Ohm’s Law

What is Ohm’s Law?

    The relationship between current, voltage and resistance was and Ohm’s Law  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:

    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 acts are known before starting and what facts need to be known.

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.

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

Examples of Solving Ohm’s Law 

Using the simple circuit below, assume that the voltage Ohm’s Law  supplied by the battery is 10 volts, and the resistance is 5 Ω.

To find how much current is flowing through the circuit, cover the “I” in the triangle and use the resulting equation.

Using the same circuit, assume the ammeter reads 200 mA  and the resistance is known to be 10 Ω. To solve for voltage, cover the “E” in the triangle and use the resulting equation.

Remember to use the correct decimal equivalent when dealing with numbers that are preceded with milli (m), micro (µ) or kilo (k).  In this example had 200 been used instead of converting the value to 0.2, the wrong answer of 2000 volts would have been calculated.