Ohms Law and Resistors
Written by Bryce Ringwood   

This program provides a means of determining the Voltage, Current or Resistance in a series circuit. It also calculates the power dissipated in the resistor.


Used to calculate dropper resistor values and just about everything in an electrical circuit. Use to calculate power in a load.

Simple Circuit Resistors


Data Entry Section

Enter the unknown quantity as zero. Press "Calculate" Button to get the third quantity. In this program, SI suffixes can be used on input, and are used on output. For example, you can enter 1k for 1000 Ohms - and so on.

Enter /Calculate Voltage V Volts                  

Enter / Calculate Current I in Amps              

Enter/Calculate Resistance in Ohms               

Additional Results

Power dissipated in Watts     

Conductivity in Siemens        



Example - Mains Dropper

A "Pye Piper"  radio contains 5 Valves UY41, UL41, UBC41, UF42 and UCH42. The sum of the heater voltages is 121 Volts, and a mains dropper resistor is needed to drop the supply from 230 Volts. The filament current is 0.1 Amp.  The voltage drop is clearly 230 - 121 volts = 109 Volts. From Ohms law, we calculate the resistance (unknown) = 1090 Ohms. The power dissipated = 10.9 Watts. Practically, you may have to settle for a 1000 Ohm resistor rated at 20 Watts.


Ohm's law is a basic property of materials.

The law was named after the German physicist Georg Ohm who, in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire.


\displaystyle{ V = IR}
\displaystyle{ I = {V\over R}}
\displaystyle{ R  = {V\over I}}
\displaystyle{ P = IV}

$V$ = Applied Voltage
$I$ = Current
$P$ = Power 
$R$ = Resistance

By the way, you will see the terms "amperage" and "wattage" (and hear them too). The correct expressions are "current" and "power".

For a program in 'C' for Ohm's law, see experimenters corner.


Horowitz P, Hill W "The Art of Electronics"  - Cambridge University Press, 1988 pp 33-34


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