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The relationship between voltage, current and resistance is defined by Ohm's law, and power by Joule's law.
Ohm's Law:  
Joule's Law:  


These formulae can also be conveniently placed in a formula wheel. 
Given any 2 of the above values, it is possible to calculate the others:
Electrical Calculator Enter two values to find the other values. 
Series Resistor Calculator
Enter resistor values to calculate the total resistance. 
Parallel Resistor Calculator
Enter any two of the following values to calculate the third. 
[Updated: 27 Jan 2013] 11 
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