Key Terms
Electric current
The rate at which charge flows through an area.
Formula
I = delta-Q / delta-t
SI unit
1 ampere = 1 coulomb per second (1 A = 1 C/s)
Typical drift velocity
~10^-4 m/s (extremely slow) Electrical signal speed: ~10^8 m/s (fraction of light speed)
Drift velocity formula
I = nqAv_d
Ohmic materials
Resistance stays constant regardless of voltage or current. Examples: copper, aluminum.
Non-ohmic materials
Resistance changes with voltage, current, or temperature. Many real devices are non-ohmic.
Resistivity (rho)
An intrinsic material property; tells you how much a given material resists current flow, independent of shape or size.
Conductors
Lowest resistivity (~10^-8 omega-m range) Examples: silver, copper, gold, aluminum
Semiconductors
Intermediate resistivity; varies strongly with impurities Examples: silicon, germanium, carbon
Insulators
Highest resistivity (~10^9 to 10^15 omega-m range) Examples: glass, rubber, teflon
Reason
Atoms vibrate more at higher temperatures, causing more collisions with electrons.
Superconductors
Zero resistance below a critical temperature. Mercury goes superconducting at ~4.2 K.
Direct current (DC)
Charge flows in one direction only. Batteries produce DC.
Alternating current (AC)
Charge flow periodically reverses direction. Most household and commercial electricity is AC.