Physics 4 – Nov 6, 2018 Agenda: About Charge Coulomb’s Law Do Now – Review of vector addition: Add 50 N @ 35 and 60 N @ 110 Objective: 5.1 Electrostatics Assignment: p205 #1-9 Agenda: About Charge Coulomb’s Law Electric Field Diagrams

About Charge Charge is a fundamental quantity of nature and is a feature of all matter because matter consists of electrons and protons. 1) Charge comes in two varieties: positive and negative (arbitrary names) Electrons are negative, protons are positive. Neutral matter has approximately equal numbers of electrons and protons https://www.youtube.com/watch?v=NsxhbgCrrSQ

About Charge 2) Charge is quantized. The charge of one electron is the smallest amount of charge you can have. All charges are some multiple of 1.6 x 10-19 C. Charge is measured in Coulombs and given the symbol Q or q. 3) Charge is conserved. Charge is neither created nor destroyed Touching two objects together instantly equilibrates and evenly distributes their charges. https://www.youtube.com/watch?v=NsxhbgCrrSQ Explain what is happening

Coulomb’s Law Two charges will exert an electrical force on one another. Opposite charges attract Same charges repel Force is proportional to both charges and inversely proportional to the square of the distance between the two charges. Proportionality constant is k = 9.0 x 109 N m2/C2 (To make the units work) k = 1/4𝝅𝝐o O = permittivity of vacuum = 8.85 x 10-12 C2/Nm2 𝑭=𝒌 𝑸 𝟏 𝑸 𝟐 𝒓 𝟐 A force just like any other. Added just like any other.

Electric Field Diagrams Mechanism for how charges act over a distance: Electric field. Direction of electric field is in the same direction as the force on a small positive test charge q+ Strength indicated by arrow length Direction: tail of field lines begin at + and radiate outward. Direction: Arrow heads point toward a – charge and radiate inward. Similar to gravitational field

Electric field formulas F = qE As a result, there is a formula of electric field based on Coulomb’s Law. 𝑬=𝒌 𝑸 𝒓 𝟐 Can calculate electric field knowing the force and test charge or the magnitude of the electric field present at a given distance from an isolated charge.

Exit slip and homework Exit Slip – What is the electrostatic force (magnitude and direction) between a positive charge of 1.3 x 10-3 C and a negative charge of 6.2 x 10-3 C when they are separated by a distance of 15.0 cm? What’s due? (homework for a homework check next class) P205 # 1-9 What’s next? (What to read to prepare for the next class) Read 5.1 p 196-205