EGR 1011 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 C Negative electron (e - ) Q = -1.6 X C Neutral neutron.. 0 C Valence electrons Atom

EGR 1012 Relation of Coulombs to Electrons The charge on 1 e - is Q = -1.6 x C How many e - exist in Q = 1 Coulomb?

EGR 1013 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 1014 Example Media of Current Flow Wire – made of a conducting material i.e. a material that provides little opposition to the flow of charge (low resistance)

EGR 1015 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 1016 Resistance of a Material R is the resistance (in Ohms, Ω)  is a property of the material called resistivity L is the length of the material (in cm) A is the cross-sectional area of the material (in cm 2 ) What are the units of 

EGR 1017 Observations As A increases, R Why? As L increases, R Why? Increases Decreases Increases Decreases

EGR 1018 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 1019 Monster Cables If gold is not the best conductor, why do “Monster Cables” advertise “24k gold contacts” on their connectors?