It’s the Law … Ohm’s Law

A lot of people don’t start with electronics because they think that it involves a lot of mathematics and they are correct. However, you don´t need a higher degree in maths, you are already halfway if you can find the on-and-off-switch of your pocket calculator.

If you still have doubts then read on and you will see how easy it is.


The most important law in electronics is Ohm’s law, named after the German physicist Georg Ohm. You probably heard of it in physics class and forgot about it immediately. It states:

The current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them.

That is quite a mouth full, isn’t it? What does it actually say in plain english? It says:

When you increase the voltage in a circuit, the current will increase proportionately.
When you decrease the voltage in a circuit, the current will decrease proportionately.
When you increase the resistance in a circuit, the current will decrease proportionately.
When you decrease the resistance in a circuit, the current will increase proportionately.

In mathematics, we can write those 4 lines in a simple equation: I = V/R. Where I is Current, V is Voltage and R is Resistance. If we tweak this equation a bit then we get it as the better known:

V = IR

Already it makes more sense but let’s take a closer look to make it completely clear:

We can write Ohm’s law as the mathematical equation V = IR or Voltage = Current x Resistance. That means that we can calculate the voltage of a circuit if we know the current and the resistance.

Lets try:

In this circuit the resistance is 150Ω and the current is 10mA or 0.01A. Knowing this we should be able to calculate the voltage of the battery.

V =IR → V = 0.01 x 150 → V = 1.5

So our battery has a voltage of 1,5Volts.

To calculate the current, the equation is I=V/R or Current = Voltage ÷ Resistance.

We know that the Voltage is 3V and the Resistance is still 150Ω so lets calculate the Current.

I = V/R → I = 3 ÷ 150 → I = 0.02A or 20mA

So our Current is 20mA.

If we want to calculate the  Resistance, the equation is R = V/I or Resistance = Voltage ÷ Current.

R = V/I → R = 3 ÷ 0.03 → R = 100

So our resistance is 100Ω.

As you can see, calculating the one of the 3 variables is easy as long as you know the value of the two others and if you can remember the right equation. There is a trick to remember the equations:

This is called Ohm’s Triangle and it works pretty easy. Just put your finger on the variable you want to know and the triangle will show you the right equation to calculate it.

And that’s all you need to know to get started!

Well……….. Errrrrrr………..

There is a small problem with ohms law. You can only directly apply it to Resistive circuits. A resistive circuit is a circuit containing only resistors, ideal current sources, and ideal voltage sources. So no capacitors, inductors,…

But don’t worry about that, it will become clear in future tutorials. For now the all important thing is V = IR. It’s the law!


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