Thin and Thick Wires

In this wiki page, you will learn about thin and thick wires and how they physically operate.

Wiki created by Ryan Keefe (rkeefe3)

The Big Picture

Let's start out by saying this: no matter how much the wire's thickness changes, if a wire has a current running through it, it will always have the same number of electrons passing through it every second. This value cannot change within the wire just because of thickness. What can change however is the magnitude of the electric field caused by the wire. Because the electric field due to a wire can be formulated by this:

[math]\displaystyle{ E(R)=\frac{\lambda}{2 \pi \epsilon_0 R} }[/math]

Since electric field is inversely related to the radius of the wire, the thicker the wire, the less the magnitude of the wire. Funny enough though, if you take a wire (we will call this wire A) with a current and replace a section of the wire with a thinner wire (we will call this wire B) and measure the magnitude of the electric field at thick sites of the wires, we will find that wire A has a larger magnitude than wire B. Why is this? Well, analyzing wire B, one can find a large gradient of surface charge across the section of thin wire replacing the thick wire, meaning this area is acting like a funnel for electrons. This funnel means that the overall speed of the electrons going through the wire. This is true for the whole wire. As stated above, every wire has a constant number of electrons passing through it every second, also known as electron current. Because of this slow down due the thin wire, the value of [math]\displaystyle{ \lambda }[/math], which is charge per unit length, goes down making the electric field magnitude go down as well.

Example

In the circuit below, all the wires are made of the same material, but a section of the wire is thin.