Ohm's Law For Direct-Current Circuits

We'll start the discussion of circuit mathematics and the use of Ohm's law for DC circuits since it is more intuitive and widely understood. The relationships between the four circuit parameters: Voltage (E), Current (I), Resistance (R), and Power (W) are summarized in a convenient form in the Ohm's Law Wheel shown here..

This wheel summarizes the formulas used to calculate any one of these parameters if two of the others are known. Each segment of the circuit is assumed to have only a voltage source, a single load (resistor), and two wires connecting them. The identifying symbols and an arrow to show the direction of the current flow complete the picture. Power flows continually from the voltage source to the load resistor in the closed circuit.

This relationship is defined by Ohm's Law:

E = IR

E = Voltage

I  = Current (amps)

R = Resistance (ohms)

The triangle illustration shown above shows you how to use any two of these values to determine the other in a given circuit. If you need to find current, cover the "I" with your finger and you'll have E/R (volts divided by ohms.) If you want to determine resistance, cover the "R" and you'll have E/I (volts divided by amps).