Lesson 04 – Ohm’s Law

What is the relationship between current, voltage and resistance?

The current (I) through a conductor is directly proportional to the potential difference (V) across the conductor. Mathematically the relationship can be written as I = V/R where R is a constant defined as resistance and measured in Ohms.

The triangle on the left is an easy way to remember Ohm’s law. The following three formulas is represented in the triangle where V=Voltage measured in volt, I=Current measured in Ampere and R=Resistance measured in Ohms:
V = I x R 
I = V/R
R = V/I

So how is this useful? If we know two of the entities the third can be calculated with one of the three formulas above.

Let’s look at a practical example.
We want to run a LED off a 9 volt battery. We can’t just connect a LED to the battery because the LED can typically only handle 2 volt across it with about 20 mA of current running through it. We need to put a resistor in series (in line) with the LED to limit the current flowing through the LED.

How do we calculate the value of the resistor? We need the current flowing through it and we need the voltage over the resistor and then we can calculate the resistance. We have the required current of 20 mA for the LED and because the resistor is in series (in line) with the LED it also flows through the resistor.

Do we have the voltage over the resistor? We have the voltage of the battery and we know that the LED needs 2 volt over it to be switched on. The voltage over the resistor is just the battery voltage minus the LED voltage, Vr = Vb – Vl. Vr = 9-2 volt = 7 volt. 

We can now calculate the value of the resistor as R = V/I = 7/0.020 = 350 Ohm. If we use the 5% range of resistors (later more about that) there is no resistor with that value and we have to select the first available resister bigger than 350 Ohm in the range and that is 360 Ohm.

Ohm’s Law can be extended to electrical power and we then get the Power Triangle. The following three formulas is represented in the triangle where V=Voltage measured in volt, I=Current measured in Ampere and P=Power measured in Watts:
P = I x V
I = P/V
V = P/I

If we know the Voltage over a component and the current flowing through it we can calculate the dissipated power in the component. If we use the above example we can calculate the power rating of the required resistor which is also a very important parameter when you want to buy a resistor.