I usually don’t use a lot of math when doing electronics, but Ohms law is extremely useful!
The law was found by Georg Ohm and is based on how voltage, current and resistance are related:
Look at the drawing above and see if it makes sense to you that:
- If you increase the voltage in a circuit while the resistance is the same, you get more current.
- If you increase the resistance in a circuit while the voltage stays the same, you get less current.
Ohm’s law is a way of describing the relationship between the voltage, resistance and current using math:
V = RI
- V is the symbol for voltage.
- I is the symbol for current.
- R is the symbol for resistance.
I use it VERY often. It is THE formula in electronics.
You can switch it around and get R = V/I or I = V/R. As long as you have two of the variables, you can calculate the last.
Ohm’s law triangle
You can use this triangle to remember Ohm’s law:
How to use it:
Use your hand to cover the letter you want to find. If the remaining letters are over each other, it means divide the top one with the bottom one. If they are next to each other, it means multiply one with the other.
Let’s find the formula for voltage:
Place your hand over the V in the triangle, then look at the R and the I. I and R are next to each other, so you need to multiply. That means you get:
V = I * R
Let’s find the formula for resistance:
Place your hand over the R. Then you’ll see that the V is over the I. That means you have to divide V by I:
R = V / I
Let’s find the formula for current:
Place your hand over the I. Then you’ll see the V over the R, which means divide V by R:
I = V / R
How to remember Ohms law
A simple way of remembering things is to make a stupid association with it so that you remember it because it’s so stupid.
So to help you remember Ohm’s law let me introduce the VRIIIIIIII! rule.
Pretend that your driving a your car really fast, then suddenly you hit the brakes really hard. What sound do you hear?
And this way you can remember V=RI ;)
A practical example
The best way to teach how to use it is by example.
Below is a very simple circuit with a battery and a resistor. The battery is a 12 volt battery, and the resistance of the resistor is 600 Ohm. How much current flows through the circuit?
To find the amount of current, you can use the triangle above to the formula for current: I = V/R. Now you can calculate the current by using the voltage and the resistance:
I = 12 V/600 Ohm
I = 0.02 A = 20 mA (milli Ampere)
So the current in the circuit is 20 mA.
If you don’t like calculating things yourself, check out this calculator for Ohm’s law.
Let us try another example.
Below we have a circuit with a resistor and a battery again. But this time we don’t know the voltage of the battery. Instead we imagine that we have measured the current in the circuit and found it to be 3 mA (milli Ampere).
The resistance of the resistor is 600 Ohm. What is the voltage of the battery?
By remembering the “VRIIII!” rule, you get:
V = RI
V = 600 Ohm * 3 mA
V = 1.8 V
So the voltage of the battery must be 1.8 V.