Resistance

Author: Joe H., Inflow Engineer

Now that we’ve covered how we generate usable electricity, we need to go over what happens when trillions of electrons are pushed through a wire in a fraction of a second. To do so, we have to go back to our old friend, the atom. Atoms have charged particles called protons in their centers, which are attracted to electrons. When electrons move into a wire, they are captured by the protons in the wire’s atoms. As more and more electrons are pushed into one end of the wire, the wire runs out of protons to capture electrons. If we keep trying to push electrons into the wire, one of two things is going to happen…

The first option is the atoms hold on to the electrons they already have. When this happens, the electrons being pushed into the wire have no place to go and stop moving down the wire. In this case, no current flows. Materials that do not let electrons flow are called insulators. While this might sound boring, it’s actually a very important phenomenon. For example, the plastic coating on your computer’s power cable keeps you from electrocuting yourself when you plug it in and it ensures that your computer gets the correct amount of power to operate. This principle was beautifully illustrated recently at CERN, when a small mammal chewed through a power cable and disabled the whole machine. [1]

The second option is the atoms in the wire will give up the electrons they’ve already captured and accept the new electrons. These released electrons will then travel further down the wire, propagating the current through the wire. Obviously, if we want to do anything with electrical currents, this is the type of behavior we want. Materials that allow electrons to flow freely like this are called conductors. However, in most materials, the atoms don’t just release their electrons without a fight. Much like trying to pull two magnets apart requires a bit of work, a certain amount of force is needed to push the electrons away from their atoms. This tendency of electrons to stick to atoms is called resistance. We can measure this tendency and use the unit Ohms (Ω) to describe how much resistance a material has. [2]

All normal conductors have some small level of resistance; the rare materials that have no electrical resistance at all are called super conductors (we’ll talk about these perfect conductors in a later post). Interestingly, there’s no such thing as a perfect insulator; with enough effort you can push electrons through a normally non-conductive material. However, if we know how much force (which we call voltage) is being applied to the electrons we’re using, we can pick materials that are very conductive and surround them with materials that do not conduct at the voltage we’re using. There’s one more piece to the puzzle though: materials that have more resistance than just a normal wire, but not enough resistance to completely prevent the flow of electrons at a given voltage. We call components that exhibit this “in-between” level of resistance resistors, and they are incredibly useful items.

Resistors let us manipulate the amount of current flowing through a section of wire. At a given voltage, the more resistance we have, the fewer electrons will flow through. Remember in the last post, where I gave the example of a lightbulb burning out if too many electrons flow through it at once? With a resistor, we can reduce the amount of current the lightbulb gets. When we’re using batteries, which can only provide so many electrons before going dead, limiting the current to the bare minimum needed makes our batteries last longer. There are even some materials which change their resistance under certain conditions. A light dependent resistor, for example, decreases its resistance when exposed to light. Want a coffee maker that turns on when the sun rises? Use a light dependent resistor and you can make one. As we move forward, we’re going to see many more uses for resistors.

You may have noticed that I did not give an explanation of exactly what an ohm is, unlike the last post, where we talked about amps. That’s because we define that value of 1 ohm in relation to both current and voltage, so we need to talk about voltage in a bit more detail first. We’ll do that in the next post and then look at how current, voltage, and resistance are all related. This will lead us to Ohm’s law, one of the fundamental tools of electrical engineering. We’re just starting to get to the interesting part, so be sure to check back for the next post.

[1] http://phys.org/news/2016-04-weasel-large-hadron-collider.html

[2] Named after Georg Ohm, who first published research on the topic. https://en.wikipedia.org/wiki/Georg_Oh

 

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