The amount of current flowing through a conductor is proportional to the voltage applied across the conductor. This is true of all conductors with the exception of super-conductors. Mathematically this is stated as:
V = k ⋅ I
- V is the voltage (in volts) drop across the conductor
- I is the current (in amps) flowing through the conductor
- k is the constant of proportionality
In this case, the constant of proportionality is replaced by the resistance (in ohms) of the conductor. The equation becomes:
V = I ⋅ R
Given two of the three values, it is possible to calculate the unknown value.
This equation can be rearranged to allow the calculation of the current:
I = V / R
or the resistance:
R = V / I
See Driving LEDs with Current Limiting Resistors for an example of using Ohm’s Law.