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Alternating current, direct current & what is frequency?

Current that keeps changing is direction is called alternating current, (AC) whereas the one that never changes its direction is called direct current (DC). Power can be transmitted with low heating losses in the form of AC (not DC). The frequency of the AC in India is 50 Hz. This means the current changes it's direction 100 times per second. Created by Mahesh Shenoy.

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Video transcript

when I took a slow-motion video inside my house I noticed this tube light flickering is this a camera problem no because if I take the same slow-motion video outside in sunlight I don't see any flickering so clearly the tube light must be flickering for some reason but why to answer this question in this video we will explore the two different kinds of electric current and so current comes in two flavors one is called the DC which stands for direct current direct current and then the other one is called AC which stands for alternating current alternating current so what is the difference between them direct current is a one directional current meaning you can imagine the current just flows in one direction if you could visualize the electrons inside this wire and you can kind of visualize them as you know when the current starts moving the electrons start moving they're moving in the same direction with the same speed one particular direction like this okay and since the current value is the same all the time we will see that the bulb keeps glowing with the same amount of brightness constantly all the time one direction of current DC your batteries usually provide this kind of current okay what about alternating current well this is where the current keeps changing its direction continuously once the current would flow this way then the current would change and flow the opposite direction then again it'll go back and forth back and forth again if you could visualize the electrons over here now you would see the current is going the electrons are going back and forth back and forth like this alright but why does that make our bulb flicker why does that happen well let's look at these electrons a little bit carefully and let's do it in slow motion now as the electrons are going forward notice when it's about time to change the direction they first come to a stop and then change the direction they go back and again go to a stop and then change the direction go forward stop backward stop forward stop and so on every time the current stops electrons stops the current goes to zero and then the current goes to zero the filament over here cools down a little bit and when the filament cools down a little bit the bulk dims a little bit why doesn't the bug completely switch off when the current goes to zero well that's because as you can see the current comes back up very quickly so it stops only for a fraction of a second and so during that fraction the coup the well cools down just a little bit and then again heats up cools down and heats up cools down and heats up and that's why we see the bulb flickering like this and why do you think it flickers a hundred times a second well that's because it turns out in India the current that we get from our main supply no changes is Direction 100 times a second so let me just write that down that seems important so the current changes direction we just write current changes Direction changes direction about a hundred times per second all right and that means the current goes to zero a hundred times in one second and therefore when the current goes to zero that means our filament will cool down about 100 times a second and as a razor will see the bulb dimming and then coming back up about a hundred times a second and of course something very similar is happening with the tube light it's flickering because of the alternating current all right this might bring now more questions to your mind like why do we choose 100 times a second or why do we even use alternating current in the first place all right we'll address these questions towards the end of the video but let me quickly first talk about the standard way of representing how quickly the current changes its direction the standard way of doing that is by coming up with a number called frequency frequency what is the frequency represent it's representing the same thing a little different way instead of talking about the number of times the current changes direction it talks about the number of times our number of cycles cycles the current goes through per second so number of cycles per second it turns out that there is an advantage of doing it this way because when we you know look into the mathematics of this it turns out this is a very nice number to use in our formulae so what is the what is the meaning of the word cycle over here so whenever things are going back and forth their motion is repeating itself that that particular motion which starts repeating is what we call a cycle let me be more specific so let's say let's bring back our electron we know the electrons is going back and forth so it goes forward then change the direction goes back changes direction again goes forward and so on and so forth right this is what the electron is doing now notice once it has gone forward and then comes back and once it changes direction then the motion repeats itself is the same as before again goes back and changes direction motion repeats itself therefore since the motion repeats after after over here after it has to change its direction we will say this is one cycle that make sense this is one cycle so cycle is the portion which starts repeating itself and after this again the next cycle begins and then once it the electron comes back and changes the direction that cycle ends in the next cycle begin so this is cycle number two and then this is cycle number three and the frequency basically represents how many cycles how many cycles is this electron going through every second now here's a question given that we know electron changes Direction a hundred times in one second can we figure out how many cycles it is going through in one second these numbers are related to each other can you pause the video and think about this all right hopefully you've tried since