It is a common assumption that electrical energy travels very fast. It is an equally common assumption that the speed, or velocity, of electricity is a single measurement. But actually, electrical current travels with relative slowness. In this article, learn how science defines and calculates the speed of an electric current.
Understanding how electricity flows
To start with, let’s take a look at what happens when you plug in an average, ordinary floor light and it starts to glow. What makes that happen?
Interestingly, an electric current isn’t a single entity. It is always in motion and is made up of billions of moving parts including atoms and free electrons. The atoms are large and stay in a fixed position. The free electrons are small and much quicker, relatively speaking. As the free electrons zig and zag around, bouncing off one atom and then another, slowly but steadily making their way down the chain of atoms.
Each free electron is bouncing on its own and also taking help from other bouncing free electrons that bump them along in a forward direction. (Here, it may help to visualize teeny tiny bumper cars all bumping each other forward in the same direction.) Eventually all those free electrons generate enough built-up momentum that they all arrive at their destination, and – PRESTO. The light comes on!
3 Velocities make up the flow of an electric current
Unless you are a research scientist or an electric engineer, you may not know that there are three different velocities that are factored into any calculation of how fast electricity itself flows. These three different velocities are:
1. Electron velocity: the speed of each electron.
2. Drift velocity: the speed of the free electrons moving forward together.
3. Signal velocity: the speed of the electric current itself.
The actual formula used today to calculate the signal velocity is as follows: I = n*A*v*Q
You may also see the formula written as follows: v = I/(n*A*Q)
Here is the key that explains what each element stands for in this formula:
– I = the electrical current
– n = electrons/m3 (electrons per cubic meter)
– A = a cross-section of the wire
– Q = each free electron’s charge
– v = is the electrons’s collective drift velocity
Let’s say we are working to calculate the flow of electric current through a copper wire, which is the standard type of wire used for electrical wiring today. To calculate the number of free electrons present in that copper wire, you use the formula: n = 8.5 * 1028 per m3.
To calculate the charge of each free electron inside that wire, you use the formula: Q = 1.6 * 10-19C.
Putting it all together: how fast does electricity flow?
So now let’s put it all together using a common example. In this example, we will assume the following:
– The current is 14 ams
– The wire is copper
– The cross-section of the wire is 3 * 10-6 m2.
This tells us that the electron speed is 3.4 * 10-4 m/s. This works out to approximately one-third mm/second. But what is the speed in plain language? It works out to be about 4.1 feet (1.2 meters) per hour. Yes, you read that right – per hour. With a speed this slow you wouldn’t think you’d ever get your light bulb to light up! So what gives?
It is true each electron is drifting rather than speeding towards its destination. But together, the create a chain reaction whereby a continual flow of current is generated.