Lenz’s Law | Magnetic forces, magnetic fields, and Faraday’s law | Physics | Khan Academy

Lenz’s Law | Magnetic forces, magnetic fields, and Faraday’s law | Physics | Khan Academy

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– So, right over here depicted a square loop of a conductor. Let’s say it’s a wire and it’s stationary and it’s sitting in a magnetic field. And I’ve drawn a few
vectors that represent the magnetic field and
you can see at least on the surface that is defined
or that is contoured by the wire that the magnetic
field looks constant. So, if we just had this scenario nothing too special going on but it becomes interesting
if I were to actually change the flux going through the surface. So, both of these pictures right over here they actually show the same scenario where we have increased the flux. We have increased the flux at these points on the surface defined by
the wire at every point the magnetic field has
now gotten stronger. So, we have increased the flux. So, let me write that, the flux. We use the Greek letter
phi used to denote flux. The flux of the magnetic field. The flux of the magnetic
field has gone up. And we know from Faraday’s law that when you have a change in your flux that that’s going to induce
a current in the loop. And so an interesting question is what direction is that current
actually going to go in? We have two options. The current could go. The current… Let me find a
nice color for the current. The current could go in
the clockwise direction or it could go in the
counter clockwise direction. So, which of these two do you think the current will actually go with? Well, let’s think a little bit about it. We know that a current flowing through a wire actually on it’s own
will induce a magnetic field above and beyond a magnet
field that’s already there. So, let’s think about the
type of magnetic field that this orange current
would actually induce. So, if it’s going in the
counter clockwise direction remember we use the right hand rule. I can take my right hand point my thumb in the
direction of the current. So, let me see if I can do that. So, my thumb in the
direction of the current and then my hands are going to loop or I should say my
fingers are going to loop in the direction of the magnetic field. So, when I do that with my right hand, so my right hand looks
something like that thumb in the direction of the
current right over here we see that it shows the
magnetic field being induced that would wrap around like that. Or if we were to show
it, if we were to sample it points right on the surface the magnetic field that would be induced would look something like this and I’m just doing it
at some sample points. So, notice it would be additive to the existing magnetic field. In fact, it would increase
the flux even more. In fact, at these points these vectors would increase even more. These vectors, let me
see if I can draw that. These vectors would increase even more. Well, what would that do? If the flux increases even more then the current is going to increase. The current is going to increase even more which is going to make the
flux increase even more and which makes the
current increase even more and you would have this never ending cycle where the current keeps increasing, the flux keeps increasing
and you would have this energy that’s
appearing out of nowhere which would violate the law
of conservation of energy. And so that’s a pretty good argument for why you would not… That’s a pretty good
argument for why we would not expect the counter clockwise scenario. We would not want the
current that’s been induced to induce a magnetic
field that goes in the same direction as our increase in flux. So, just by deductive reasoning we know that this is going to be the scenario and let’s see if we think
about what happens here. We can do the right hand rule again. We take our right hand point our thumb in the
direction of the current point our thumb in the
direction of the current and we see if we do that
with our right hand. Well, now this would
induce a magnetic field that would decrease the flux. So, it would produce a magnetic field. The current right over here would produce a magnetic field that’s going downward. The current in that direction would also when you take your right hand and you were to put it along here. Once again if you’re going
in the counter clockwise, or sorry if you’re going
in the clockwise direction over here that too when you do your right hand rule right over here your fingers would coil around that way. And so once again when
you look at the surface it would produce, it would
induce a magnetic field that is going in that direction. And so it would have the
induced magnetic field from the induced current will go against the change in flux. And this makes sense because
we won’t go into this never ending positive feedback loop where the current keeps getting
strong and stronger and the flux keeps increasing and increasing and increasing. And this idea that the orientation of the current that is induced will produce a magnetic field that counter acts the change in flux. This is Lenz’s law. Lenz’s Law. And once again if that wasn’t true then you would have a violation of the conservation of energy. So, in general if someone says okay well, this is the change in flux is happening in a certain direction. Well, to think about which way the current would flow
you just have to say well what type of a magnetic field would each direction
of the current produce? And that magnetic field
should go in a direction that goes against your change in flux. So, if the flux is increasing the magnetic field that’s induced by the induced current should
make the flux decrease. If the flux was decreasing then the induced magnetic field by the induced current should make the flux decrease less or should be additive to the flux.

