Electric Forces: Introduction Physics 12

Clip of the day: 0 Minute physics! 0

Demos: 0 Paper and Balloon/comb 0 Balloon on wall 0 Homemade “Electroscope” 0 Electroscope 0 Rods (from chem lab) 0 Van de Graf generator 0 Pith balls 0 ov/bychptr/chptr7_electricity.htm ov/bychptr/chptr7_electricity.htm

How does an electroscope work? 0 An electroscope consists of a vertical metal rod from which two parallel strips of gold leaf hang down. 0 The gold leaves are enclosed in a glass bottle to protect them from drafts of air. 0 When the metal rod is touched with a conductive charged object the gold leaves spread apart in a V. 0 This is because the charge on the object is conducted through the metal rod to the leaves. 0 Since they receive the same sign charge they repel each other and spread apart.

Electrostatics 0 Aka static electricity 0 Is the study of electrical forces between stationary or hardly moving charges 0 Defined in terms of its effect (has no colour, mass, length, etc.)

In everyday life…. 0 Taking off a sweater over a shirt there is a crackling sound 0 Static cling of clothes out of the dryer 0 A pen rubbed with a piece of cloth will pick up small pieces of paper 0 Cling wrap sticking to everything 0 Getting shocked by another person 0 Getting a small electric shock from a cat that has rolled on a synthetic carpet

Chemistry Review??????? 0 What is an atom? 0 Basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force. 0 How can you tell a hydrogen atom from a carbon atom? 0 Atoms of each element are distinguished from each other by the number of protons that are present in their nucleus. An atom containing one proton is a hydrogen atom. An atom containing 6 protons is a carbon atom.

How can an atom orobject be charged? 0 The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral. Positively Charged Negatively Charged Uncharged Possesses more protons than electrons Possesses more electrons than protons Equal numbers of protons and electrons

Why do some things become more easily charged? 0 The presence of different atoms in objects provides different objects with different electrical properties. 0 One such property is known as electron affinity. 0 Electron affinity refers to the relative amount of “love” that a material has for electrons. 0 High electron affinity, then that material will have a relatively high “love” for electrons.

Friction: one way to create charge 0 The frictional charging process results in a transfer of electrons between the two objects that are rubbed together. 0 For example: 0 Rubber has a much greater attraction for electrons than animal fur. 0 As a result, the atoms of rubber pull electrons from the atoms of animal fur, leaving both objects with an imbalance of charge. 0 The rubber balloon has an excess of electrons and the animal fur has a shortage of electrons. 0 Having an excess of electrons, the rubber balloon is charged negatively. 0 Similarly, the shortage of electrons on the animal fur leaves it with a positive charge. 0 The two objects have become charged with opposite types of charges as a result of the transfer of electrons from the least electron-loving material to the most electron-loving material.

Examples: 0 Higher electron affinity (-) 0 Lower electron affinity (+)

Charging by induction 0 Is a method of charging a neutral object, using a charged object, without establishing physical contact between them. 0 The neutral object becomes polarized 0 There is a redistribution of the centers of positive and negative charges within the object by the movement of electrons across the surface of the object 0 While there are the same number of protons and electrons within the object, these protons and electrons are not distributed in the same proportion across the object's surface. 0 While there is a separation of charge, there is NOT an imbalance of charge. When neutral objects become polarized, they are still neutral objects.

Coulomb’s Experiment 0 He used his torsion balance to measure the magnitudes of the electrical forces between charged objects 0 He was able to prove that an attractive and repulsive force between charges existed and that the magnitude depends on the separation between them. 0 watch?v=FYSTGX-F1GM watch?v=FYSTGX-F1GM

What did he find? 0 The electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects 0 The electrical force between two charged objects is inversely proportional to the distance of separation between the two objects. 0 Increasing the separation distance between objects decreases the force of attraction or repulsion between the objects. 0 Decreasing the separation distance between objects increases the force of attraction or repulsion between the objects.

Coulomb’s Law 0 F e is electrical force (N) 0 k is the Coulomb constant 0 q 1 is the electrical charge on object 1 (C) 0 q 2 is the electrical charge on object 2 (C) 0 r is distance between the objects (m)

Note: Coulomb’s Law 0 Determines magnitude of force only 0 Sign is based on charge interactions

0 Electric Force is the force felt by separated (positive or negative) charges. 0 Opposite charges attract 0 Like charges repel Electric Force: +- ++

Note: Electrical Force 0 Positive and negative does not indicate direction! 0 Positive indicates a repulsive force 0 Negative indicates an attractive force

Electric Force 0 Positive force will result when: 0 Two positively charged particles interact 0 Two negatively charged particles interact 0 Negative force will result when: 0 One positively and negatively charged particle interact

Conservation of Electrical Charge 0 When two charged objects touch, they transfer charge from one another and the total charge of their system must be conserved. 0 Example: 0 One object with a charge of +4 and another object with a charge of-12 (total charge is -8). 0 When they touch, the charge is evenly distributed between each of them (electrons are transferred from the -12 to the +4). 0 When they separate, their total charge still remains -8. Thus, the objects will each have a charge of -4.

Reminder: 0 1 coulomb (C) = microcoulombs (µC)

Example: 0 A positive charge of 6.0 x C is 0.30m from a second positive charge of 3.0 x C. Calculate the force between the charges. F e = k q 1 q 2 r 2 = (9.0 x 10 9 N m 2 /C 2 ) (6.0 x C) (3.0 x C) ( 0.30m ) 2 = = 1.8 N (repulsion so + force)

Try it : 0 Coulomb’s Law WS#1 0 Page Questions 1-5

Pith Ball