UBlearns where all announcements are made Syllabus is online read carefully Homework same as PHY101 Office Hours from our point of view Slides same as PHY101 Not all finished Posted at ublearns What s important Physics problems tend to require two parts Equations over rated Concepts under rated Simple Example A stone is thrown straight up at a speed of 10 m s How long does it take before it starts to fall Online courses Less demand on course scheduling More demand your scheduling Scheduling makes life easier not harder Set up a fixed schedule for yourself and stick with it Bottom line you are asked to learn something well in a short period of time Be smart about how you do it It goes a long way as much as the knowledge itself if not more Chapter 15 Electric Forces and Electric Fields To make use of forces strength volume and variation How about a force 10000000000000000000000000 00000000000000 1 x 1040 times stronger than gravitational force Plain English ten thousand billion billion billion billion times stronger How about a force that can be either attractive and repulsive Confusing but very useful Electric force and magnetic force Or electromagnetic force Basic objects electrons and nuclei or protons 15 1 Properties of electric charges Greeks 700 B C to 18th century existence of charges Franklin 1750 two kinds of charges Buffalo once upon a time the anticipated center of the industrial world Two types We are all wired up Send messages in your body Use to monitor body EKG Positive charge protons p 1 60 x 10 19 Coulomb mp 1 67 x 10 27 kg mn mass of neutron Negative charge electrons e 1 60 x 10 19 Coulomb me 9 11 x 10 31 kg Side note electrons also protons also spin at the same rate Like charges repel unlike charges attract 15 2 Conductors and Insulators Materials allowing charges to move freely conductors Typically metals and some solutions Power lines Materials do not allow charge movement insulators Typically ceramics dry wood plastics Handles for screw driver and pliers electrical shocks are not pleasant Semiconductors chips in computers flash drives LEDs and lasers Superconductors one day it will be useful for everyday things Use to manipulate body Defibrillator Pacemaker Tasers Clean water de ionized is an insulator Dirty water e g drinking water and distilled water is a conductor You and I conductors we have to be Charging by conduction Charged rod touches the sphere and transferring some charge Scope is left w same or opposite charge as rod Example What happens when the two conducting spheres are put in contact and then separated Because plastics are insulators changes can t move around The surface molecules get polarized Charging by induction Charged rod is brought near the sphere The sphere is briefly grounded allowing charge to flow on or off sphere 15 3 Coulomb s Law Make your equation understanding the concepts only reason for this is you remember longer and do better F k q1q2 r2 For HW k 8 9875 x 109 Nm2 C2 Coulomb Constant Different number of charges in different positions Example Force on electron in Hydrogen atom Qp 1 6x10 19 C Qe 1 6x10 19 C r 1x10 10 m Gravitational force between the electron and proton 3 6 x 10 48N F 9x109 1 6x10 19 1 6x10 19 10 10 2 N 2 3x10 8 N Direction To the left Coulomb force 2 10 40 gravitational force 3 The question is always force on one particular charge force has to be on something Focus on that charge find ALL of ITS pairs and deal with one pair at a time Add the forces or subtract could be vector sum at the end mg plus electric force can possibly balance charge q2 Three charges at the corners of an equilateral triangle Three charges at the corners of an equilateral triangle Calculate force on 2 C charge due to other two charges Calculate force on 2 C charge due to other two charges Calculate force from 7 C charge Calculate force from 3 5 C charge Add VECTORS Calculate force from 7 C charge Calculate force from 3 5 C charge Add VECTORS F7 F7 F k q1q2 r2 Q 2 0 C Q 2 0 C 9 6 6 9 10 2 10 7 10 N 25 3 F7 5 10 N F7 Q 7 0 C 5m F3 Q 3 5 C 9 109 2 10 6 3 5 10 6 N 25 F3 2 5 10 3 N F3 Q 7 0 C 5m Q 3 5 C 15 4 The electric field F7 Any time there is a charge there is a thing around it Q 2 0 C Invisible but can be felt like gravity If another charge is close to it the charge will be either pushed away or pulled towards it F3 This thing has to be either pointing away Q 7 0 C 5m or pointing to it Q 3 5 C Convention F3 2 5 10 3 N F7 5 10 3 N E electric field unit N C Why field E Why don t we step off a roof directions E What is the value of gravitational acceleration g If we know m we know F mg without worrying about F Gm1Mearth r2 q Once E is found for a spot you can bring any q to a spot and F qE qq For two point charges F k 1 2 2 r q E From Newton s law of gravity we got g also called the gravitational field From the Coulomb s law we get E Both are fields something there that is invisible but can do things q For HW once you know what the field is at a location you will be asked to find a force including direction once you put a charge there Parallel Plates For HW the question is always how strong a field is at a given location recall what is said about force earlier 2 C E Example Two charges at opposite corners of a square length 1 m What is the electric field at the center of the square 0 q A Charge density 0 1C 1 8 85 10 12 C 2 N m 2 4 k e Permittivity of free space A very common way to generate a uniform electric field 15 5 Field Lines 25 q r2 1 0 11 N C 11 32 1011 N C E k 0 10 8 N C E 2 9 1011 N C If the lines are close to each other the field is strong otherwise weak When there are more charges than 1 total fields will be the vector sum of all fields 15 6 Conductors in electrostatic equilibrium E Regardless of the shape of a conductor the electric field is always zero inside the conductor All charges are on its surface Charges get stuck at sharp corners and can fly out from the tips too when it gets too crowded there