Laws of Electricity

Ohms Law is the principle all electronics is built on. Ohms law states that Voltage is equal to the value of Resistance times the Amperage of a circuit. Ohms law labels Voltage as “E”, and Resistance as “I”. The formula for ohms law is:

            E = I * R

You will need to learn and understand this formula as a part of your electrical studies and never forget it. In this formula you must have Amperage and a Resistance to find the voltage. This initial formula ONLY works for series circuits.

For example, a circuit has a resistance of 10 Ohms, and Amperage of 10 Amps. So:

        E = I * R

       E = 10 * 10

       E = 100 Volts

Now, what happens if you have a circuit that you know the Amperage and the Voltage, because you measured these, and you want to know the Resistance of the circuit? You can change the formula around so that:

       R = E / I

Now, if your circuit is getting 120 Volts, and the Amperage is 2 amps your resistance would be calculated as:

       R = E / I

R = 120 / 2

       R = 60 ohms

The third scenario would be that you have measured the resistance and you know the voltage but you need to find the amperage that the circuit will draw. You once again change Ohms law around to get:

       I = E / R

An example of this is you are installing an air conditioning unit, you know that it takes 240 Volts to run this unit. You have measured the resistance of the unit and find that it is 4 Ohms. You need to find out how big the circuit breaker and the wire must be. So,

       I = E / R

       I = 240 / 4

       I = 60 Amps

You find out that the breaker must be able to handle 60 Amps.

Now, you will have a chance to practice these basic calculations using the worksheets at the end of this book for section 3.1.

Here are some basic rules that apply to all circuits:

Series Circuits (One path for electricity to flow):

Amperage remains constant all the way through the circuit.

Voltage is split between all the loads in the circuit, but adds up to source.

Resistance adds to the total resistance.

Parallel Circuits (Multiple paths for electricity to flow):

Amperage adds to total amperage.

Resistance is less than the lowest resistance in the circuit.

Voltage is equal to source for each branch.