The electric current is the ordered flow of charge through a conductor, and it is one of the fundamental quantities in all of electricity.
If in the previous chapter we said that Voltage is the cause (the pushing force), then Electric Current is the effect (what moves).
Having voltage without current is like having a full water tank with the tap closed: there is potential energy, but no flow. The moment we connect a conductor between two points with different potential, free electrons begin to move in an orderly fashion.
We call this orderly flow Electric Current.
Current Intensity (I)
To quantify this flow, we use the quantity Intensity.
The definition is simple: Intensity is the amount of charge passing through the cross-section of a conductor per unit time.
Or, in its differential form (more rigorous):
The Ampere (A)
The unit of measurement in the International System is the ampere. If we remember that charge is measured in coulombs (C) and time in seconds (s):
Remember what we saw before: the magnitude of one coulomb is equivalent to the charge of about
That is, in a metallic conductor with direct current, measuring one ampere means that 6.24 trillion electrons pass through a cross-section per second.
Direction of Electrons and Conventional Direction
Here we encounter one of the most annoying “historical legacies” in physics.
- Real Direction: We know that electrons have a negative charge. Therefore, physically, they travel from the Negative Pole (where there is an excess) to the Positive Pole (where there is a deficit).
- Conventional Direction: However, when Benjamin Franklin and the pioneers were studying this, they did not know about the electron. They assumed that charge was a “positive fluid” flowing from Positive to Negative.
What do we do today? In engineering and circuit analysis, we still use the Conventional Direction. We assume that current flows out of the positive terminal and into the negative terminal.
Don’t worry. The math works out exactly the same. Simply think that, instead of negative charges moving to the left, positive “holes” are moving to the right. The energetic effect is the same.
The Speed of Current
Exam question: When you flip the light switch, the bulb turns on instantly. Does that mean electrons travel at the speed of light?
The short answer is no.
- The Electrical Signal (the electric field) does travel at nearly the speed of light (
). It is the order to “Move!”. - The Electrons (the matter) are extremely slow. Their drift velocity is only a few millimeters per second.
The tube of balls analogy: Imagine a tube filled with ping-pong balls from end to end. If you push a ball in from one side (the switch), an ball comes out the other side (the bulb) instantly. The motion is transmitted instantly, but the specific ball you pushed will take a long time to reach the end.
Current Density (J)
Knowing how many Amperes are flowing is important, but to design a cable, it is not enough.
Suppose 100 people pass through a hallway.
- If the hallway is 10 meters wide, they go comfortably.
- If the hallway is 50 centimeters wide, there will be pushing, friction, and heat.
The same happens in electricity. It matters not only the Intensity flowing, but also through how much surface area we are making it flow. This is the Current Density.
It is measured in Amperes per square meter (
Why is J important?
Current density, together with the material’s resistivity, determines Joule heating.
- If we try to pass a large current through a very thin wire, the density J skyrockets.
- The power dissipated per unit volume increases.
- The wire heats up, the insulation melts, and we can cause a fire.
Therefore, when sizing a cable, it is not enough to look at the current and cross-section. The insulation, installation method, ambient temperature, grouping, and voltage drop also matter.
- For 10 Amperes in a reasonable cable, the density can be acceptable.
- For 10 Amperes in a very thin wire, the density skyrockets, and the conductor will heat up enormously.
General rule (very simplified): In domestic copper installations, we usually operate in moderate current density ranges to avoid overheating.
Conclusion
We have now defined the two main quantities:
- Voltage (V): The energy per unit charge (the cause).
- Intensity (I): The amount of charge per second (the effect).
Now we need the third leg of the stool. What prevents the current from being infinite when we apply a voltage? Why do some materials allow more current to pass than others under the same voltage?