100 thoughts on “Lenz’s Law | Magnetic forces, magnetic fields, and Faraday’s law | Physics | Khan Academy

  • Renรฉ Alanรญs Orta Post author

    As I understand, if the loop of wire moves in the same direction as the vectors from the sationary magnetic field, it means the wire never cuts any magnetic line, thus no current should be generated. Could you pleas explain what am I missunderstanding? Shouldn't the magnetic lines be horizontal rather than vertical?

  • Atul Mahindra Post author

    Why we have increased the flux ?

  • Mohammad Masroor Post author

    pretty good explanation

  • Justin Y. Post author

    i dont get it

  • singer dream Post author

    thank you VERY much. khanacademy is always so helpful after everyone fails

  • Sarah Low Post author

    If that pink arrow slows down the turquoise arrow…. Then wouldnt the loop start to "slow down" kinda thing?

  • Prince Alif Post author

    This was an absolute waste of 5:45 min, yes I cant explain it any better but only for now. As I get the get the theory I think I can explain a lot better than this. The conclusion this video disappointed me.

  • Ruposree Chakraborty Post author

    its an interesting explanation

  • Aditya Barman Post author

    On point! Got it finally ๐Ÿ˜€

  • Smit Patel Post author

    y u so awesome

  • Shubham Saraf Post author

    superb explanations. If you could simplify the LENZ'S LAW (the statement) and provide that to us, would be helpful.

  • Jack Daniel Post author

    Why is he using the technical current direction for a physics video? For the physical current direction the LEFT hand need to be used!

  • Amanda Magana Post author

    thank you!!!
    -UCLA student with physics final in two days and still doesn't understand lenz's law

  • Williams Alain Post author

    pays $$$ for college. comes to youtube to learn for free.

  • Agh Post author

    Omg omg omg you'r the best !!!!! i finely got it !!!!

  • David SL Post author

    Why does the created current only exist for a short period of time then? Shouldn't the current be continuous to create a constant force that is equal to the flux?

  • Paul Carkhuff Post author

    When looking at this more carefully I realized 2 things. First, the right hand rule will only work if one is NOT standing in the center of the hoop, and if one does NOT reach across the hoop to grab the other side of the hoop. secondly, and more interestingly, the magnetic flux lines travel in one direction, where as the electric flux lines travel in 2 directions, like a spiral. So the electrically charged particles travel around the wire, but also down the wire. Whereas the magnetic flux lines only travel around in a circle. Is this correct? Does this have to do with the fact that magnetic flux lines are always bipolar, whereas electric charges, like electrons, are always mono polar? Are electric flux lines, and magnetic flux lines in fact 2 different phenomena? We are usually taught that these are 1 phenomena that we call electromagnetic, but I'm seeing 2 very odd phenomena, especially regarding electric charges traveling in a spiral down, and around a conductive wire. This in turn makes me wonder how a flux line leaving the north pole of a magnet can circle around and enter the south pole and then exit the north pole again. With the reverse happening at the south pole. What is going on here? Are the magnetic flux lines leaving a magnet traveling in a spiral through space as they exit the bipolar magnet, like the electric flux lines traveling down a wire via electrons? This is getting into quantum physics because we would want to know what exactly are flux lines. I've heard that they are in fact photons, and that flux lines don't really exist. A Khan video, or 2 explaining this would be very helpful. Perhaps there are and I just haven't found them out yet. Thank you:)

  • Mom's Spaghetti Post author

    Nice drawing

  • Arianator GUY Post author

    Thank you sir.

  • Emily Chow Post author

    this makes so much sense thank you T.T saved my final exam mark

  • jessicamoley123 Post author

    Thank you so much for this, it makes so much more sense now.

  • darkflamemaster Post author

    The hand looks real artistic ^^

  • Ali Aljibari Post author

    I finally understand. Thank you.

  • Anton Igartua Post author

    What software do you use to make this drawings?

  • แฟ ฬ”ฯ€ฮฟฮบฯแฟฯ„ฮฎฯ‚ Post author

    Conventional or electron flow current ? Assuming electron flow otherwise it's contradictory to my view on things

  • WISSAM SALEEM HISHMAT Post author

    Hello sir
    I have a question please, which is :
    Let us have a circle to put it on a horizontal paper,
    if there is an induced current passes counterclockwise through the circle, while the magnetic field is going upward (out of the )paper). So, what would be the reason :(the increasing of the magnetic field passed) , or ( decreasing of the magnetic field

    Please, can you explain that on you tube ?
    I appreciate .

