of charge. Until you give the charge a path to flow through to recombine, such as your body.

Math Root

Two positive point electric charges,q1 and q2, repel each other with a force that falls off like the inverse square of distance. (a positive and a negative charge attract. with the same force.

F = kq1q2/r2

Where the force is in newtons, N
the charge q is in coulombs, C
and the distance is in meters, m.

and where k is a constant equal to 9 x 109 Nm2/C2

A charge and a dipole have a force between them of

F = kqp/r3

This force falls off faster with distance than the force between two charges, thus it can start off stronger at short distances and become weaker at longer distances.

A dipole is an equal and opposite charge q and -q separated by a small distance d with a dipole moment p = qd.

The force between two dipoles p is equal to

F = kpp/r4

The force between a charge and a dipole it induces in a neutral body is proportional to

F a q/r5

Since when you increase the distance the induced dipole gets weaker as the inverse square of distance and the force between a dipole and a charge then also falls off as the inverse cube of distance. The combination gives inverse fifth.

The force between two non charged bodies whose charge distribution is fluctuating about zero but which has dipole moments occasionally and another similar uncharged body is attractive and falls off proportional to the inverse sixth power.

F a 1/r6

 Scientific Explorations with Paul Doherty © 2000 25 July 2000