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Electric Charge

Electric Charge. Charge (Q) An intrinsic property of matter that establishes a force of attraction or repulsion between two particles. Unit is Coulombs (C) Positive proton (e + ) Q = +1.6 X 10 -19 C Negative electron (e - ) Q = -1.6 X 10 -19 C Neutral neutron.. 0 C. Atom.

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Electric Charge

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  1. Electric Charge • Charge (Q) • An intrinsic property of matter that establishes a force of attraction or repulsion between two particles. • Unit is Coulombs (C) • Positive proton (e+) Q = +1.6 X 10-19 C • Negative electron (e-) Q = -1.6 X 10-19 C • Neutral neutron.. 0 C Atom Valence electrons EGR 101

  2. Relation of Coulombs to Electrons • The charge on 1 e- is Q = -1.6 x 10-19 C • How many e- exist in Q = 1 Coulomb? EGR 101

  3. Current, I • The DIRECTED flow of charge through a conductor • Provide an external energy (e.g. a battery) • Current = I = ΔQ/Δt • ΔQ = the amount of charge that has moved past a point in the conductor (Coulombs) • Δt = elapsed time (seconds) EGR 101

  4. Example Media of Current Flow • Wire – made of a conducting materiali.e. a material that provides little opposition to the flow of charge (low resistance) EGR 101

  5. Classification of Materials • Conductors • Materials with few electrons in the outermost (valance) band • The electrons can be freed with very little external force • Insulators • Materials with complete valance bands • It takes a great force to free these electrons • Semiconductors • Materials with half-complete valance bands EGR 101

  6. Resistance of a Material R is the resistance (in Ohms, Ω) • is a property of the material called resistivityL is the length of the material (in cm)A is the cross-sectional area of the material(in cm2) • What are the units of r ? EGR 101

  7. Observations Increases Decreases • As A increases, R Why? • As L increases, R Why? Increases Decreases EGR 101

  8. Class Activity (10 minutes) • Assume you want buy some wire and select the material with the least resistance per length.a) How much lower is the resistance of a 3mm diameter wire over a 1 mm diameter wire of the same length and made out of the same material?b) Which of these materials should your wire be made out of:a) iron b) copper c) aluminum d) gold?Why? EGR 101

  9. Monster Cables • If gold is not the best conductor, why do “Monster Cables” advertise “24k gold contacts” on their connectors? EGR 101

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