I'm studying resistance as part of my coursework but i am totally lost as
to how to define it and was wondering if you could give me a simple
statement that could define it in an understandable and easy manner. the
factor i have chosen to investigate is how length can affect
resistance-"as the length increases so does the resistance". this is a
statement i am working around but i still need help. can you possibly also
explain what a variable resistor is?
Electrical resistance is proportional to the length of a wire divided by the
cross sectional area. If the lenth double then so does the resistanc, if the
are doubles then the resistance is half the initial value. The is a
proportionality constant known as the electrical resistivity, usually
represented by the Greek letter rho (lower case). rho depends on the
material of the wire and other variables such as temperature.
Hope this is sufficient to solve your problem.
Dr. Harold Myron
"Resistance" is a resistance to the flow of current through a part of a
circuit. Two important concepts are energy and charge. As charge flows
through a standard material, some of the energy in the moving charge is lost
to the structure of the material as heat. For many materials, the voltage
drop across the material is proportional to the resulting current through
the material. The ratio of these values, (voltage drop across
material)/(current through material), is the resistance. A larger
resistance results in a smaller current.
Now for why resistance of a material is proportional to its length. Suppose
one volt will get one ampere of current through 1 cm of material.
Resistance is (1V)/(1A) = 1 Ohm. A 3 cm piece of material will be like
three 1 cm pieces connected end-to-end. Assume one ampere is flowing
through. A potential drop of 1 volt is necessary across the first
centimeter. A drop of 1 V is necessary across the second, and again across
the third. Any charge that passes through all three has its potential
decrease by a total of 3 volts. Resistance of the entire 3 centimeters is
(3V)/(1A) = 3 Ohms. As length increases, so does resistance.
A variable resistor is a long resistor that can be adjusted. The connection
at one end is stationary. The other connection can be moved to various
points on the apparatus, altering the length of resistor that current
actually passes through. When the current doesn't have to pass through as
great a length, the resistance decreases.
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Update: June 2012