Resistance and Ohm’s Law Explained

How do resistors work?

The atoms in a material like copper wire are always vibrating around just a little bit, and this is because of the heat energy they have. When electrons try to move through the wire, sometimes they’ll bump into an atom that’s in the way, and effectively the flow of current gets resisted. As this happens, some of the kinetic or movement energy from the electrons gets converted into heat. This is the fundamental principle behind how electric heaters and incandescent light bulbs work. But it’s not just metals that have the property of resistance, resistance can exist simply from the fact that some materials just don’t have a suitable arrangement of atoms for electrons to flow through. And some materials just don’t have enough free electrons floating around for large amounts of current to flow. Keep in mind this is a huge simplification and this is not how actual atoms and electrons are going to look and behave at the subatomic level. Nearly everything on earth has some resistance to electrical current, and metals tend to have the least resistance. We measure the amount of resistance with a unit called ohms. The symbol is the greek letter omega. To give you a sense of scale, a resistance of under 1 ohm is considered to be a very low resistance. That’s something that you’d expect to see from a piece of wire that’s good at conducting electricity. 1 million ohms, or 1 megaohm, is generally considered to be a very high resistance.

Ohm’s law

In textbooks you usually see it written as V=I times R. Or voltage = current times resistance. If you use a little algebra you can rearrange the equation to calculate any of the variables as long as you know the other two. Although it’s important to understand that all these versions of the equation are exactly the same thing, our LED circuit is going to be using this version, so let’s focus on that. Let’s say we have a 10 volt power source, and we want to make sure that no more than 10mA flows from it. We can use ohm’s law to figure out what resistor will accomplish this. The answer is really simple, just take the voltage, divide it by the desired current, and we get the answer of 1000 ohms. So now we can either use the resistor color code, or a resistor calculator app to figure out what a 1000 ohm resistor looks like, and it turns out to be brown, black, red. The 4th color band all the way on the right refers to the tolerance of the resistor. A real world 1000 ohm resistor might actually have a resistance of 1020 ohms, or 998 ohms, and for most circuits you play with at home +/5% will be good enough. So let’s double check our math in real life. I’ve got my power supply set to 10 volts, it’s hooked up to a 1k resistor, and as you’d expect, 10mA is flowing from the power supply. It’s also important to know that ohm’s law is a linear relationship, meaning that for a fixed resistor value, if you double the voltage, you double the current. Here’s 20 volts going into the same 1000 ohm resistor, and as you’d expect, the current doubles to 20mA. I want you to understand that only pure simple resistors obey Ohm’s law. The relationship between voltage and current for most electronics is a lot more complicated than this. In a lot of cases things will work fine up until their recommended voltage level, and if you exceed that then things suddenly blow up. But for now, resistors are good enough to help us limit current in a simple LED circuit.