Electric Charge Chapter 16 in your book

Electric Charge Repel Attract Two kinds of Electric Charge Positive (+) and Negative (-) Like charges… Unlike charges… Repel Attract

Law of Conservation of Charge Electric charge is ALWAYS conserved Charge cannot be created or destroyed but can be transferred between objects

Prediction If you rub a balloon on you hair, it will become charged. Explain this process using the law of conservation of charge.

Millikan Oil Drop Experiment Millikan sprayed oil drops between charged metal plates. The oil drops were negatively charged by friction. By adjusting the voltage on the plates, he could make the drops rise and fall.

Millikan found that the amount of charge on objects was always a multiple of some fundamental charge (e). In other words, charge is quantized. e turned out to be the amount of charge on an electron. e = 1.602176  10-19 coulombs Coulomb is the SI unit of charge.

Practice 1) How many electrons are in -6.72*10-18 C of charge? 2) Is it possible to have 3.56*10-18 C of charge? Explain.

Charging For a substance to be neutral, it must contain the same number of protons and electrons. Substances can only gain or lose electrons to become charged. Why? If a substance gains electrons, what will its charge be? Why? If a substance loses electrons, what will its charge be? Why?

Practice A metal sphere is electrically neutral. It is touched by a positively charged metal rod. As a result, the metal sphere becomes charged positively. Which of the following occur during the process? List all that apply. The metal sphere gains some protons. Electrons are transferred from the sphere to the rod. The metal sphere loses electrons. The overall charge of the system is conserved. Protons are transferred from the rod to the sphere. Positive electrons are moved between the two objects.

Insulators and Conductors What is a Conductor: A material through which electric charges move freely. Examples? What is an Insulator: A material through which electric charges cannot move freely. Examples?

Practice Consider these two spheres. They have both been charged. In one case the charge is isolated in three distinct locations; in the other case, the excess charge is evenly distributed about the surface of the sphere. Which one of these spheres is made of an insulating material and which is made of a conducting material? Label which is which and support your answer with an explanation. A B

Methods of Charging Friction: rubbing two objects together to transfer charge from one object to the other. Conduction: charge is transferred between objects due to direct contact Polarization: Movement of polar molecules in one object due to presence of charge in nearby charged object Induction: a charged object polarizes a second object; the 2nd object is grounded and charge escapes

Charging by Friction John Travoltage Sweater and Balloon When two objects are rubbed against each other, electrons will transfer from one object to another object. John Travoltage Sweater and Balloon

Charging by Friction When two objects are rubbed against each other, electrons will transfer from one object to another object. Which object receives electrons and which object gives electrons as well as how much charge transfers depends on the type, roughness, temperature, and other properties of the material. The “Triboelectric Series” helps predict the direction of the flow of charge.

Triboelectric Series When two materials are touched together, materials nearer the top of the graph will tend to lose electrons and have a positive charge while materials nearer the bottom will tend gain electrons and have a negative charge.

Practice When you pull a cotton sweater off your skin, electrons are transferred from _______ to ________. As a result, your skin becomes _____ charged and the sweater becomes _____ charged.

Charging by conduction When two objects touch each other, electrons will transfer from the object with excess electrons to the object with a deficit of electrons. John Travoltage

Polarization When a charged object is brought near a neutral substance, the charged object will repel similar charges in the neutral substance and attract opposite charges in the neutral substance. The substance will still be neutral, but the charges in the substance will be polarized Sweater and Balloon

Polarization

Charging by induction A charged rod is held near a metal sphere. Why do the charges in the metal arrange themselves as shown? The metal sphere is connected to the ground with a conductor. Why did some of the electrons move off the sphere?

The conductor connecting the sphere to ground is removed The conductor connecting the sphere to ground is removed. What type of net charge does the sphere now possess? The negatively charged rod is removed. Why do the charges move into the positions shown?

Charging by Induction http://www.physicsclassroom.com/mmedia/e statics/isop.cfm http://www.youtube.com/watch?v=o0hXYfvnN D0

So how can neutral objects be attracted to charged objects? Demo time: water and balloon, 2x4 and balloon. Free Write – Using your knowledge of electrostatics, explain the two demonstrations (Water and Ruler, Wood and Ruler).