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Course: Physics library > Unit 13
Lesson 4: Magnetic flux and Faraday's lawLenz's Law
Lenz's Law - seeing that the magnetic field induced by a current induced by a change in magnetic flux (Faraday's Law) counteracts the change in flux.
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at4:40u hav shown that using right hand rule we get magnetic field in the opp direction of flux. but this production of field will not only be inside in the loop but also outside the loop i.e. there will be circular path of field around the sides of rectangle. what will happen to this extra field?(20 votes)
The portion of the field that exists outside the loop will not affect the magnetic flux inside the loop, which is why Sal didn't discuss it. It will 'co-exist' without having any effect.(28 votes)
Does the induced magnetic flux just 'try' and decrease the increase in flux or is it actually able to negate the entire change.(14 votes)
Is the magnetic field created by the current the same magnitude as the original magnetic field, ie does it cancel it entirely?
I'm also a little confused by what the overall outcome is with this setup. If the current creates a counteracting magnetic field that would mean a change in flux, which would induce a new current going the opposite direction, which would then create another change in flux... and on and on. Does this back-and-forth go on forever, or does it reach some static equilibrium eventually?(9 votes)- It reaches equilibrium almost immediately
You know how the lights dim a little bit when your refrigerator or air conditioner turns on?
That's because a lot of current flows while the motor gets going
Once it gets going, Lenz' law says it will create a back EMF that will oppose the original.
The sum of that back EMF and the original lead to a lower total voltage to the motor once it reaches speed, and less current flowing through the motor, and then the lights get bright again.(7 votes)
How the current will go on increasing in the counter-clockwise situation? (Reason)
and should I consider it as an exception for right hand thumb rule?(6 votes)- Hello Yashi,
Regarding the situation shown at3:00. Here Sal is exploring what would happen if the current was to flow in a a counter clockwise direction. This was a though experiment - not real life. Such a situation could never happen in the real world as it would violate the laws of thermodynamics. It would lead to perpetual motion machines and free energy - nice ideas but they will never happen....
To answer your question, no. this is not an exception to the right hand rule. It is just an exploration that lead to the correct answer - current flows in opposition to that which caused it aka Len's Law.
Regards,
APD(9 votes)
Won't a decrease in flux make the current decrease which would make the flux decrease even more. Doesn't this violate the conservation of energy?(5 votes)- Lenz' law says that the direction of the induced current will oppose the change in flux that created it. It's not a matter of increasing and decreasing, it's a matter of direction. A decreasing flux creates an EMF that creates a current and that current will be in whatever direction it has to be to try to keep the flux from decreasing. An increase in flux will lead to a current that will try keep the flux from increasing. In both cases, it's consistent with conservation of energy. If it worked the other way, it would violate conservation of energy because an increasing flux would cause a current that would cause the flux to increase more, which would cause a current that would cause the flux to increase more...(5 votes)
At the end, is the flux going to reach its original value?(6 votes)- Could you make me understand the query better? Especially what you mean by 'origibal value'?(3 votes)
In the second scenario Sal discussed, wouldn't the decrese in magnetic flux reverse the direction of current in the loop?(3 votes)- Hello Kartikeya,
Correct!
The current will do whatever is necessary to oppose the change in magnetic flux. As Sal pointed out, to do otherwise is to violate the conservation of energy.
Regards,
APD(8 votes)
wish it was elaborated what current would do if flux decreases(2 votes)- At2:33, Sal states that the magnetic field is additive...is this because the fingers loop clockwise (using RHR-1) ?(2 votes)
It is using right hand rule #2, which relates current in a wire with magnetic field. The magnetic field wire due to the current in the wire will be directed upward, in the same direction as the original magnetic field that caused the current in the wire.(1 vote)
- My textbook states that Len's law refers to induced currents and not to induced emf...what does this actually mean?........... Whereas in the topic of mutual induction it is stated that induced emf in the secondary coil is such that it opposes the time rate if change of current in the the primary coil..... Why they are using emf in the mutual induction instead of current then?(2 votes)
- Hello Sumaira,
Perhaps the answer is the context. I suspect your textbook talks about an ideal transformer with no load attached. I would be very confusing to talk about current in this situation! EMF in this unloaded circuit is considerably easier to understand.
Please leave a comment below if I missed the point of your question.
For what it's worth I have a textbook in my hand that reads "Lenz' law states that the direction of the voltage buildup in the coil is such that if the coil ends were short circuited, it would produce current that would cause a flux opposing the original flux change."
Ref: Electric Machinery Fundamentals 3rd ed Chapman
Regards,
APD(1 vote)
4:40Video transcript
- 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.