The most fundamental formula in electronics is Ohm’s Law. Ohm’s law explains the relationship between voltage, current and resistance. Ohm’s law states that Voltage (E) equals current (I) multiplied by resistance (R). In modern times, voltage is often represented as a V, but on the test Voltage is represented with an E.
Using Ohm’s law, we can calculate either voltage, current, or resistance, as long as we have two of the three values.
Here is a triangle representation that is useful to help calculate Ohm’s law. Voltage is on top, and current and resistance are on the bottom of the triangle.