Ohm’s law is one of the most important theories and a corner stone principle of the electrical industry. We are going to have a look at what Ohm’s law is and how it applies to the electrical world.
Firstly, let’s look at the 3 key components that form an electrical circuit. They are Voltage, Current and Resistance.
We’ll be using an analogy of water flowing in a hosepipe to describe what each part is.
Voltage:
A good way to think of voltage is as pressure. The pressure in a hosepipe is what drives the water through the pipe. If you increase the pressure and you’ll increase the flow.
Just like the pressure drives water along the pipe the Voltage drives the current through a circuit.
The symbol for Voltage is V and it’s measured in Volts which has the unit symbol V.
Current:
To put it simply, it’s the ‘stuff’ that’s flowing. In the case of the hosepipe it would be the water ‘which is made up of billions of molecules’ that are driven around by the pressure.
In an electric circuit, the ‘stuff’ would be the current ‘which is made up of billions of electrons’ being driven around the circuit by the Voltage.
The symbol for current is I and it’s measured in Amps with the symbol A.
Resistance:
Partially blocking the end of the hosepipe with your thumb would reduce the amount of water flowing through the pipe. Your thumb is in effect ‘resisting’ the pressure and reducing the flow of water.
In an electrical circuit, a piece of equipment such as a lamp or heater would have the same effect of resisting the voltage and would reduce the amount of current flowing in a circuit.
The symbol for resistance is R and is measured in ohms, symbol Ω ‘omega’.
So, remember the Voltage is the force / pressure driving the Current around the circuit and Resistance is a force that restrict the flow of current.
Just as important as being familiar with the key components of an electrical circuit we need also to be familiar with all the units of measurement and the symbols used.
Ohm’s Law
During the 1820s Georg Ohm discovered that there is a proportional relationship between Current, Voltage and Resistance in an electrical circuit.
Which basically means that If you know the value of any two key components in an electrical circuit you can calculate the third.
Ohm’s Law Triangle
V for voltage at the top:
I for current at the bottom left
R for Resistance at the bottom right.
Like with all formula triangles if you know the value of any two components you can calculate the third using this Ohm’s Law Triangle.
So, to use the ohm’s law triangle…
Cover V to calculate voltage: V = I x R: To calculate current, cover I so it’s I = V ÷ R and to calculate resistance, cover R so it’s R = V ÷ I
To calculate Voltage using the Ohm’s Law triangle:
Step 1 – Refer to your triangle
Step 2 – Cover the value which we are trying to calculate ‘V’
Step 3 – This leaves us with I and R at the bottom.
Values at the bottom of the triangle are multiplied together to calculate the top value.
V = I x R : Voltage = Current x Resistance
To calculate Current using the Ohm’s Law triangle:
Step 1 – Refer to your triangle:
Step 2 – Cover the value which we are trying to calculate ‘I’:
Step 3 – This leaves us with V at the top and R at the bottom.
Values at the top of the triangle are divided by the remaining value at the bottom of the triangle
I = V over R ‘I = V ÷ R’. Current = Voltage ÷ Resistance
To calculate Resistance using the Ohm’s Law triangle:
Step 1 – Refer to your triangle:
Step 2 – Cover the value which we are trying to calculate ‘R’:
Step 3 – This leaves us with V at the top and I at the bottom.
Values at the top of the triangle are divided by the remaining value at the bottom of the triangle
R = V over I ‘R = V ÷ I’. Resistance = Voltage ÷ Current
Ohm’s Law Applications…
As we have seen there are three key components to an electrical circuit. Voltage, Current and Resistance and if we know the value of any two of these, we can calculate the third using Ohm’s Law.
Here we need to calculate the Voltage.
We know that the Current = 2A and the Resistance = 7Ω.
Using Ohm’s Law, we can see that V = I ‘current’ x R ‘resistance
So, the voltage is 2 x 7 = 14 V
Here we need to calculate the Current.
We know that the Voltage = 12V and the Resistance = 3Ω.
I = V ‘voltage’ ÷ R ‘resistance’
So, the current is 12 ÷ 3 = 4 A
Here we need to calculate the Resistance.
We know that the Voltage = 36V and the Current = 4A.
Using Ohm’s Law, we can see that R = V ‘voltage’ ÷ I ‘current’
So, the resistance is 36 ÷ 4 = 9 Ω
This is Part One of our Electrical Foundation Series
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