The Current I = V x R

If we keep the same resistance in a circuit but vary the voltage, the current will vary. The circuit in Fig. 2-1 demonstrates this idea. The applied voltage V can be varied from 0 -12 V, as an example. The bulb has a 12-V filament, which requires this much voltage for its normal current to light with normal intensity. The meter I indicates the amount of current in the circuit for the bulb.

With 12 V applied, the bulb lights, indicating normal current. When V is reduced to 10 V, there is less light because of less I. As V decreases, the bulb becomes dimmer. For zero volts applied there is no current and the bulb cannot light. In summary, the changing brilliance of the bulb shows the current is varying with the changes in applied voltage.

For the general case of any V and R, Ohm's Law is






where I is the amount of current through the resistance R connected across the source of potential difference V. With volts as the practical unit for V and ohms for R, the amount of current I is in amperes. Therefore,






This formula says to simply divide the voltage across R by the ohms of resistance between the two points of potential difference to calculate the amperes of current through R.