Resistor: Reduce Flow or Velocity of Charge
In a DC electric circuit, current flow will be decreased as
resistance in increased. Does the resistance reduce current by slowing
down the electron flow per unit of time or does it reduce the number of
electrons available to flow? Yes, the result in either case would be
reduced current but the cause of the effect is the question.
Short answer: flow
In principle, as you point out, it could happen either way. Typically, though,
resistors are made from a combination of conductor and insulator. Higher
resistance means thinner pieces of conductor. For fixed voltage, the speed at
which the electrons drift remains the same, as does the number density of
conducting electrons within the conductor. But, as the pieces of conductor get
thinner, there are fewer TOTAL conducting electrons, so the current decreases,
and vice versa. Think of it, for fixed voltage, like traffic on a highway system:
the cars (electrons) move at the speed limit (drift velocity) within their lanes
(conductive material within the resistor), and the density of cars per square
foot of highway is fixed. But, by adding more lanes or more roads (thicker
pieces of conductive material), more traffic (current) can flow.
In the Drude model of conductivity (which treats electrons classically),
the conductivity of a piece of material is proportional to:
(number density of free electrons ) * (mean free time between collisions) *
(cross sectional area) / (length)
As you point out in your question, the resistance ( ~ 1 / conductivity) can
be increased by finding a material with lower number density of free electrons,
or longer mean free time between collisions (which corresponds roughly to the
In practice, it is easier to make a finely tuned resistor by adjusting the cross
sectional area and length parameters. For example, you could make a resistor from
a thin, long strip of aluminum foil. You could increase the resistance by making
the foil longer or thinner. There is a type of resistor, called a metal film
resistor, that works in precisely this way.
Another type of resistor is the carbon-composition resistor. It is made by taking
a mixture of carbon dust (a conductor) and insulator powder, mixing the two
together, bonding the material, and sticking wires on either end. The resistance
is varied by changing the amount of carbon relative to insulator. By including less
carbon, the effective cross sectional area of conducting material gets reduced, so
the resistance increases, and vice versa.
The electrical resistance of an object is a measure of its opposition to the
passage of a steady electric current.
Different materials cause this to happen by different effects.
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Update: June 2012