Electric Charge
• Charge (Q)
– An intrinsic property of matter
that establishes a force of
attraction or repulsion between
two particles.
– Unit is Coulombs (C)
Atom
• Positive proton (e+)
Q = +1.6 X 10-19 C
• Negative electron (e-)
Q = -1.6 X 10-19 C
• Neutral
Valence electrons
neutron.. 0 C
EGR 101
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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
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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
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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
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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
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Resistance of a Material
R
L
A
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  ?
EGR 101
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Observations
• As A increases, R
Increases
Decreases
Why?
• As L increases, R
Increases
Decreases
Why?
EGR 101
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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
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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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