Static Electricity

Static Electricity All Around Us The shocks you get from walking across a carpet and touching a metal doorknob look like small lightning bolts – in fact, that is exactly what they are.

The Greek philosopher, Thales, noticed that when he rubbed amber with fur, the amber attracted small pieces of straw and wood shavings. The fur also attracted some materials.

Scientists used the word charged to describe material that attract other materials after rubbing. They said that the material carried an electric charge. Since the charges remained stationary on the surface of the charged object, scientists called it static electricity. The study of static electricity is called electrostatics. Materials that do not carry a charge are said to be neutral.

Hundreds of years after Thales, scientists studied the attractions in more detail and discovered that many combinations of materials, when rubbed against another object, will attract small pieces of paper and straw. When two objects are rubbed together, the electrons move from one object to the other resulting in charged objects. The object that is more attractive to the electrons will receive the electrons.

A list of materials that places materials in order of their tendency to lose electrons is called an electrostatic series. This list allows you to predict what will happen when two objects on the list are rubbed together. Material Tendency to lose e- Ebonite Least likely to give up electrons Rubber Polyethylene Dacron Cotton Silk Nylon Wool Glass Cat fur Most likely to give up electrons

Law of Electric Charges Two objects with like charges with repel each other Two positive charges will repel one another Two negative charges will repel one another Objects with opposite charge will attract each other.

Category A repels other objects from category A. Category B repels other objects from category B. Category A attracts objects from Category B. Benjamin Franklin named the charges positive and negative.

Testing for Charge A metal leaf electroscope can detect the presence of charge. A typical electroscope consists of a metal rob with a knob at the top, and a pair of thin metal leaves at the bottom.

In an uncharged electroscope, the leaves hang straight down. When a negatively charged object touched the electroscope, the electric charges travel down the rod to the leaves and the leaves spread out – because they are both negative and they repel each other.

Uses of Static Electricity The common uses of static electricity are in Xerography or electro-photography, Air filters, Automotive paints, Spray paintings. Most of the modern photocopy machines use static electric charges to make photo copies of documents. Recently invented laser and LED printers also use static electricity.

Everyday Electrostatics Fine mist of negatively charged gold particles adheres to positively charged protein on finger print. Negatively charged paint adheres to positively charged metal.

Methods of Charging Friction Conduction Induction When two substances are rubbed together, one of them will lose electrons to the other. The object that has lost electrons will become positively charged. The object that has gained electrons will become negatively charged. Conduction Direct contact of objects is necessary. Electrons flow from one object to another. Induction A neutral object need only come close to a charged object to become negatively charged. No contact is necessary. This is a rearrangement of charges.

Conduction (touching) Friction (rubbing) Induction (no touching needed)

Electric Discharge Charges build up on an object because electrons have moved from one object to another. Once built up, the charges do not flow. They remain at rest until they can be discharged – through a shock.

Sometimes electric discharge is slow and quiet, while other times it is loud and quick. One of the most dramatic examples of static discharge is lightning! During a storm, particles move around in the clouds. This separates the charges, resulting in positive and negative parts of a cloud.

If a negatively charged part of the cloud passes near a part of the earth it will become charged by induction. Electrons jump from the cloud to the earth. As the electrons jump through the air, they produce intense light and heat. The light you see as a lightning bolt. The thunder you hear is the expansion of air due to the build up of heat.

Lightning rods, or grounding rods, are used on tall buildings to provide a path for the electricity to follow to the ground.

Grounding Grounding a conductor means to connect it, through some conducting material, directly to the ground (earth). Earth is not an excellent conductor, but because it is so massive, it can accept or give up electrons without any significant change to its overall charge.

Grounding electrical appliances around the home prevents serious and possibly fatal shocks. The third prong on plugs that fit into wall sockets connects the grounding wires. This is a simple technique that prevents fires and saves lives. Electronic instruments must be grounded because they are sensitive to electrical surges. Electrical wiring in houses, businesses, and industries are always grounded.

Insulators Insulators are materials that do not allow charges to move freely on or through the object.

Insulators Amber Cotton Ebonite Fur Glass Paper Plastic Porcelain Rubber Silk Sulfur Pure water Wood Wool

Conductors Materials that allow charges to move freely on or through the objects are called conductors.

Good Conductors Aluminum Copper Gold Iron Magnesium Mercury Nickel Platinum Silver Tungsten

Fair Conductors Silicon Carbon Earth Human body Humid air Nichrome Salty water

The Van de Graff Generator