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Current (Chapter 2: Theory)

In this article, we will cover two key points regarding current:

- What is current?

- What is conventional current and why is it in the opposite direction to electron flow?


What is Current?

The authors define electric current as follows: "Electric current is the total charge which passes through some cross-sectional area per unit time".

Therefore, the average current would equal:

change in charge/change in time.

Thus, if the current changes, the instantaneous current can be found by taking the change in time to approach zero. So, I = dQ/dt.

Charge is measured in Coulombs and current is measured in Amperes (Coulombs per unit time).

It is also important to note that the charge-carriers can be other particles beside electrons. In a metallic conductor, electrons are the charge-carriers, whereas in other circumstances ions or holes can carry charge.


Conventional Current

Arguably the "Father of Electronics", Benjamin Franklin, during his pioneering work on electronics, had a convention of assigning positive charges to the mysterious things that were moving and doing work in electric circuits. Later, a physicist known as J.J Thomson discovered that these mysterious charges were actually negative.


Above is British physicist J.J Thomson. Above him is American polymath Benjamin Franklin.


If we wish to find the number of electrons flowing in a cross-section of a copper wire in 1 second, when 1 A of current flows, we would have to do the following:

I = Q/t

1 A = (1C/1S). An electron has a charge of -1.602x10^-19. So if we wish to to find the number of electrons in 1C of charge we would do 1/-1.602x10^-19 to get -6.24x10^18 electrons.


This number is negative and there can only be two possibilities why this is the case:

1) The charges flowing are positive.

2) The electrons flow in the opposite direction to the defined current.




Since we know that the flowing charges are negative, 2) must be the case. Since, positive charges flowing in one direction is the same as negative charges flowing in the other direction, the earlier equations (created by Franklin and others) still hold true.


Sticking Point: When using equations like I = Q/t, Q is given as a positive number, so I is the magnitude of the conventional current, not the electron current. If you take Q as negative, I will be negative which does not make sense. In other words, we adopt conventional current as we deal with the old formulas.

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