Andres La Rosa
Portland State University
Lecture Notes
PH-212
CURRENT and RESISTANCE
Steady currents of conduction-electrons moving through metallic
conductors
Atom
Closed
shell
Electrons in the closed shell
do not participate in the
current
Now we apply an electric field Eext
Eext
+
+
+
+
+
negative charges
moving to the right
Equivalent
to
positive charges
moving to the left
Definition of current
Equivalent
to
Closed
shells
Each atom contributes with 1-free electron:
Cu, Au
Each atom contributes with 2 free-electrons: Zn, Cd
Current Density
j
DRIFT SPEED
RESISTANCE
and
RESISTIVITY
RESISTIVITY ( ρ ), CONDUCTIVITY ( σ ),
ELECTRIC FIELD ( E ), RESISTANCE ( R ) Shell
Microscopic Viewl
CIRCUITS
B
Al
+
+
-
-
Single-loop circuit
+
ε
l
+
-
P
Battery,
electric generator,
or solar cell
ε
ε
is not a force
The unit of
l
ε is VOLT
l
VP +
ε
− r i − R i = VP