About the Basic concepts of voltage & current

[Vtile]

^ I were too hang-up with the "what happens in moment of connection" so I didn't see the obvious. Doesn't matter, this starts to be in the area of offtopics offtopic, without any practical use. Hopefully OP (or someone else) get something out of it.

There is no wires, only conductors with resistances (impedance). 

[3db]

My teachers used to emphasise "At constant temperature" when teaching Ohms Law.

3DB

[Ratch]

Hello everyone.
  What I'm about to ask is the most basic questions about electronics, Since I can't find the proper answer from internet, I thought to post it. I'm confused with some of the simplest concepts of voltage, current  & Ohm's law.

1) while dealing with LED's we mostly use resistors along with it right??

Correct, do you know why?

So if a 3v,20mA LED is provided with a standard 9V battery

It is the voltage and current capacity that is important, not the battery type.  If the voltage source cannot supply enough current to run the LED, then it will not work for that application.

we have R= 9-3/.02 = 300ohm resistor, so here what does the resistor actually do ?? does it reduces the voltage from 9v to 3v  or limits the current up to 20mA  or both ??

It does both, for the purpose of dissipating the excess electrical energy as heat, so as to protect the diode from absorbing the excess electrical energy.

2) So some components like LEDs can draw lots of currents when connected directly right??

Do you mean hooked up to a voltage without the proper current limiting?  You are not clear on that point.  Yes, you can burn out you LEDs that way.

But i have also heard that, a circuit or component consumes the current that is required by it.

Current or charge flow is not "consumed".  It is changed to a higher or lower rate.

Does this implies that i can use a 12v 500mA  or 12v 1A  supply for a same circuit or should i look for any other specifications ??

You can use a 100 amp capacity voltage source to run a tiny LED if you want to do so.  Just make sure you put enough resistance in the circuit to limit the current to what the LED can take without burning out.  The calculations you made above should do it.

Ratch