Hi Kiran,
Almost every material which conducts electricity has some
resistance associated with it. Some materials are superconductors, and
they have identically zero resistance. You can find out more about
superconductors by searching these questions and answers.
The resistance of resistors in series just add together. An
example of this is that if you had a length of wire with a resistance
of R (measured in Ohms), then connecting a second length of the same
kind of wire to the end of it will give a wire that's twice as long and
which has a resistance of 2R. So the length of the wire affects the
resistance in a linear fashion.
Similarly, connecting resistors in parallel reduces the total
resistance. The result here is that the resistance of your wire is
inversely proportional to its cross-sectional area.
Other effects on the resistance then come from the resistivity of
the material. Some metals conduct electricity better than others.
Aluminum conducts electricity much better than iron, for example.
Contacts can corrode, also increasing the total resistance in
real-world applications. Aluminum oxidizes and iron rusts, although it
is usally only at the contacts where the biggest problems from
corrosion occur.
Tungsten is a popular resistive material for making light bulb
filaments out of -- it has a very high melting temperature and a very
low vapor pressure when hot, so it doesn't sublimate all that fast when
heated to glowing hot in a vacuum in a light bulb.
The resistivity of many materials depends very much on the
temperature. For metals, the resistivity generally decreases with
decreasing temperature (some metals, like lead and niobium and mercury,
even become superconducting at very low temperatures). Other materials,
like semiconductors and graphite, become more resistive at lower
temperatures.
Materials may also change their resistivity when in a magnetic
field. These effects are also very temperature- and geometry-dependent
(they can be seen quite strongly in some thin films and thin wires,
when the magnetic field is arranged appropriately) -- this effect is
called "magnetoresistivity".
Impurities and structural defects in the metallic crystal have effects on the resistivity too.
Tom
(published on 10/22/2007)
