## STATIC ELECRICITY

BACKGROUND INFORMATION

It is essential to teach about static electricity during a time of the school year when the weather is dry if one wants the static electricity experiments to work. The electrons need dry air in which to buildup and make their "jumps". That is why there is more "static cling" when taking clothes out of the dryer on a dry day than on a humid one. Complete circuits can be taught on any day because electrons move through the wire and are not affected by the weather.

In 1752, Benjamin Franklin showed that lightning is a form of static electricity. Franklin demonstrated this by flying a metal-tipped kite during a thunderstorm. The voltage differential between the clouds and the ground was sufficient to charge the string, and this caused the fibers on the string to stand on end. It is the same way that hair stands on end after being brushed on a dry day. He also showed that electricity moved quickly through certain materials and that a pointed surface attracts electricity more than a flat surface.

Atoms are composed of positively charged protons, neutral neutrons, and negatively charged electrons. When the number of electrons in the atom is equal to the number of protons, the atom is said to be neutral or have no charge. If there are more electrons than protons in the atom, the atom has a negative charge. If there are more protons than electrons in the atom, the atom is said to have a positive charge.

Positive and negative charges at rest on an object are called static electricity. Static electricity is an electric charge built up in one place. If you rub only one end of a plastic comb, then only that end becomes charged. The electric charges stay in one place. An object can become charged in several different ways. When electrons are transferred between two objects that are rubbed together, the objects become charged due to friction. Conduction is another way that objects can become charged. It is the direct contact between two objects. An object can also be charged by an already-charged object without there being contact between them. This object is said to be charged by induction.

Normally a body is neutral since the number of electrons is almost equivalent to the number of protons. If a person rubs their shoes across a carpet in dry weather, the body builds up negative charges that are released from the carpet. The charge that develops may be released through a spark as the body goes near an uncharged or a positively charged object. When the charged object (the body) loses its charge, the body becomes neutral once again. An electric discharge is the loss of static electricity as the electric charges move off the object.

Lightning is another example of an electric discharge that most children have experienced in their life. During a storm, particles contained in the clouds are moved about by the wind. Charges become separated during the storm and there are buildups of positive and negative charges in different areas of the clouds. As a cloud that is negatively charged moves near the Earth's surface, the negative charges near the surface move away (likes repel) from the cloud. The positive charges move closer to the surface (opposites attract). This is an example of induction since the Earth becomes electrically charged by the cloud movement. The electrons can either jump from cloud-to-cloud or cloud-to-ground. The result of the transfer of electrons is a huge spark known as lightning.

Some common materials that teachers can use to demonstrate static electricity are as follows:

 Plastic rod Thermometer Plastic Clear Ruler Glass Rod Fake fur Rubber Hard Comb Cotton Balloons Polyester Tissue Paper-Paper Flannel Thread Wool Petri Dishes with Lids Silk

You can also look at a chart that has common electrostatics listed. A list in The Dick and Rae Physics Demo Notebook written by D. Rae Carpenter, Jr. and Richard B. Minnix is very useful.

The list looks like this:

Rabbit's fur

Glass

Mica

Wool

Cat's fur

Silk

Cotton

Wood

Amber

Resins

Metals (Cu, Ni, Co, Ag, etc.)

Sulfur

Metals (Pt, Au)

Celluloid

NEGATIVE

Since silk is lower than glass on the chart, the glass will give-up electrons to the silk. By doing this, the glass will stay more positive as it gives up electrons to the silk.