Electricity
Electricity Describes all phenomena caused by positive and negative charges Electrical charge is caused by protons and electrons Electrons and protons are subatomic particles found in the atom Protons carry a positive charge and electrons carry a negative charge
A negatively charged object has more electrons than protons This means that the object has gained electrons. A negatively charged object has more electrons than protons This means that the object has lost electrons
We measure the amount of electrical charge in coulombs (C) 1 coulomb is equal to 6.25 x 10 18 electrons 2 types of electricity: Static electricity Dynamic electricity
Attraction and Repulsion Electrically charged matter behaves the following way: 1) electrical charges that have the same sign (ex. 2 positives or 2 negatives) repel each other They push away from each other 2) electrical charges that have different signs (ex. A positive and a negative) attract They pull toward each other
+ + -- repulsion attraction
The force at work in attraction and repulsion is called the electrical force Law of conservation of charge: electrical charges can neither be created nor destroyed; they can only be transferred from one body to another.
Conductors and Insulators Most objects are electrically neutral but charges can be transferred from one object to another Depending on how objects react to receiving electrical charge, they can be classified as a conductor, insulator or semiconductor
A conductor is a substance that allows electrical charges to flow freely Metals and electrolytic solution are usually conductors Charging a metallic conductor in a circuit allows electrons to move in one direction When electrodes are put in an electrolyte the solution, the positive ions will move to the negative electrode and the negative ions will move toward the positive electrode
An insulator is a substance that tries to block the flow of electrical charges Non metals are usually insulators wood, plastic, rubber, paper, air, pure water etc. When an insulator is charged the charges do not move A semi conductor is a substance whose conductivity varies (depending on different factors) Metalloids are examples of semi conductors often used in electronics
Static Electricity Static electricity is related to electrical charges that are usually at rest refers to the insulated charges Electrically charged objects don t stay charged They can gradually lose their charge (example: by combining with the water molecules in the air)
Charged objects can sometimes recover their neutral state more quickly when 2 objects of opposite charge come near each other or touch This is called electrostatic discharge sometimes gives off a spark (caused by electrons that have passed through the air and heated it making it light up)
Charging an Object 3 ways to charge an object: 1) Friction 2) conduction 3) induction
Charging by Friction Charging by friction occurs when two different materials rub against each other, causing a transfer of electrons The material with the greater affinity (pull) for electrons becomes negative, and the other material becomes positive. E.g. Clothes in a dryer or Rubbing a balloon on your hair.
Charging by Friction
Charging by Friction Negative Positive Rubber Plastic Ebonite (hard rubber) Cotton Paper Silk Fur Wool Glass Acetate The electrostatic series shows which material will be + and which will be when different materials are rubbed together. The higher it is on the list, the more likely it is to gain electrons Example: Balloon (rubber) and wool are rubbed together. What will happen? 16
Charging by Conduction Charging by Conduction occurs when a neutral object is placed in contact with an alreadycharged object. Negatively charged rod Neutral Ball The two objects will share the charge between them You will get two similarly charged objects but the charge will be weaker than the original object
Electrostatics: Charging Objects by Conduction Negatively charged rod Neutral Ball
Electrostatics: Charging Objects by Conduction ` Similarly Charged Rod and Ball
Electrostatics: Charging Objects by Conduction ` Overall the total negative charge remains constant. It is now shared equally. Both objects now have an overall charge of - 4 What would happen if you used a positive rod to touch the metal sphere? The negative charges from the sphere would transfer to the positive rod, distributing the negative charges evenly, leaving both with an overall positive charge
If the objects are spheres of the same size, and are made out of conductive material, they will have the equal amounts of charge after contact. The charge is located on the surface (not the interior) because the electrons repel each other. 22
Charging by Induction Induction happens when a charged item is placed near (but not touching) a neutral item The charged item causes the charges in the neutral item to separate through repulsion. + - + - + - + - + - + - 23
In a and b, a negatively charged rod is placed close to the neutral metal sphere the negatively charged electrons in the sphere are repelled to the far side of the sphere, leaving the atoms on the near side positively charged owing to their missing electrons.
In c, a copper wire is connected to the negative side of the sphere and an electrical ground allowing some of the free electrons to flow into the ground. In d and e, the copper wire and the negatively charged rod are removed leaving a metal sphere with a uniform distributed positively charge
Summary Method Before During After Friction Two neutral objects Friction pulls electrons away from one of the objects and transfers them from one to another Two objects with opposite charge + - Conduction Induction One charged object and one neutral object - - One charged object and one neutral object The charge is shared between 2 objects when they come into contact The proximity of the charged object causes the charges in the neutral object to separate - - - - Two objects with the same charge but weaker - - One charged object and one object carrying a partial positive charge on one side and a partial negative charge on the other + - + - + -