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Saturday, March 13, 2010

What is the conventional current?

We have seen in a previous post that when you have a complete circuit an electric current will flow. We have also see that an electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal of the power supply.

Fig 1 below shows how the electron flows from the negative terminal to the positive terminal of the power supply.

Fig 1

However in a circuit the direction of the electric current is not defined as moving from the negative terminal to the positive terminal just as the flow of electrons. It is defined as moving from the positive terminal to the negative terminal of the power supply as shown in fig 2 below.

Fig 2

So why is it that despite knowing that the electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal as shown in fig 1we set the direction of the electric current as being from the positive terminal to the negative terminal as shown in fig 2.

The answer lies in convention. A convention is like an assumption. When electricity was first  discovered it was assumed that electricity is a flow of positive charges that the positive terminal to the negative terminal. Hence in all physics books at the time the electric current was taken as moving from the positive terminal to the negative terminal.

However it was discovered later on that in a circuit only electrons move. they move from the negative terminal to the positive terminal of the power supply. However the concept of conventional current was so ingrained that it could not be altered.

Hence as from now we will say that electric current is a flow of positive charges even though we know that it is in fact due to the flow of electrons.