Electrical Electric Current | Current Direction | Introduction to Electrical...

Electric Current | Current Direction | Introduction to Electrical Engineering

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The idea of electric current derived from or stars with the idea of electric charges. Therefore, before talking about what is current, we need to understand the concept of electric charges, and where it comes from.

electric current

The charges are the infinitesimal particles in an atom. There are two types of electric charges, positive (+), and negative (-). Generally positive (+) charges are heavyweight, therefore they don’t move, and stick together in a place called nucleus in an atom. (But moving positive (+) charges sometimes refers to the positive $ion^{+}$ in a solution, illustrated below. For example, in NaCl solution, $Na^{+}$ ion contains positive (+) charge, and $Cl^{-}$ ion contains negative charge.) On the other hand, negative (-) charges are comparatively very lightweight, and therefore move around the nucleus in a circular path name orbital. Positive charges in an atom are protons, and negative charges are electrons.

There is also a particle in atom name nutron, which we really don’t need to care about in electrical engineering, because it has no charge. So these are the basic idea of charges, and where its come from. Now let’s talk about Electric Current (I).

Current | Definition of Electric Current

The formal definition of electric current is the rate of flow of electric charges pass a point. Symbol I. The unit of measurement is amperes (A),or amp.

As figure shows that the valence electrons on the Copper metal start flowing for an external force of Batteries. The external force is called voltage, which is applied across two terminal of the copper wire. The reason of this force, the electrons break out the metallic bond of copper metal, and start flowing to the positive side of the battery. The negative side of the battery repulses electrons of the metal, and the positive side attracts the electrons of the metal. This flow remain continued until the electrons totally fills the positive holes of the battery. 

current direction

Now to determine current I, suppose, we put an imaginary rectangular shape on the copper wire as shown in the figure above. And we put an eye on the point that how many electrons (negative charge $q^{-}$) pass the point per unit time. If we find $q^{-}$ number of charges pass the point per second, then we measure the current: $$I=\frac{q^{-}}{sec}\text{ amp}.$$

1 amp of current (I) equals a flow of 1 coulomb of charge per second. I must be the negative here. Because we calculate the number of negative charges pass the point per second. Which direction is from negative side of the battery to the positive side.

Direction of Current (I)

By convention, I in a circuit is considered to flow from a more positive point to mare negative point, where electron flow is in opposite direction. …

In 1947 Ben Franklin discovered that there are two types of charges,positive, and negative. But on that time no one knows about electron. Electron was discovered by a famous physicist J. J. Thomson in 1897, approximately after 150 years of discovering charges. But many before exploring electron, direction of I refers to the direction of positive I, means direction of flow of positive charges (such $Na^{+}$).

current
Figure: Electrolytes NaCl solution

Direction of positive charges (such as $Na^{+}$) in a close loop is from more positive side to more negative side. This direction for electric current is conventional since 1947. So, direction of positive I is still considered as standard for the direction of electric current and hasn’t been changed.

But it should be remembered that the flow of current is always caused by the flow of negative charges or electrons, because positive charge in an atom can’t flow.

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