Ohm's Law

By: Jonathan Z. Kremer

Ohm's Law, named after Georg Ohm, a German mathematician and physicist B. 1789 D. 1854 - Bavaria, defines the fundamental relationship between power, voltage, current and resistance. These are the very basic electrical units we work with, and are the very same principles that apply to direct current, alternating current, and radio frequency alike.

Ohm's law states that the direct current flowing in an electrical circuit is directly proportional to the voltage applied to the circuit. In other words, in a DC circuit, 1 volt across a 1 Ohm resistor will cause 1 amp of current to pass through the resistor, at 1 Watt of power. Based on this law, any given voltage, resistance, current, or wattage can be found by knowing any 2 of the 4 factors. The basic formula used is: E = RI. Numerous deviations from this simple, linear relationship have been discovered, which can be seen from the wheel below.

Ohm's Law Wheel

E = Voltage (sometimes "U" or "V" are used instead of "E", and measured in "volts")
I  = Current (measured in "amperes")
R = Resistance (measured in "ohms")
P = Power (measured in "watts")

To make your calculations even easier, use the handy and useful "Ohm's Law Calculator".

Alternating current (AC) varies cyclically in both magnitude and direction, as opposed to direct current, whose direction remains constant. The formulas above, if used for AC current, must include an efficiency value, an impedance value, and power factor, in order to be accurate. For small home appliances, or where only a "ballpark" figure is needed, the standard formulas above can be used as is, because there would not be a great difference.

Example of Use

Let's say...you have a small room heater that blows out the circuit breaker whenever it's on for more than an hour. Why? Well...you know that the voltage coming to your home is 220V and the heater has a sticker on it, rating the power at 2500 watts. After checking, the outlet you are plugging the heater into is controlled by a 10A breaker. So now what? Pick out the appropriate formula, from the "Ohm's Law Wheel" above to find the current (or even easier, stick everything into the calculator). The formula we need is:    I = P/E   so we have    I = 2500 /220.    I = 11.36A.    Now you see that your heater is pulling over 11A. on a 10A breaker, causing the breaker to jump after a while. To figure out the resistance of the heating element, simply pick out one of the fitting formulas from the wheel above to find R.     R = E/I R = 220/11.36     R= 19.36 ohms. Due to the fact that we are using AC current, all the above results are not exact.