Objectives: The students will be able to: 1.Demonstrate that charged objects exert forces, both attractive and repulsive. 2.Explain that charging is the separation, not the creation, of electric charge. 3.Distinguish between an insulator, a conductor, and a semi conductor and give examples of each.

Induced Charge; the Electroscope Metal objects can be charged by conduction:charged by conduction

Electric Charges and Forces Explain why?

Induced Charge; the Electroscope They can also be charged by induction:induction

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... electrons of the can are pushed away from the rod.  top of the can: positive & attraction > repulsion metal can buttom of the can: negative induced charges attraction repulsion

Attraction of uncharged objects Similarly, when charged rod is close to paper scrap molecules of paper align.  attraction between the rod and + charge > repulsion between the rod and - charge. paper + – + – + – + – + – + – + – + – attraction repulsion

Methods of Charging Charging by Induction: bringing a charged object close to a neutral object will induce the same charge in the neutral object -when moved away, the neutral object goes back to being neutral negative

Induction & Polarization The displacement of charge in an isolated conductor when placed near by an electrically charged body Separation of positive and negative charges

Polarization Uncharged insulatorthe polarized insulator

Conservation & Quantization of Charge Conservation: Total electric charges in any closed system is a constant. Charge can be transferred from on object to another. Quantization: minimum amount of charge: e, the basic unit, the charge of the electron or proton. Any Q = integer x e e = 1.602x C or 1 C ~ 6x10 18 protons(+) or electrons(- )! mC,  C, nC…

Induced Charge; the Electroscope Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

Induced Charge; the Electroscope The electroscope can be used for detecting charge:

Methods of Charging Electroscope: a device used to detect and identify static electric charges -metal conductors insulated by a rubber collar

Methods of Charging Electroscope: a device used to detect and identify static electric charges -metal conductors insulated by a rubber collar -charges spread throughout metal parts so both leaves have the same charge -like charges repel so the leaves move apart

Induced Charge; the Electroscope The electroscope can be charged either by conduction or by induction.

When a + charged rod is put near neutral object, ______________ is induced on the side of the object near the rod and _____________ is induced on the side away from the rod. The rod can attract the netural object because _________ between rod and – induced charge > the ________ between rod and + induced charge. How does a positively charged rod attract a neutral object? negative charge positive charge attraction repulsion

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Triboelectric series => Triboelectric Effect When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged. MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE

Induced Charge; the Electroscope The charged electroscope can then be used to determine the sign of an unknown charge. determine the sign

Summary: How Can You Charge Objects? There are 3 ways objects can be charged: 1.Friction 2.Conduction 3.Induction **In each of these, only the electrons move. The protons stay in the nucleus**

Friction Charging by friction occurs when electrons are “wiped” from one object onto another. Ex. If you use a cloth to rub a plastic ruler, electrons move from the cloth to the ruler. The ruler gains electrons and the cloth loses electrons.

Conduction Charging by conduction happens when electrons move from one object to another through direct contact (touching). Ex. Suppose you touch an uncharged piece of metal with a positively charged glass rod. Electrons from the metal will move to the glass rod. The metal loses electrons and becomes positively charged. Period 3 stopped here

Induction Charging by induction happens when charges in an uncharged object are rearranged without direct contact with a charged object. Ex. If you charge up a balloon through friction and place the balloon near pieces of paper, the charges of the paper will be rearranged and the paper will be attracted to the balloon.

Conservation of Charge When you charge something by any method, no charges are created or destroyed. The numbers of electrons and protons stay the same. Electrons simply move from one atom to another, which makes areas that have different charges.

Objectives: The students will be able to: Apply Coulomb's law to determine the magnitude of the electrical force between point charges separated by a distance r and state whether the force will be one of attraction or repulsion.

Two types of charge: Positive Charge: A shortage of electrons. Negative Charge: An excess of electrons. Conservation of charge – The net charge of a closed system remains constant.

