An electric current is a flow of charges. But there are two kinds of charges: positive and negative. It turns out that a 1 C s^-1 net flow of positively charged particles in one direction produces the exact same electromagnetic effect as a 1 C s^-1 net flow of negatively charged particles in the opposite direction. As fate would have it, scientists have chosen to define the direction of current to be the direction of flow of positive charges. This came to known as the conventional current.

In actual fact, in metals, the current is carried by electrons. So while we describe the electrical current to be from the positive terminal of the battery (through a resistor) to the negative, the electrons in the copper wire actually flow from the negative terminal of the battery (through the resistor) to the positive terminal.

Some students cannot understand why we don’t redefine conventional current to be the flow of negative charges instead, now that we know it is the electrons that carry the current in circuits. I can think of two reasons: (1) It is too troublesome. All the textbooks on electromagnetism have to be rewritten. And what about Fleming’s Left/Right Hand Rules, Right Hand Grip Rules etc? (2) It is not true that current is always carried by electrons anyway. For example, in an ionized gas (plasma), or in an electrolyte, the current is carried by both positive and/or negative ions. In a semiconductor (e.g. silicon, germanium), the current is carried by both valence holes (+ve) and/or conduction electrons (−ve).

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