I like the pinball machine analogy best. The analogy can actually be seen
in a simple physical manifestation - a piece of wood with dowels (pins)
stuck in it.
Balls - conduction electrons
Pins - scattering centers in a solid
Slope of pinball machine - electric field
Height difference of back and front of machine - voltage
Horizontal length of machine - length of resistor
Balls slowly drift down the machine at about constant velocity.
The accelerate between collisions with the pins.
Increase the number of pins - increase the density of scattering centers,
which increase the resistivity
Increase the length of machine - increase the length of resistor, which
decreases the electric field, which decreases the drift velocity, which
increases the resistance
Increase the width of the machine keeping the density of balls the same -
increases the current for a fixed voltage so decreases the resistance.
Decrease the slope of the machine - decrease the electric field, which
decreases the drift velocity, which decreases the current
Dr. Lawrence D. Woolf; General Atomics, 3550 General Atomics Court, Mail
Stop 15-242, San Diego, CA 92121