Resistors and Ohm's Law

If there is one thing that every single electronic engineer needs to know, its Ohms law, it describes the basis of all electronics. Published in 1827 as the book Die galvanische Kette, mathematisch bearbeitet ("The galvanic circuit investigated mathematically") Ohm's Law states

That the current through a conductor between two points is directly proportional to the voltage across the two points.

To put it slightly simpler, if there is current flowing through a conductor, then that current is proportional to the voltage across that conductor. The proportional bit is the resistance. Basically, if you have a resistor, with a voltage across it, the current will be a certain amount (calculated using ohms law). If that voltage was doubled, the current would also double.

The equation representing this law is:

V = I x R

V = voltage across the conductor
I  = current flowing through the conductor
R = the resistance of the conductor

Example

Lets say that we have a constant 5v supply, and need to draw a constant 10mA from that power supply. We need a resistor, and we can use Ohm's law to figure out the value. Start with the Basic equation:

V = I x R

Then rearrange it so resistance is the subject:

R = V / I

Replace V and I with the values shown above (remember that 10 mA is actually 0.010A).

R = 5 / 0.010

R = 500Ω