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Name: Ken
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Question:
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.



Replies:
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.

More detail:

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 drift rate).

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.

Douglas Stanford


Ken

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.

Please see the following URL for more details:

http://en.wikipedia.org/wiki/Electrical_resistance

Sincere regards,

Mike Stewart



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