Relation between voltage and currnet also a resistor.

This is a long and interesting story.It is the heart of electronics.Crudely speaking, the name of the game is to make and use gadgets that have interesting and useful I-versus-V characteristics.
Resistors(I simply propotional to V),Capacitors(I propotional to rate of  change of V),
Diods(I flows in only one direction),thermistors(temperature-dependent resistor),Photo resistors (light dependent resistors),Strain gauges(strain-dependent resistor) etc are examples.
We will gradually get into some of these exotic devices;for now,we will start with the most mundane(and most widely used )circuit element,
The resistor.
It is  an interesting fact that the current through a metallic conductor is propotional to the voltage across it.the formula of resistor is R=V/I.
Resistors are available with resistances .resistors are easy to use.
Power in resistors.
The power dissipated by a resistor(or any other device)is P=VI.
Using ohm,s law , you can get the equivalent forms P=I^2R and P+V^2/R.

HOW to check resisters: