1. Electric Charge Charge (Q) Positive proton (e+) Negative electron (e-)
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
Atom
Valence electrons
EGR 101

2. 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 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)
Use flow of cars analogy or water
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
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 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
With Area, it’s like adding another lane on the road for traffic to flow
With Length, it takes more work to move electrons a longer distance
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?
a) R = rho*L/A
For a cylinder A = pi*d2/4
So R1 = rho*L*4/(pi*32) = rho*L*4/(pi*9)
R2 = rho*L*4/(pi*12) = rho*L*4/pi
b) Since rho of copper is less than rho of gold, the copper is a better conductor and buying gold would be a waste
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