What is electrical resistance, Ohm’s law and resistor?
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| What is Ohm’s law and resistor? |
So you can see how it would be useful if there was something that could resist the flow of electrical current. Something that the flow in a controlled way. That device is called a resistor, and here are some examples of what resistors can look like.
We have got a very basic resistor over here, which is the kind of resistor that most hobbyists would use at home when constructing circuits.
This is something you and expect to see in a small device like your phone. And this big resistor is the type of thing you do use a large power supply.
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 has not just metals that have the property of resistance. It 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.
What is Ohm Symbol?
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. Sorry, I had to put it in the video somewhere. We measure the amount of resistance with a unit called ohms. The symbol is the Greek letter Ω (omega).To give you a sense of scale, a resistance of under 1 Ω (ohm) is considered to be a very low resistance. That is something that you do expect to see from a piece of wire that is good at conducting electricity. 1 million ohms, or 1 megaohm, is generally considered to be a very high resistance. That is something that you might expect to see from a bad conductor of electricity like this dried out a piece of carrot.
This thing that I am using to measure resistance is called a multimeter, and it can measure the resistance of almost anything. I will explain a separate article on multimeters. And I recommend you watch it as soon as possible to learn more about this important tool in electrical and electronics.
How to find resistor value with Ohm's law its color,
Now if you are playing with electronics at home, you will be using resistors that look like these. They have colored bands on them, and there is a special code that lets you translate the colors into a resistance value. For example, these red, violet, brown and gold bands mean this is a 270 Ω (Ohm) resistor. It can also find with Ohm's law.Now you can memorize the color code, but it’s a lot easier to just use one of the many resistor calculators out there. Just search for resistor color calculator on Google or on your phone is an app store. By having resistors with specific resistance values we can carefully control the amount of current that flows in a circuit.
Let’s start out with everyone is first simple resistor circuit, using a resistor to limit the current going through an LED. Make sure you have already watched my LED tutorial and have bought some LEDs and resistors, which I am going to link again in the video description section. In order to do the math for this circuit, you need to know about the mathematical relationship between voltage, current, and resistance. Here is an old comic that I have always liked that illustrates the relationship on an intuitive level.
Ohm’s Law equation,
More formally, we use this equation. 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 12 volt power source, and we want to make sure that no more than 10mA flows from it. That you 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 1200 ohms.
So now we can either use the resistor color code, or a resistor calculator app to figure out what a 1200 ohm resistor looks like, and it turns out to be brown, red, red. The 4th color band all the way on the right refers to the tolerance of the resistor. The real world 1200 ohm resistor might actually have a resistance of 1260 ohms or 1140 ohms. For the most circuits, you play with at home +/-5% will be good enough.
Ohm’s law and current
So let’s double check our math in real life. I’ve got my power supply set to 12 volts, it is hooked up to a 1.2k resistor, and as you can 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 is 24 volts going into the same 1200 ohm resistor, and as you can 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.
Let’s start out with a 9 volt battery, a resistor, and an LED connected with the correct polarity. And notice that it doesn’t matter which way we connect the resistor - unlike the LED, polarity doesn’t matter for resistors. We want to find out what resistor will let us safely use 9 volts with this LED. In my previous video about LEDs we talked about forwarding voltages, and for this particular white LED the forward voltage is 3 volts. That means that when the LED is on, there is going to be a 3 volt drop across it.
What is the voltage across the resistor?
Remember that voltage is all about differences in electrical potential between two points. Our power source is a 9 volt battery. So we have got 9 volts between here and here, and we have got 3 volts across the LED. This must mean that we have got 6 volts across this resistor because of 9 - 3 is 6.So we have got our voltage. Now the current in this circuit is going to be whatever we want to it to be. But the recommended maximum current for this LED is 20mA, so we are going to use that. And notice that I am using conventional current here which moves from positive to negative. That’s what you are going to see in every single electrical engineering situation. The theoretical physics classes might use negative to positive electron flow.
Calculate with Ohm’s law,
So let’s calculate with Ohm’s law now. 6 volts divided by 20mA gives us a proper resistance value of 300 Ohms. Now you don’t have a 300 ohm resistor in your parts collection. But a 330 ohm resistor will be good enough. If you are messing around with LEDs at home it doesn’t matter if you get the current wrong by 10%.So here I have my 9 volt battery and a 9 volt battery clip. The red positive wire is going to one side of my 330 ohm resistor, and that’s going to the LEDs anode. Then I am just connecting the negative wire from my battery to the LEDs cathode. 9 volts, roughly 20mA, and no exploding LEDs. If we increase the resistance to, let’s say, 18 kΩ (kilo-ohms). We will get less current, and as you expect, the LED is dimmer.
In general, this is the equation (ohm’s law) you can use to calculate the resistor for a simple LED circuit. But there is a limitation.
