Electrostatics Chapter 23 Week 1 2 What s Happening Clicker use will start on Friday maybe Today we begin the study of charge with make believe clickers There will NOT be a quiz this week There WILL be a quiz a week from Friday WebAssigns are now active Probable First Observation Electricity Idiot If lightening had actually traveled down the kite string old Ben Franklin would have been toast Probably never happened but good story A Quick Experiment Experimental Procedure The sequence of Experiments 1 Identify the two rods 2 Treat each rod 3 Bring one rod near to the other 4 PREDICT WHAT WILL HAPPEN 5 VOTE ON POSSIBILITIES 6 Observe what happens 7 Did we learn anything Pivot Allowable Predictions In the future we will use clickers for this 1 Rods will attract each other 2 Rods will repel each other 3 Nothing will happen 4 Something not listed above will happen Experiment 1 Rubber rod Pivot Rubber rod 1 2 3 4 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen Experiment 2 Rubber rubbed with skin of dead rabbit Pivot Rubber rubbed with skin of dead rabbit 1 2 3 4 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen Experiment 3 Glass rubbed with wool Pivot Glass rubbed with wool 1 2 3 4 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen What is the effect of DISTANCE Experiment 4 Rubber rubbed with skin of dead rabbit Pivot Glass rubbed with wool 1 2 3 4 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen What s Going On All of these effects involve rubbing two surfaces together Or pulling two surfaces apart Something has happened to each of these objects These objects have a new PROPERTY Other properties are mass color We call this NEW PROPERTY CHARGE Example Tape Separation Another Example Effect of Charge We have also observed that there must be TWO kinds of charge Call these two types positive negative We define the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE The charge on the glass rod or the dead cat is consequently defined as POSITIVE From whence this charge Easily Removed Materials Two kinds of materials Insulators Conductors Electrons and Protons are tightly bound to their positions Hard to move them around Electrons are easily removed and moved around Electrons are said to be MOBILE charges There are other kinds of materials that we will not discuss semiconductors semi metals Experiment 5 Rubber rubbed with skin of dead rabbit Pivot Metal Rod 1 2 3 4 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen Ways to charge an object Rubbing or bond breaking same thing Transfer Direct transfer Polarization Induction Neutral Object POLARIZATION Positive charge attracts negative charges Rod becomes polarized Negative end is closer to positive charge Distance effect causes attraction Induction Polarize Ground Remove Ground Positive Balloon Physics Same as before Polarization From WebAssign Three objects are brought close to each other two at a time When objects A and B are brought together they attract When objects B and C are brought together they repel From this we conclude which of the following Select all that apply Objects A and C possess charges of the same sign All three of the objects possess charges of the same sign One of the objects is neutral Objects A and C possess charges of opposite sign We need to perform additional tests to determine information about the charges on the object What happens when two surfaces touch or rub Bonding The Triboelectric Series 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 No No No No 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 Summary Rubbings What have we found There are TWO types of charge Positive Negative Like Charges Attract Un Like charges repel The force between charges increases as they are brought closer together This charge separation results from chemical bonds which are severed Forces Between Charges Q1 Q2 X X Attract Repel X X Coulomb s Law Force between charges The force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between them The force acts along the line connecting the two charges q1q2 1 q1q2 q1q2 F 2 k 2 2 r 4 0 r r 1 k 4 0 Remember Coulomb s Law The Unit of Charge is called THE COULOMB 1 q1q2 F runit 2 4 0 r 1 k 9 x109 Nm 2 C 2 4 0 Smallest Charge e a positive number 1 6 x 10 19 Coul electron charge e Proton charge e Three point charges are located at the corners of an equilateral triangle as shown in Figure P23 7 Calculate the resultant electric force on the 7 00 C charge Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal insulating rod extending from the origin to the point x d As shown in Figure P23 10 a third small charged bead is free to slide on the rod At what position is the third bead in equilibrium Can it be in stable equilibrium The Electric Field Fields Imagine an object is placed at a particular point in space When placed there the object experiences a force F We may not know WHY there is a force on the object although we usually will Suppose further that if we double some property of the object mass charge then the force is found to double as well Then the object is said to be in a force field The strength of the field field strength is defined as the ratio of the force to the property that we are dealing with Example Gravitational Field Property is MASS m Force is mg Field strength is defined as Force Property F mg Gravitational Field Strength Force Gravitational Force mg g Property mass m The Gravitational Field That We Live In M m mg Mg This is WAR Ming the merciless this guy is MEAN You are fighting the enemy on the planet Mongo The evil emperor Ming s forces are behind a strange green haze You aim your blaster and fire but Nothing Happens The Green thing is a Force Field The Force may not be with you Side View The FORCE FIELD Force Force o Big Position