  • Arka jyoti Karmakar Post author

    Thanks

  • ghalib asad Post author

    i confused about the negative sign in the formula that u not describe

  • Tyler Lewandowski Post author

    thank you soo much

  • Christian Gutierrez Post author

    when you come to Khan academy wanting to learn physics and get an art lesson in the process

  • Ali Kareem Post author

    impressive i got it

  • Esha Mukhopadhyay Post author

    How are u so damn good at every single thing u teach?????????????? bio phy maths or mayb even music… its just awsome

  • Gurusha Juneja Post author

    Infinity loop of increasing energy ๐Ÿ˜‚๐Ÿ˜‚dude nice explanation . Thanks a lot sal . It does a lot for students like us

  • shuram xargy Post author

    Your English is not better

  • Cool Videos Post author

    Awesome

  • franciscomaerle Post author

    I always thought that the flux used to induce the current and that this current just exists because of this flux. It would not necessarily intensify the flux because its existence simply relies on the flux. Like the rain and the rainbow. The rain creates the rainbow and the rainbow would not intensify the rain.

  • John Levin Post author

    Woow this is really helpful. Thank u. And u draw very good! Impressive.

  • Jane Doe Post author

    I love you

  • samarsh mental Post author

    why the loop or coil is already in a magnetic feild …the begining part ..plzzz some one help

  • listerreg Post author

    I really appreciate your videos. Nevertheless there is a small flaw in reasoning in this one. Even if a generated current would increase magnetic flux it is not said that it would cause infinite gain. That is because a change of flux is what inducing a current not a strength (density) of it. So you could have very strong externally generated flux with just a little change over time. This way flux induced by a current would be negligible. Another thing. In the moment when you will cease to increase external flux the current will drop. When a current drops induced flux also drops. Drop in a flux means generation of a current in a reverse direction.

  • narendra mishra Post author

    I truly understood the Lenz law today..thnk u sir.. Impressive explanation

  • Davide Post author

    An amazing explanation.. thank you a lot to make me understand that law, I'm so grateful!

  • Maria Lee Post author

    If i get into med school I have khan academy to be thankful for. Wish me luck๐Ÿ˜๐Ÿ˜๐Ÿ˜

  • Shubham Manna Post author

    Are u a muslim? Your channel name does not suggests so

  • Some Random Fellow Post author

    OMG I GET IT NOW ITS SO EASY

  • Yuan Xiang Post author

    Canโ€™t thank you enough!

  • Michael Haggerty Post author

    phenomenal video … the positive feedback loop analogy was the key insight!

  • pranilmasti Post author

    Why does electromagnetism follow the right hand rule? Or say left hand rule for conventional or electron current whichever what decides the either direction?

  • Ishu Gangwar Post author

    Nice

  • Aslam Khan Post author

    thanks sir , I will never forget that you have said " this is the violation of law of conservation of energy " and from this statement I have understood the lanz,s law very successfully.

  • ุฅุณู…ูŽุงุนูŠู„ Post author

    2:33 it would be what?

  • surya nath Post author

    Wonderful explanation…

  • Gahl Yogu Post author

    3 loops , each have 6 magnetic field lines …… 666!

  • 18Y6C35 YANG YUESHAN Post author

    to represent a increase in magnetic field, aren't you supposed to increase the number of arrows? By lengthening the arrows it only represents that the length that the magnetic field increases. I m so confused now.

  • Makhanlal Kar Post author

    Thank you sir!!! You have increased my confidence in Physics. People always try to demotivate me but you have motivated me to persue my favourite subject.

  • Angel Yazaki Post author

    ugh sorry im having to ask this: but how does the right-hand rule for counterclockwise and clockwise on the left-side coil on clockwise picture (4:24) point at the same direction (therefore same hand gesture) but counterclockwise produce an additive force but subtractive force for the clockwise one?

  • Josue Acevedo Post author

    2:32 :v

  • Aaron Spiller Post author

    What is the drawing program you use?

  • Krishna gupta Post author

    Awsome video

  • angelsintahoe Post author

    So it only OPPOSES the change in flux if it is INCREASING therefore the induced magnetic flux would go the opposite direction, otherwise (if the induced magnetic flux is decreasing) it adds to it = does not oppose it??? Is this correct?

  • Nitin Chopra Post author

    don't you need to know the original direction the current is flowing before you increase the magnetic flux?

  • J. Post author

    93 Perpetual Motion believers dislike this video!

  • Vikram Krishna Post author

    excellent teaching ๐Ÿ™‚

  • h y Post author

    When current goes in counter clockwise,how magnetic decreases, it just changes the direction??~can anyone explain pls

  • h y Post author

    Field

  • Abd Alkader Abdeen Agha Post author

    This video is a piece of art, this is legit explanation, don't know how to thank you.

