EGR 101 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

• Neutralneutron.. 0 C

Valence electrons

Atom

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 material

i.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

A

LR

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

EGR 101 7

Observations

• As A increases, R

Why?

• As L increases, R

Why?

Increases

Decreases

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?