+ n n n n n n Neutral Atom Number of electrons = Number of protons Nucleus Negative Atom Number of electrons > Number of protons -2e = -3.2 x C - - Positive Atom Number of electrons < Number of protons +2e = +3.2 x C

Inquiry Activity Electric Field Hockey phET

Electric Forces Like Charges - Repel Unlike Charges - Attract - + F F + + F F

Coulomb’s Law Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.

Coulomb’s Law Coulomb’s Law – Gives the electric force between two point charges. k = Coulomb’s Constant = 8.988x10 9 Nm 2 /C 2 q 1 = charge on mass 1 q 2 = charge on mass 2 r = the distance between the two charges The electric force is much stronger than the gravitational force. Inverse Square Law

Coulomb’s Law Coulomb’s law: This equation gives the magnitude of the force.

Coulomb’s Law The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same.

Coulomb’s Law Unit of charge: coulomb, C The proportionality constant in Coulomb’s law is then: Charges produced by rubbing are typically around a microcoulomb:

Coulomb's Law The force between two charges gets stronger as the charges move closer together. The force also gets stronger if the amount of charge becomes larger.

Coulomb's Law The force between two charges is directed along the line connecting their centers. Electric forces always occur in pairs according to Newton’s third law, like all forces.

Coulomb's Law The force between charges is directly proportional to the magnitude, or amount, of each charge. Doubling one charge doubles the force. Doubling both charges quadruples the force.

Coulomb's Law The force between charges is inversely proportional to the square of the distance between them. Doubling the distance reduces the force by a factor of 2 2 = (4), decreasing the force to one- fourth its original value (1/4). This relationship is called an inverse square law because force and distance follow an inverse square relationship.

Coulomb’s Law Charge on the electron: Electric charge is quantized in units of the electron charge. Unit of charge is a Coulomb (C)

If r is doubled then F is : If q 1 is doubled then F is : If q 1 and q 2 are doubled and r is halved then F is : ¼ of F 2F 16F Two charges are separated by a distance r and have a force F on each other. q1q1 q2q2 r F F Example 1

Example 2 Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) 3μC 40g 50cm 3μC 40g

The electric force is much greater than the gravitational force

Homework

Objectives: The students will be able to: Apply Coulomb's law to determine the magnitude of the electrical force between point charges separated by a distance r and state whether the force will be one of attraction or repulsion.

Summer July 06PHYS632 E&M44 Coulombs Law In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using a torsion balance two charged mulberry pithballs repelled each other with a force that is inversely proportional to the distance. where k=8.99*10 9 Nm 2 /C 2 in SI unit k ~ Nm 2 /C 2 q2q2 q1q1 r Repulsion Repulsion Attraction Lab Experiment Spheres same as points as points - - Point charges

Superposition of electric forces Net force is the vector sum of forces from each charge q1q1 q2q2 q3q3 q F3F3 F2F2 F1F1 Net force on q: F = F 1 + F 2 + F 3 F

Principle of Superposition Three charges In a line In the previous example we tacitly assumed that the forces between nuclei simply added and did not interfere with each other. That is the force between two nuclei in each penny is the same as if all the others were not there. This idea is correct and is referred to as the Principle of Superposition. Example of charges in a line –Three charges lie on the x axis: q 1 =+25 nC at the origin, q 2 = -12 nC at x =2m, q 3 =+18 nC at x=3 m. What is the net force on q 1 ? We simply add the two forces keeping track of their directions. Let a positive force be one in the + x direction. 1 2 x 3

Example Three charges in a line. Three charged particles are arranged in a line, as shown below. Calculate the net electrostatic force on particle 3 (the -4.0μC on the right) due to the other two charges.

Example Three charges in a line. Three charged particles are arranged in a line, as shown below. Calculate the net electrostatic force on particle 3 (the -4.0μC on the right) due to the other two charges.

Solving Problems Involving Coulomb’s Law and Vectors The net force on a charge is the vector sum of all the forces acting on it.

Homework

Closure Kahoot

Elaboration  Assignments Determination of Charge P Charging by Induction