I know the total number of times the electron changes direction in one second what we can do to figure out the frequency is let's see how many times the electron changes the direction in one cycle okay so let's look at the first cycle electron is going forward and then Bob changes its direction over here so the electron changes direction over here once and then it comes back comes back comes back finishes no it finishes its cycle when it changes its direction again all right why because what is next cycle cannot begin unless you change your direction so the electron finishes its cycle the first cycle after changing his direction again so it changes direction once it change directions second time so in one cycle the electron is changing its direction two times that's important same case with the second cycle as well goes forward forward forward butch in easy Direction goes back back back back back change direction and gets ready for the next cycle and it goes forward change direction goes back change direction so in one cycle the electron is in each direction two times so as write that one cycle the electron changes direction changes direction two times so if the electron changes direction 100 times how many cycles is it well that is the cycle is half of the number of times it changes direction right so if it is if this is 100 if the electron is changing is Direction 100 times and that means how many cycle must should be it has to be 100 by 2 5 cycles sorry 50 cycles 50 cycles and so in this case we will say the frequency is 50 cycles per second let's write that over here 50 cycles per second and cycles per second is also often called Hertz named after the scientist Heinrich Hertz and so nowadays if the current changes direction or if the current goes to zero a hundred times a second that means the frequency is half of that 50 Hertz and since this is the standard chosen in India this is the Indian Standard that means everyone everywhere in India we get about 50 Hertz frequency of alternating current this is an Indian Standard in United States this is this number is about 120 times per second which means the frequency would be half of that 60 Hertz in the United States all right that's pretty much it now let's come back to the question of the curious questions that you might have before we end the video one question you might be wondering is why hundred times a second why not any other number well if the number is too low let's say it was like two or three times per second then the flickering will be very obvious to us right and in that case it will be very inconvenient so it can't be too low and all that if you make it very high then there are some other problems that start coming and that's why it needs to be high enough but not too high and that's why it's about hundred in India or up 120 in in the United States another question like in the last question we will tackle is you may be wondering why even go for alternating current in the first place why can't we just go with direct current what's the problem with that that's an actually very interesting question because it turns out it's got something to do with efficiency of power transfer so let's look at in a little bit detail so here is let's say your power station through which the electricity is coming from from the electric poles to your houses now since the power station is very very far away that means the electricity has to travel a long long distance and since we we have lots of houses lots and lots of houses the electric current required to be very high so that means these these are transmission cables would have to carry a lot of current sorry they have to carry a lot of current and that was a problem with carrying a lot of current in the transmission wire is that it heats up the wire and it causes a lot of heating heating is loss of energy and that electrical energy is getting converted to heat that means although most of the you know the electricity is being wasted as heat who's gonna pay for that you me or the electric company it's gonna be bad it's gonna be you know very very inefficient thing we can't do business over there that's why we go for alternating current not direct current now you might say how does alternating current solve this problem well when you're using alternating current what we do is we send very low amount of current through these wires very low amount of current okay and therefore the heating losses will be very low and that solves our problem well of course now you might say okay if you send low current then how will we light up our houses all that we're something called a transformer comes in you may have heard of this will not look in the details but the what a transformer does is it takes in that low amount of current and then it increases and gives you a high current that's the speciality of a transformer it's not magic of course it's it's science we need to understand how the transformers work and everything which we will do in enough videos of higher grades but as of now just remember just think of it this way the transformer can take in very low current and convert it into a very high current this way we can save a lot of power loss now the main thing over here is that transformers only work on alternating current so you can only use this trick with alternating current and not the direct current and that's the main reason why we use alternating we supply alternating current because it gives weekends do it at a very low in you can do it a little very low current and that gives us very low power loss alright that's pretty much it so let's see if we can go ahead and summarize and recall what we learned first thing can you recall what AC and DC stand for and what exactly are they what's the difference between them secondly what is it what is the meaning of the word frequency what does it mean to say frequency is 50 Hertz can you more importantly explain why if frequency is 50 Hertz that means that the current is changing its direction hundred times per second try to explain this say to your friend or to someone in your family it's a super important concept over here finally can you now explain why we go for AC when it comes to power transmission to all our houses and not DC if you have difficulties answering these questions nobody's feel free to go back and re-watch that part of the video