Ohm’s Law

George Simon Ohm and Ohm’s Law The relationship between current, voltage and resistance was studied by the 19th century German mathematician, George Simon Ohm. Ohm formulated a law which states that current varies directly with voltage and inversely with resistance. From this law the following formula is derived:


I = E / R OR CURRENT = VOLTAGE / RESISTANCE


Ohm’s Law is the basic formula used in all electrical circuits. Electrical designers must decide how much voltage is needed for a given load, such as computers, clocks, lamps and motors. Decisions must be made concerning the relationship of current, voltage and resistance. All electrical design and analysis begins with Ohm’s Law. There are three mathematical ways to express Ohm’s Law. Which of the formulas is used depends on what facts are known before starting and what facts need to be known.


I = E / R E = I * R R = E / I


Ohm’s Law Triangle There is an easy way to remember which formula to use. By arranging current, voltage and resistance in a triangle, one can quickly determine the correct formula.






Using the Triangle To use the triangle, cover the value you want to calculate. The remaining letters make up the formula.





Ohm’s Law can only give the correct answer when the correct values are used. Remember the following three rules:

• Current is always expressed in amperes or amps
• Voltage is always expressed in volts
• Resistance is always expressed in ohms