In electrical science, Ohm's Law is so useful that it helps engineers to calculate either current, voltage, or resistance, if any two of these factors are given. Actually Ohm's Law is the relationship between current, coltage and resistance. Ohm's Law says the current is directly proportional to the voltage between two points on a circuit, and inversely proportional to the resistance between them. In formula, it is simply I = V/R, where I represents current, V represents voltage, R represents resistance. Thanks to our common sense of math, we can get V=  I*R and R= V/I from the original fomula I = V/R, and that is why we can find I, V, or R if any of these two are given / measured.        But be CAREFUL!!!!! This is only true when temperature is at a constant value!!!! Why? Because the resistivity of any material is affected by temperature. Remember electrical energy loss is in terms of heat? So when a circuit runs quite a while, the temperature of the circuit goes up and so the resistivity in the circuit also changes!!!!!!        Let's do a practice question. So under constant temperature, what is the voltage in a circuit that has a current of 2A and resistance of 300 Ω?        OK, sicne current and resistance are given, we can get V=  I*R from I = V/R, put the numbers in and get the following:      V=  I*R = 2A * 300 Ω = 600V        That is a very high voltage!!! Remember, human will probably die when contacting a circuit that has a voltage of 36V or above.