DC Circuits, Part 1: Establishing Conventions

20
Mar
2015
Posted by: AK Tutoring  /   Category: Physics   /   No Comments

An electric current is made of moving charged objects. You’ve probably already seen a very simple diagram of a voltage- (or EMF-) driven current flowing through metallic conducting wire:Slide1

In such cases, electrons within the metal’s atoms are free to move and flow from the negative terminal to the positive terminal:Slide2

Despite the actual flow of electrons through the wire from the negative terminal to the positive terminal, there’s a catch you’ll have to remember. In most classes and applications, we use the conventional current – the flow of protons – rather than the actual electron flow:Slide3

Using the conventional current might seem strange, especially when we’re most often considering metallic wire in which electrons would flow, but there are several reasons for the switch. First, current isn’t always made of negative charges. While moving electrons form the current within metal wire, this isn’t true for all other materials. One might ask, in that case, why not just define current how it would actually be – positive charges, negative charges, whatever! This leads us to the second, and most compelling, reason for using the conventional current: Our rules thus far, especially the all-important right hand rule, work for conventional current, but would need to be reversed for electron flow. Imagine the confusion switching things around for different materials!

Think back to our previous discussion on electric potential energy, electric potential, and voltage: We’ve been saying that electric fields are directed from higher electric potential to lower electric potential for positive charges.  This would mean that we are talking about positive charges making up the current! Let’s stick with conventional current, and once you get things down, feel free to switch things around and see if they still make sense to you!

Now that we’ve established our convention, we can move forward onto much more complicated (and realistic!) circuits! Get comfortable with the conventional current, review your electric potential energy, and we’ll move onward next time!

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