Electric Forces: Introduction

Electric Forces: Introduction Physics 12

Clip of the day: • Minute physics! • http://www.youtube.com/watch?v=9YUtFpLpGfk

Demos: • Paper and Balloon/comb • Balloon on wall • Homemade “Electroscope” • Electroscope • Rods (from chem lab) • Van de Graf generator • Pith balls • http://www.wfu.edu/physics/demolabs/demos/avimov/bychptr/chptr7_electricity.htm

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

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

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

Chemistry Review??????? • What is an atom? • 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. • How can you tell a hydrogen atom from a carbon atom? • 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? • The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral.

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

Friction: one way to create charge • The frictional charging process results in a transfer of electrons between the two objects that are rubbed together. • For example: • Rubber has a much greater attraction for electrons than animal fur. • As a result, the atoms of rubber pull electrons from the atoms of animal fur, leaving both objects with an imbalance of charge. • The rubber balloon has an excess of electrons and the animal fur has a shortage of electrons. • Having an excess of electrons, the rubber balloon is charged negatively. • Similarly, the shortage of electrons on the animal fur leaves it with a positive charge. • 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: • Higher electron affinity (-) • Lower electron affinity (+)

Charging by induction • Is a method of charging a neutral object, using a charged object, without establishing physical contact between them. • The neutral object becomes polarized • 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 • 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. • 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 • He used his torsion balance to measure the magnitudes of the electrical forces between charged objects • He was able to prove that an attractive and repulsive force between charges existed and that the magnitude depends on the separation between them. • http://www.youtube.com/watch?v=FYSTGX-F1GM

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

Coulomb’s Law • Fe is electrical force (N) • k is the Coulomb constant • q1 is the electrical charge on object 1 (C) • q2 is the electrical charge on object 2 (C) • r is distance between the objects (m)

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

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

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

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

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

Reminder: • 1 coulomb (C) = 1 000 000 microcoulombs (µC)

Example: A positive charge of 6.0 x 10 -6C is 0.30m from a second positive charge of 3.0 x 10 -6C. Calculate the force between the charges. • Fe = k q1 q2 • r2 = (9.0 x 109 N m2/C2 ) (6.0 x 10 -6C) (3.0 x 10 -6C) ( 0.30m )2 =0.162 0.09 = 1.8 N (repulsion so + force)

Try it : • Coulomb’s Law WS#1 • Page 638 • Questions 1-5

Pith Ball

Electric Forces: Introduction