Basic Electricity – Resistance and Ohm’s law

Basic Electricity – Resistance and Ohm’s law


In this video I’m going to talk about electrical
resistance, ohm’s law, and how to pick a resistor to limit
current in an LED circuit. In previous videos I talked about how voltage
can behave like a pushing force, pushing electric current
around a circuit. But in one example I connected an LED straight
to 7.5V, way too much current flowed, and the LED blew up. So you can see how it would be useful if there
was something that could resist the flow of electrical current. Something that could tame the flow in a controlled
way. That device is called a resistor, and here
are some examples of what resistors can look like. We’ve got a
very basic resistor over here, which is the kind of resistor that most hobbyists would
use at home when constructing circuits. And over here we have a tiny surface mount
resistor. This is something you’d expect
to see in a small device like your phone. And this big resistor is the type of thing
you’d use large power supply. So how do these resistors work? Remember how in my video about current, I
talked about electrons jumping from atom to atom, all at the
same time, like a conga line? Well in reality this process is not 100% efficient. 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. 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 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. That’s something that you might
expect to see from a bad conductor of electricity like this dried out 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 have a separate tutorial on multimeters,
and I recommend you watch it as soon as possible to learn more about this important
tool. Now if you’re playing with electronics at
home, you’ll be using resistors that look like these. They have
colored bands on them, and there’s 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 ohm resistor. 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 in your phone’s app store. By having resistors with specific resistance
values we can carefully control the amount of current that
flows in a circuit. Today, let’s start out with everyone’s first
simple resistor circuit, using a resistor to limit
the current going through an LED. Make sure you’ve 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’s an old comic that I’ve always liked
that illustrates the relationship on an intuitive level. 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 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. 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 forward 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. So… 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’ve
got 9 volts between here and here, and we’ve got 3 volts across the LED. So this must mean that we’ve got 6 volts across
this resistor, because 9 – 3 is 6. Ok so we’ve 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’re 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, theoretical physics classes might
use negative to positive electron flow. So let’s apply Ohm’s law now. 6 volts divided by 20mA gives us a resistance
value of 300 ohms. Now I don’t have a 300 ohm resistor in my
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%. Ok, 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
LED’s anode. Then I’m just connecting the negative wire
from my battery to the LED’s cathode. 9 volts, roughly 20mA, and no exploding LEDs! Finally! If we increase the resistance to, let’s say,
18 kiloohms, we’ll get less current, and as you’d expect, the LED
is dimmer. In general, this is the equation you can use
to calculate the resistor for a simple LED circuit. But… there is a limitation! I’ve got another power supply here set to
give me 140 volts, and that’s enough to mess you up so don’t do
this at home. Let’s put 140 volts into this equation, we’ve
got 3 volts for our white LED, and we want to
stick to the 20mA current limit. So we get a resistance value of 6,850 ohms. I’ve got a 6.8k resistor in my
parts collection, which is very close to our theoretical value, so let’s see what happens. Huh. Now instead of the LED getting toasty, the
resistor gets too hot. So what’s going on here? To answer
that you need to learn about electrical power, which will be the subject of my next video. Finally, I can’t make a video about ohms without
mentioning Ohmnilabs! It’s a company run by a few
friends of mine that make some pretty cool robots. Check them out at ohmnilabs.com. Thank you for watching, subscribe and check
out the video description section to learn more about
electronics!

100 thoughts to “Basic Electricity – Resistance and Ohm’s law”

  1. How were you able to get the recommended max current? Is it included in every device or there's a need to calculate it?

    Also, what happens if for example, the current is 40mA rather than 20mA?

  2. what are some things around the house that have 75 ohm resistors? too lazy to order… or too poor lol. I wanna RGB mod some TV and I need them…

  3. I'm working on a project which requires twenty-nine 3mm LED's (20 milliampere), I've currently got two 9 volt batteries which I'm hoping will do the job, and I'm planning to use a 470 ohm resistor within the circuit… any guide as to how this can be achieve successfully will be much appreciated as I'm completely new to this.

  4. Hi I'm going to use a green led in my car and obviously i need to drop the voltage from 12v to 3v , by my rough calculations i need a 450ohm resistor, am i right?

  5. CAN SOMEONE PLZ HELP:

    idk understand how can an LED light have 3V? like what does that mean? but I understand how the battery has 9V of potential electrical energy. and I don't understand what he means by there being 6V across the resistor? what does "Across the resistor" mean?

  6. Oh, Hi @GreatScott!, you really got some likes… not as good as the dried carrot but better than the watt joke. Nice work.

  7. If the resistor is on the positive side, shouldn't it burn the led anyways? as it gets the full current and voltage from battery?

  8. Do electrons actually move at approximately the speed of light? Not quite. It is an electric field that "moves". In a one meter wire, with one amp current, electron particles take 24 hours to move the length of the wire.

