Question

why is the basis for representing a practical inductor as an ideal inductor in series with a resistance

Answer 1

The dominant unideality of the inductor is the fact that the copper wire has some resistance too.

Answer 2

Every single material has its own resistance. That's why you put a resistance in series with an inductor. To eliminate the error between the practical and the paper calculus

Answer 3

but what i actually wanted to ask was
what is the basis for representing a practical inductor as an ideal inductor in series with a resistance and a practical capacitor as an ideal capacitor in parallel with a resistance. what is the reason for the resistance to be placed in series in an inductor and in parallel for a capacitor?

Answer 4

Inductor is built from a (usually) long wire and therefore there is some resistance always. As the frequency of the signal increases, the impedance of the inductor increases and the current will decrease (if the voltage is kept constant). If the resistor representing the unideality was in parallel, then the impedance of the inductor would never go over the value of the resistance.

A capacitor is basically constructed by placing two metal plates near each other. There are no "long wires" with significant resistance. The main unideality of capacitor is the leakage current, especially in electrolytic capacitors. If you charge an electrolytic capacitor and leave it alone for a night, it will be empty.

To be precise, there is also series resistance in capacitor. If you connect a plastic 1 uF capacitor in the mains voltage, it will get a little warm, though the parallel resistance is very small.

Here is a pretty accurate model for an electrolytic capacitor:
http://en.wikipedia.org/wiki/Electrolytic_capacitor#Electrical_behavior_of_electrolytics

And for inductors, there is also other unidealities, for example, the parallel capacitance (between the coil windings):
http://www.intusoft.com/articles/inductor.pdf

Hope this helps :-).