Ohm’s Law is the basis for all electronic theory. It is a simple equation.
V = I * R
This says that Voltage(V) is equal to Current(I) multiplied by Resistance(R). It is a simple ratio.
Electricity is basically the flow of electrons in a circuit. Voltage is the measurement of the potential for these electrons to flow between two different points. The higher the voltage the more vigorously the electrons will want to flow. Electrons will flow through a conductor from higher potential to lower potential. Voltage is the measurement of this potential difference between two points. The higher the difference the more electrons will flow.
Current is given in Amperes or Amps. It is a measurement of the flow rate of these electrons. The higher the current the more electrons are flowing past in a given amount of time.
Resistance is an impediment to the flow of electrons. The higher the resistance the more difficult it is to flow electrons through a circuit. All conductors have some level of resistance. We sometimes build devices that have a known resistance so that we can control and regulate the flow of current. These are called resistors, and you will see many of them in just about every circuit. We measure resistance with a unit that we call an Ohm we symbolize Ohms with an Ω.
Now let’s go back and look at Ohm’s Law. Basically it means that the voltage difference between any two points in an electrical circuit will be equal to the current flowing between those two points multiplied by the resistance.
It can also be given as…
I = V / R
This means that the current flowing between any two points in an electrical circuit is equal to the voltage difference between those two points divided by the resistance between those two points.
For example. If the battery in the above diagram is a 9V battery and the resistor is a 100 Ohm resistor then the current that will flow in the circuit is 0.090 Amps or 90 milliamps (mA).
If we wanted to determine how big a resistance we’d need to flow 20mA in the same circuit we’d take the voltage divided by the current that we want to get the resistance. 9/0.020 = 450 Ω.
That is really all there is to it. Obviously things are usually more complicated than a single battery and a single resistor but this is the basis for all electronic theory. We’ll discuss more complex examples in other tutorials. For now make sure that you really understand this concept.