  • Lavender Post author

    Amazing๐Ÿ‘๐Ÿ‘๐Ÿ‘

  • Technical Raj Ashish Post author

    thanxx to clear my concept

  • Biswaraj Nath Post author

    simply Perfect….Thank You so much

  • Sospicion Post author

    Thank you sir !

  • Rashid Siafi Post author

    khan Bhai bsc i year ki physics padao please samaye parivartan dharaye…

  • Yuarian Smith Post author

    My teacher just skipped all of this and went straight to the equations and questions, I want my money back!

  • AngelTheAssassin Post author

    ๐Ÿ‘๐Ÿ‘๐Ÿ‘๐Ÿ‘๐Ÿ‘๐Ÿ‘๐Ÿ‘

  • MUNESHWAR TEJASHWINI Post author

    The law of conservation of energy is stated as lenz law. is it correct?

  • Pounding Saturday Post author

    thanks, this helped me understand the concept. also you can draw some bang up hands!

  • Daniel Downs Post author

    Perspective my dear Wattson perspective

  • zwep Post author

    If there would be a positive feedbackloop in the counter-clockwise direction… why is there no negative feedbackloop in the clockwise direction..?

  • Random Internet Profile Post author

    Okay, I've been watching Khan academy for about 4 years now. Does this dude have a degree in everything? Because he's not just reading scripts, he's teaching. He's an expert in mathematics, biology, chemistry and physics. I'll assume he's an expert in engineering too. Who are you? Show yourself!!

  • Tome Post author

    It makes everything so much easier

  • Niraj Gurjar Post author

    There is any other reason for this phenomenon to occur instead of law of conservation….. If yes please tell me

  • strat proj Post author

    ๐Ÿ‘

  • Jainil Shah Post author

    Why do we care about flux in the loop and not flux on the edges of the loop or outside of it.

  • Tina Xie Post author

    how do you know the direction of the change in flux

  • Min'Kookie่–„่ท็ณ–'ๆžœ Post author

    After the video, all I want to say is look at those right hands that he draw! Wow, I can't even draw hands properly lol

  • andrew z Post author

    Awesome video! but I have a small question: I did not understand that you said energy is created if the magnetic field created by the current is in the same direction of the magnetic flux, which violates the law of conservation of energy. What energy is being created?

  • Gursimran Chhina Post author

    Why did he immediately jump to assuming the current was positive?

  • nour ghazal Post author

    Can I do the same exp with the left hand ? Someone plz answer.

  • ps3gaming52 Post author

    who else is grinding for ap phys 2

  • Jukebox300 Post author

    This is such a great explanation! Now I've made a left-hand rule.

  • nawar khaild Post author

    ู…ู†ูˆ ุตู ุณุงุฏุณ ๐Ÿ˜‚

  • Josรฉ Cruz Post author

    I'm not sure how the right hand rule on the left hand side of the upper image would result in fingers curling downward if the thumb faces in the direction of the orange arrow

  • Tariq Saba Post author

    I am confused. Could have been better if u wrote out the formula?

  • montyandlucylove Post author

    Khan Academy does art lessons now?

  • train song Post author

    Thank you so much

  • TheInsaneComputer Gamer Post author

    Learnt this at Sixth Form College and twice at university with different schools (mathematics and engineering).

    Despite that, this is the best and simplest explanation I have seen.

  • Krishnaraj Thadesar Post author

    amazing video i got it in seconds. Great !!!!! this is what u call an EXPLANATION

  • Hashir Khan Post author

    i don't get it for the current carrying wires the magnetic field would be going in a circle yet u drew them as arrows either going just up or down how is this possible?

  • Patricia Nyrรถnen Post author

    Best video on lenz law on youtube. Great! Elegant. Ah ha ๐Ÿ™Œ

  • abhishek raj Post author

    Khan academy is superb in physics.๐Ÿ‘๐Ÿ™Œ๐Ÿ‘๐Ÿ™๐Ÿ™๐ŸคŸ๐Ÿ™‹๐Ÿ™‹๐Ÿ™‹….a lot of thanks from ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ๐Ÿ‡ฎ๐Ÿ‡ณ

  • Elusive Truth Post author

    So….let me summarize your conclusions:

    "…we would not want the current that is induced to induce a magnetic field that goes in the same direction as our increase in flux…" as this would create free energy in an infinite feedback loop in violation of Law?

    Hmmmm….many of the greatest discoveries throughout history were impossible. I propose there are forces which we have yet to comprehend and all of our mighty achievements are equivalent to monkeys poking a stick into their own feces!

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