Introductory Physics Volume Two

3.11 Summary 79
§ 3.11 Summary
Definitions
Electric Current:
Current Density
I = dq
dt
J = I A
Facts
Ohm’s Law: For many materials current density is proportional to
the electric field.
⃗J = σ ⃗ E
with σ the conductivity of the material. ρ = 1/σ is called the resistivity.
∆V = IR
with R the resistance of the device.
Theorems
Electric Power:
Kirchhoff’s Loop Rule:
P = I∆V
∑
∆V = 0
Kierchhoff’s Junction Rule:
∑
Iin = ∑ I out
Resistors in Parallel:
Resistors in Series:
1
R eff
= 1 R 1
+ 1 R 2
Capacitors in Parallel:
R eff = R 1 + R 2
Capacitors in Series:
C eff = C 1 + C 2
1
C eff
= 1 C 1
+ 1 C 2