Ohm's Law

 

The relationship between Voltage, Current and Resistance in any DC electrical circuit was firstly discovered by the German physicist Georg Ohm. Ohm found that, at a constant temperature, the electrical current flowing through a fixed linear resistance is directly proportional to the voltage applied across it, and also inversely proportional to the resistance.

This relationship between the Voltage, Current and Resistance forms the bases of Ohms Law and is shown below.

 

 Ohm's Law Relationship

 

By knowing any two values of the Voltage, Current or Resistance quantities we can use Ohm's Law to find the third missing value. Ohm's Law is used extensively in electronics formulas and calculations so it is “very important to understand and accurately remember these formulas”.

 

To find the Voltage ( V )

[ V = I x R ]      V (volts) = I (amps) x R (Ω)

 

To find the Current ( I )

[ I = V ÷ R ]      I (amps) = V (volts) ÷ R (Ω)

 

To find the Resistance ( R )

[ R = V ÷ I ]      R (Ω) = V (volts) ÷ I (amps)

 

It is sometimes easier to remember Ohms law relationship by using pictures. Here the three quantities of V, I and R have been superimposed into a triangle (affectionately called the Ohm's Law Triangle) giving voltage at the top with current and resistance at the bottom. This arrangement represents the actual position of each quantity in the Ohms law formulas.

 

Ohms Law Triangle

 

Transposing the above Ohms Law equation gives us the following combinations of the same equation:

 

Then by using Ohms Law we can see that a voltage of 1V applied to a resistor of 1Ω will cause a current of 1A to flow and the greater the resistance, the less current will flow for any applied voltage. Any Electrical device or component that obeys “Ohms Law” that is, the current flowing through it is proportional to the voltage across it ( I α V ), such as resistors or cables, are said to be “Ohmic” in nature, and devices that do not, such as transistors or diodes, are said to be “Non-ohmic” devices.

 

Ohm's Law Example

For the circuit shown below find the Voltage (V), the Current (I), the Resistance (R)

 

Voltage   [ V = I x R ] = 2 x 12Ω = 24V

Current   [ I = V ÷ R ] = 24 ÷ 12Ω = 2A

Resistance   [ R = V ÷ I ] = 24 ÷ 2 = 12 Ω

Georg Simon Ohm, Alessandro Volta, André-Marie Ampère