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Electric Charges and Forces 25.1 I) Developing a Charge Model Experimenting with Charges A) Neutral B) Attraction/repulsion C) Like/unlike II) Electric Properties of Materials A) Charge can be transferred β only by contact, discharging B) Conductors and Insulators (1) Conductors move charge easily (2) Insulators can be charged, but charge cannot move easily 25.2 Charge I) Convention for positive and negative began with Ben Franklin A) Positive B) Negative II) Atoms and Electricity A) Electron Cloud (1) Only know its size and shape β consists of negatively charged electrons (a) Diameter = 10-10 m B) Nucleus (1) Positively charged protons, neutral neutrons C) Charge is an inherent property of electrons and proton (1) Like mass (2) Comes in discrete values (+π and β π) III) The Micro/Macro Connection A) Charge is represented by the variable π (or π) B) π = πππππ‘πππ π β ππππππ‘ππππ (π) C) Neutral doesnβt mean no charges, rather it means no NET charge D) Quantization (1) Positive charge is established by the absence of electrons, not an abundance of protons IV) Conservation and Charge Diagrams A) Charge is neither created nor destroyed B) As one object looses electrons the other in contact will have to gain the same number of electrons as lost by the first 25.3 I) Insulators and Conductors Charging (contact) A) Insulators β charged by rubbing B) Conductors β allow charge to spread out on their surfaces (electrostatic equilibrium) II) Discharging (contact) III) Charge Polarization (momentary) IV) Electric Dipole (established/maintained) V) Charging by Induction (established/maintained) 25.4 I) Coulombβs Law Force between two charges can be quantified A) Cavendish experimentation similar to that used to determine πΊ B) Equation: πΉ1 ππ 2 = πΉ2 ππ 1 = C) πΎ = 9.0 π₯ 109 πΎ|π1 ||π2 | π2 Nβm2 C2 D) Forces exist in pairs, as always in relation to Newtonβs third law E) Multiple forces can be summed for one individual charge, Superposition Principle 1. May be written in vector form 2. May be written in component form II) Units of charge A) Coulomb: 1.6 π₯ 10β19 πΆ = 1π B) Force may also be described in terms of Permittivity constant (1) π0 = (2) πΉ = 1 = 8.85 π₯ 4ππΎ 1 |π1 ||π2 | 4ππ0 π2 10β12 C2 Nβm2 (3) Examples 25.3 and 25.4 25.5 The Field Model I) The Concept of a Field A) Mathematically, field describes a function that assigns a vector to every point in space B) fields as a model explain many physics phenomena II) The Electric Field A) Two postulates (1) Source charges alter the space around them by creating an electric field, πΈβ . (2) A separate charge in the field experiences electric force πΉπππππ‘πππ B) πΈβ (π₯, π¦, π§) β‘ πΉππ π ππ‘ (π₯,π¦,π§) π III) The Electric Field of a Point Charge A) πΉππ πβ² = B) πΈβ = C) πΈβ = πΉππ πβ² 1 ππβ² 4ππ0 π 2 , away from π 1 π = 4ππ π2 πβ² 0 1 π πΜ 4ππ0 π 2 , away from π IV) Representing the Field A) Finding a multitude of forces and field strength at various positions, allows us to draw a Field Diagram B) The arrow shows the direction and the strength at various positions C) Though drawn as rays, the values of electric field do not extend forever, rather exist only at one point.