  9. So let’s say I have a 8ohm amp that is 10 watts per channel, can I run it to a 4ohm speaker that is 30 watts rms. Without breaking the amp or blowing the speaker

  10. I humbly disagree, it is easier to memorize the colour bands. I did it as a schoolboy in the eighties and it is still with me, forty years later.

  11. So you say keeping the current off by 10% is not a problem? im asking because i have 2x AA batterie in series with 1.6V and my LED is recommended for 3V and 20mA. so i don't need to get a resistor at all? Im just double checking. 3.2V-3V = 0.2V –> 0.2V/0.02A = 10 Ohm. with 10% Current diviation i would be at 3.2V-3.3V and therefore 0V/0.02A=0 πŸ™‚

  12. I don't understand, I generated 10V using self made battery using coper and zync and use water as solution, I attached the 3V led and it barely glows, the 10V drops down to 2.4V. how do I get more Amps or what do I need to do to keep the Voltage to 10 or at least not to drop so low?

  13. why don't you have a video on how to find the resistance value of a resistor with their bands

  14. so with atoms moving due to heat and power drops due to heat transfer. is that why they super cool things now i.e super conduction

  15. Great video, all are great in fact. I'm just starting out with electronics and your videos explain things by well. Thanks. Subscribed.

  16. Your videos are Awesome! It took me 2 years to fully understand all of these things myself, if I would have found this channel beforehand it would have taken 2 days. I hope your videos get to as many enthusiasts as possible!
    Thank You!

  17. Sir if we want to connect 3LEDs in
    Series
    With 9V battery which value resister is recommended.
    Also if we connect them in parallel then which Resistor is recommended.
    Plz reply or make a detailed video in this scenario
    Thanks

  18. Hi, Thanks for the video, very informative. Do you know if there is a LED that works with 7uA at 1.1V? Thanks,

  19. As per electron flow we need to connect resistor from Negative side of battery and LED, but we are connecting on positive side.

    So this electron theory is right or wrong? as per concept due to electron flow we got electricity. What is correct? Or any other charge is there.

  20. OK I've looked at the comments and didn't find what I was looking for . So I guess I'm the one who will say …….

    " Resistance is futile " . After reading the comments I wouldn't worry about being assimilated .

    It would cause a Downgrade in the Collective .

  21. voltage drop minimize current manage current = carbon resistor 2 wire wend resistor .3 un flamable resistor v variable resistor resistor not short they break . checking resistor color and code need any one in computer hardware

  22. Someone in here asked a good question: Does it matter where in this circuit one places the resistor? (1) Why or why not? (2) Where should the resistor go? (3) In DC, where do the electrons really come from–positive side or negative side?

  23. An easy way to remmeber the equation is a triangle E at the top I bottom right R bottom left. Then you can work out which values in what place. I have discalculear ( like dslexia only with numbers) so anything to make maths easiear. You also forgot the 4th band on the resistor. Gold +/- 5% Silver +/- 10% no band +/- 20%

  24. Why do you and everyone else say that resistors resist current (amps) when they also affect voltage?
    This is why it is so confusing.

  25. 7:23
    i don't get this.
    if all the resistance in this case is is taking the voltage and dividing by the current, you could still get exactly the same resistance with vastly different values for voltage and current. 3v and 10mA would also give 300ohms. how does the resistor know exactly the right balance of volts and amps you want it to pass?

  26. Great job sir! Awesome work.. I think you should do all the videos on Youtube period. Well done! I was just fooling around and wasting time watching videos and I was very happy. Do more vid's..

  27. How many amps can you put thru a resistor? Lets say I have a 1,000 watt resistor. can I put 1,000 amps at 1 volt? Is there something that determines the amperage a resistor can take other then power?
    I just bought a 2,000 watt 100 ohm resistor. I'm using it at work to generate 3 amps on a 300 volt power supply. I was just curious if there was an amperage limit because the terminals are not beef nor is the size of the wire that is wound.

  28. What a nice explanation too good, You have a great knowledge in yours field , please keep it up πŸ‘πŸ‘πŸ‘πŸ‘πŸ‘πŸ‘πŸ’πŸ‘πŸ’πŸ’

  29. Good video for the basics of electricity. I'm a HVAC Contractor and a lot of technicians don't understand electricity like they should in the electrical and HVAC industry. Hell I have asked electricians what actually make breakers trip and all they can say is heat or over current. Both are right, but they don't understand the concept and internal components.

  30. Ok, guess i know now why i cant get four white leds to light up using two AA batteries.
    Came to learn about current, ended learning about voltage drop.

  31. this vedio cleared all my concepts feeling that i have acquired extreme knowledge
    i m very thankful to you πŸ‘πŸ‘πŸ‘πŸ‘ŒπŸ‘ŒπŸ‘Œ

  32. hi, first of all thank you for your video, it's very helpful, as many other here I'm just a beginner so that might be a stupid question, but what if I want to light up let's say 6 led of different colours, what would be the ideal resistance, current and power supply?

Leave a Reply

Your email address will not be published. Required fields are marked *