- Slides: 47
What Is Static Electricity? • A stationary electrical charge that is built up on the surface of a material
Two kinds of charges • After being rubbed, a plastic ruler can attract paper scraps. Ruler carries electric charge. It exerts electric force on paper. This charging method is called charging by friction. The interaction between static electric charges is called electrostatics.
Where do charges come from? Matter is made up of atoms. + – neutron (neutral) + + – Proton (positive charge) + – atom – electron (negative charge) nucleus
Where do charges come from? If electrons = protons neutral If electrons > protons gaining electrons, negative charge If electrons < protons losing electrons, positive charge
Electro-negativity +++++ +++ ++ + Relative electro-negativity ranking for some common materials from electron donating materials (+, glass) to -electron accepting --materials (-, teflon) ---- • • • Glass Human Hair Nylon Silk Fur Aluminum Paper Cotton Copper Rubber PVC Teflon
Where do charges come from? Rubbing materials does NOT create electric charges. It just transfers electrons from one material to the other.
Where do charges come from? When a balloon rubs a piece of wool. . . – – – wool + + + electrons are pulled from the wool to the balloon. The balloon has more electrons than usual. The balloon: – charged, The wool: +charged
Insulators and conductors Insulators: materials that do NOT allow electrons to flow through them easily. Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.
4 Insulators and conductors Conductors: materials that allow electrons to flow through them easily. Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.
Induction: The production of a charge in an uncharged body by bringing a charged object close to it When negatively charged rod is put near a metal can. . . - - - induced charges attraction top of the can: positive bottom of the can: negative ++ + + metal can electrons of the can are pushed away from the rod. - - + - repulsion & attraction > repulsion
Attraction of uncharged objects Similarly, when charged rod is close to paper scrap. . . - - - - attraction between the attraction rod and + charge > repulsion between the rod and - charge. ––– – ++ + + paper molecules of paper align. repulsion
Static Discharge Human body can not feel less than 2, 000 volts of static discharge Static charge built up by scuffing shoes on a carpet can exceed 20, 000 volts?
Gas Station Fires • Carol said a static gas pump fire is blamed for burning her daughter so badly she needed skin grafts on her legs. • Carol had put the gas pump nozzle on automatic and re-entered her car to write a check. When her then-12 -year-old daughter, wearing a sweater and jacket that may have created static electricity, reached for the nozzle, flames suddenly ignited her clothing.
A balloon has a negative charge when rubbed by a woollen cloth. 1 If the balloon can attract some paper scraps, which of the following cannot be the charge of paper scraps? A Neutral C Negative B Positive
A balloon has a negative charge when rubbed by woollen cloth. 2 During rubbing, what have been transferred between the woollen cloth and the balloon? A Electrons C Neutrons B Protons
How does a positively charged rod attract a neutral object? When a + charged rod is put near neutral object, negative charge is induced on the side of the object _______ positive charge is induced on the side near the rod and _______ away from the rod. The rod can attract the netural object because _____ attraction between rod and – induced charge > the ____ repulsion between rod and + induced charge.
Grounding What is grounding? An object is grounded when it is connected to the earth through a connecting wire. If a charged conductor is grounded, it will become neutral.
Grounding b How does grounding occur? + + + When we touch a metal ball of positive charge. . . – electrons flow from the earth to the metal ball to neutralize the metal ball. Metal ball becomes neutral.
Grounding How does grounding occur? – – – Similarly, if the metal ball is of negative charge. . . – extra electrons flow from the metal ball to the earth and the ball becomes neutral.
Why do gasoline tankers usually have metal chains at the back? When cars run, their tires and bodies are usually friction For gasoline tankers, if the charged by _______. accumulated charge is large enough, _______can be sparks explosion will occur if gasoline vapor is produced and _____ ignited. Those metal chains conduct the charge on the ground avoid the danger. bodies of tankers to the _______
Lightning • kills more than 60 people and • injures more than 400 people a year in the US • one mile every five seconds • about 20, 000 C • Voltage of up to 1. 2 x 108 volts •
Do Now Write a paragraph about the cartoon. Tell what he is trying to do. Will it work and why? What do you think will happen? Use the following words, static electricity, lightning, electrons, positive and negative charges.