Question about Ohm's Law ?

You use Ohm's Law to determine the value of the needed resistor. First you calculate the resistance you need by V = I*R (Ohm's Law), Then you calculate the wattage of this resistor by P = V*I = (I^2)(R) or P = (V^2)/R. Where V is the voltage dropped across the resistor R, I is the current through the resistor R and P is the power dissipated by the resistor R. Lets say the resistor needed is 210 ohms with a wattage of 40 milliwatts. Next you determine if it is a standard part. You can't buy a 210 ohm 5% tolerance 40 milliwatt resistor.Resistors are available with a 1/4, 1/2, 1 watt and higher power dissipation, always buy a resistor with a higher power rating than you need.You can buy a 200 ohm 5% tolerance, 1/4 watt resistor or a 220 ohm 5% 1/4 watt resistor. The tolerance means that the 200 ohm resistor can up to 5% greater in value or 5% less in value i.e. from 190 to 210 ohms. Likewise the 220 ohm resistor can be 11 ohms less or 11 ohms more than 220 ( 209 to 231 ohms). So now you have to decide if either of these will work. If not you can actually buy a 210 ohm 1% 1/4 watt resistor, these are harder to come by and more expensive. If the resistor is being used to limit current flow in a LED you should use the next higher standard value resistor. So in this case you use the 220 ohm 1/2 watt 5% resistor.