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## Class 12 Physics (India)

### Course: Class 12 Physics (India)>Unit 3

Lesson 2: Electric current and voltage

# Conventional current direction

By convention, we define positive direction of current to be in the direction a positive charge would move. Electrons (with their negative charge) move in the opposite direction of the positive current arrow. Created by Willy McAllister.

## Want to join the conversation?

• So the mountain analogy had electrons flowing from the negative terminal (the top of the mountain) to the positive terminal (the bottom). Circuit design was said to be all the stuff the electron bumps into on the way down. My question is are circuits designed with electron current in mind or conventional current? In my mind, I see a circuit designed with conventional current in mind working the opposite based on the electrons flowing in the opposite way designed by the circuit. Any clarification to why that is not so would be appreciated!
• There's an unfortunate dirty trick teachers play on beginning EE's. The idea of current is introduced by telling you about electrons and how they flow with cool mountain analogies. Then in the next breath I tell you the current arrow points the other way (by convention) (I ask you to flip the mountain onto its peak). I wish this didn't have to happen, but no teacher has figured out how to avoid doing this right at a fragile time in your learning. Conventional current is not a different kind of current, it's just a goofy way to point the current arrow. We point it into the electron current flow instead of the other way. It's the same as if I gave you a paper map and told you to hold it with South at the top. You can still read the map fine, and you can give anyone directions as long as you use cardinal directions (turn East and walk for an hour, then turn North). There's nothing "wrong" about holding your map this way.

Almost every circuit (all circuits in KA's EE subject area) are designed with conventional current in mind. It has zero impact on how the circuit works. The electrons just happen to flow in the opposite direction of the current arrows. As you get deeper into the material here this will become more and more apparent and hidden residual stress will melt away. I promise.

When you get to studying solid state electronics, (the how's and why's of what's going on deep inside a transistor) you will once again track the movement of electrons. You will have no trouble flipping between conventional vs. electron current because you will be so deep into EE it won't be a problem.

For now, my advice is to press ahead and start working with conventional current. After you do some activities with battery/resistor circuits and Ohm's Law it will become natural, and you will see how there's just one "kind" of current.
• Sal says (-) that the energetic electrons that are pumped out of the battery go back into the positive current, how then does a battery run out of energy?
• Hello Michael,,

True, the electrons go in a circle. But there is work being done to make them move. For everything that moves a small amount of the battery's energy is given away.

These videos may help:

Regards,

APD
• Okay, I've never fully understood the difference between current and voltage. From his explanation, current is like counting how many electrons pass through at once. Is voltage how hard they're being pushed/pulled through? Wouldn't that just proportionally increase the amount of current anyway?
Also, if the direction of current doesn't matter, does it matter where on a circuit you put specific parts, like resistors and whatnot?
• You got almost everything right. Current is charges flowing. To measure current and give it a number, you stand at a point and count how many charges go by in a second. If 1 coulomb of charge goes by, that's defined to be 1 ampere. Voltage is how hard the charges are being pushed/pulled. You can think of it as electrical pressure, or as the slope of a mountainside that charge is rolling down. Increasing voltage proportionally increases current. That is the idea captured in Ohm's Law.

The direction of current does matter. The idea of 'conventional current' has kind of a quirky definition, it's the direction positive charge would move. It happens that electrons move in the opposite direction of the conventional current arrow. That doesn't mean we don't care which way current flows, it just means our definition of positive current is a little unexpected. It's something all new EE's take a little time to get used to.
• Why is Conventional Current represented by an "i"?
• The symbol for current is "I" or "i". It comes from the French word for intensity, as in "intensité du courant". This is the term André-Marie Ampère used to describe current.

Another reason is that "C" is already taken by Coulomb (the unit of charge).
• What is the difference between a positive and a negative charge? I mean, why can't we reassign electron lack as negative charge and excess electrons as a positive charge?
• The naming of the charges as + and - is an arbitrary human choice. There is no sense that an electron is aware of its name, and knows nothing of "negative-ness".

There are two kinds of charge, and on (and only one) rule: "Like charges repel, unlike charges attract."

A long time ago when scientists were struggling to figure out electricity they came up with words to describe what they thought was going on. They could have come up with the system you describe, but they didn't. We still use those words today.

It is a coincidence that we use arithmetic signs (+ and -) as the names for charge. In some ways that is unfortunate, because it gives the impression charges are doing arithmetic or being "positive" or "negative". We could have gotten along just fine with other names. In fact, the original names for charge were "vitreous" and "resinous" (glass-like and resin-like). Whatever their names they follow the one rule.
• Charge is moving slowly, so why does a bulb glows so fast
I mean what does it have to do with the electric field ?
• The motion of electrons in a metal wire (the "drift current") is remarkably slow, something like a quarter millimeter per second. The reason this enough to make a bulb glow is there are SO MANY electrons moving.

Imagine the widest river you have ever seen, the Mississippi, or the Thames, or the Amazon. They are moving pretty slowly at their widest part. But if you draw a line straight across the river and count how many water molecules pass through the line in a second, you get an enormous number. Put that many electrons through a light bulb and you get a bright light.
• In the functioning of the battery , chemical reactions release electrons which want to move to the cathode but the electrolyte prevents them from doing so. Hence, it move along the circuit .
So is it safe to assume that all electrons moving in the circuit are from the battery.
If yes , then why do conductors need to have free electrons?
• Good question. It made me think. A wire (or any material) starts out neutral (same number of electrons and protons). And even when there is a current flowing, the wire has to stay pretty much neutral. If you imagine a current where you are putting a whole bunch of extra electrons into a wire you have a situation where there is a huge collection of negative charge in one location, which creates a gigantic repulsion between individual charges. That repulsion is enough to stop any battery from forcing more current into the wire.

It is actually pretty fascinating that an electric current flows in a conductor while the conductor remains essentially neutral the whole time. One electron comes in at one end, many other electrons in the wire "move over" one position (like musical chairs), and finally one electron pops out the other end. You get movement of charge (current) while the number of electrons stays the same everywhere.
• Why do most circuits have a component called Ground (GND) if the battery and copper wire exchange electrons perfectly fine?
• GND is not an actual component (even though the ground symbol looks like it might be one). GND is a designation we make to indicate which node in a circuit we choose to be the node with v = 0.

In electrical devices that have a power cord, the ground node is also connected via the power cord to eventually the actual ground outside. This is a safety feature.

In electrical devices that don't have a power cord (like a mobile phone or a flashlight), we still assign one node to be the reference node, and call it ground. If you ever use a circuit simulator program to model a circuit, it will require you to assign a ground node.