Lenz's Law

[MedPR]

If anyone else is finding this confusing or just needs a review, I just started doing these problems.

http://members.shaw.ca/rjstriemer/Physics40S/Lenz_Law_Practice.pdf

Also, #4-6 I have no idea how to get the answer probably because I don't know how to get any information out of north/south pole of a magnet.

If someone could explain those that would be great!

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[MrNeuro]

If anyone else is finding this confusing or just needs a review, I just started doing these problems.

http://members.shaw.ca/rjstriemer/Physics40S/Lenz_Law_Practice.pdf

Also, #4-6 I have no idea how to get the answer probably because I don't know how to get any information out of north/south pole of a magnet.

If someone could explain those that would be great!

magnetic fields leave the north enter the south so for #4 you'd generate a clockwise current according to Faraday lenz law ASSUMING the magnet is moving w/ an acceleration

 

[chiddler]

magnetic fields leave the north enter the south so for #4 you'd generate a clockwise current according to Faraday lenz law ASSUMING the magnet is moving w/ an acceleration

I'm not sure an acceleration is necessary. Flux is only change in mag field over time where acceleration is not necessary. Only velocity.

medpr: magnetic poles are just like + and - particles. The Efield arrows go from positive to negative. Similarly, in Bfields, the magnetic field lines go from positive to negative. As it gets closer, then more mag field lines are in the circle therefore flux increases.

 

[MedPR]

magnetic fields leave the north enter the south so for #4 you'd generate a clockwise current according to Faraday lenz law ASSUMING the magnet is moving w/ an acceleration

I'm not sure an acceleration is necessary. Flux is only change in mag field over time where acceleration is not necessary. Only velocity.

medpr: magnetic poles are just like + and - particles. The Efield arrows go from positive to negative. Similarly, in Bfields, the magnetic field lines go from positive to negative. As it gets closer, then more mag field lines are in the circle therefore flux increases.

Ok thanks. So the north pole is like a positive charge and the south pole a negative charge.

And yea, I agree that it doesn't have to be accelerating assuming it follows the same rules as moving a wire through an electric field.

Any idea on the others?

 

[MrNeuro]

Ok thanks. So the north pole is like a positive charge and the south pole a negative charge.

And yea, I agree that it doesn't have to be accelerating assuming it follows the same rules as moving a wire through an electric field.

Any idea on the others?

it does

0 acceleration = 0 change in flux = 0 emf

w/ acceleration = change in flux = induced emf

tripped me up the first time too

for the examples you have if there is 0 acceleration (so obviously not in free fall) there will be a flux the moment it enters the area and the moment it leaves w/ NO emf while its inside

 

[HollowSuperet36]

Am I wrong in thinking that flux and lens law is not something we should have to understand for the mcat. It's not on their list of topics.

 

[chiddler]

it does

0 acceleration = 0 change in flux = 0 emf

w/ acceleration = change in flux = induced emf

tripped me up the first time too

for the examples you have if there is 0 acceleration (so obviously not in free fall) there will be a flux the moment it enters the area and the moment it leaves w/ NO emf while its inside

it could be at a constant nonzero velocity.

 

[SaintJude]

Oh man, thanks for posting. It's been such a long time since I reviewed lenz's law that I forgot for a second that your thump points in the direction of the compensatory field and that your fingers then curl naturally in the direction of current. So much information!

 

[MrNeuro]

it could be at a constant nonzero velocity.

which means 0 acceleration right?

 

[MedPR]

Pretty sure it could be any velocity. The important thing is that it is moving through a mag field and needs to compensate for it. I'm sure the velocity affects the magnitude of the current, but it certainly doesn't affect the direction of the current.

 

[MrNeuro]

Pretty sure it could be any velocity. The important thing is that it is moving through a mag field and needs to compensate for it. I'm sure the velocity affects the magnitude of the current, but it certainly doesn't affect the direction of the current.

If the velocity is constant there will be no emf in a rail gun

 

[chiddler]

If the velocity is constant there will be no emf in a rail gun

why? if there is velocity, then flux is changing. therefore, there is a current.

 

[milski]

I'm not sure from where you are getting the requirement for the acceleration being non-zero. Lenz's law says that there will be an emf if there is a change in flux. Flux depends only on the magnetic field and the area that it goes through. The velocity is needed so that the flux changes - either by moving to a place where the magnetic field is different or by changing the area that is affected by the magnetic field.

You may be confused by the example of a closed loop moving through a uniform field where the loop is completely inside the field. In that case there is no emf since there is no change in the flux. That is true for both constant and non-constant velocities. Adding acceleration will not change the flux and thus there will be no emf.

 

[MrNeuro]

I'm not sure from where you are getting the requirement for the acceleration being non-zero. Lenz's law says that there will be an emf if there is a change in flux. Flux depends only on the magnetic field and the area that it goes through. The velocity is needed so that the flux changes - either by moving to a place where the magnetic field is different or by changing the area that is affected by the magnetic field.

You may be confused by the example of a closed loop moving through a uniform field where the loop is completely inside the field. In that case there is no emf since there is no change in the flux. That is true for both constant and non-constant velocities. Adding acceleration will not change the flux and thus there will be no emf.

if the velocity is changing (aka theres an acceleration) then there is a Change in flux however if the velocity is constant there will be no change in flux and hence no emf
EK has an example of this

 

[milski]

if the velocity is changing (aka theres an acceleration) then there is a Change in flux however if the velocity is constant there will be no change in flux and hence no emf
EK has an example of this

By definition, flux depends on the surface being discussed and the magnetic filed (sign and direction). Translational motion in a uniform field cannot change the flux, be it at constant velocity or accelerating. Rotation could change it, if it changes the angle between the surface and the magnetic field. Again, the type of rotation is relevant, all that it matters that the angle between the field and the surface changes.

If you post the example that you're talking about we can figure out what else is going on there.

 

[MrNeuro]

By definition, flux depends on the surface being discussed and the magnetic filed (sign and direction). Translational motion in a uniform field cannot change the flux, be it at constant velocity or accelerating. Rotation could change it, if it changes the angle between the surface and the magnetic field. Again, the type of rotation is relevant, all that it matters that the angle between the field and the surface changes.

If you post the example that you're talking about we can figure out what else is going on there.

 

[chiddler]

apples and oranges. that's different from the previous figure that is being referenced. there, constant velocity can entail change in flux.

in the example you're posting (with a constant magnetic field), the only way to achieve flux is to provide change in magnetic field or area. this can be achieved by either increasing speed (to change Bfield going through the loop) or changing area (to change Bfield going through loop).

In the previous example, simply moving the magnet back and forth will change Bfield going through the loop. therefore, acceleration is not necessary.

 

[empo]

Strong Shaftsbury link/10

 

[milski]

That would be correct even for non-constant speed. The only way that the velocity would matter would be if there was acceleration out of the xy plane.

 

[chiddler]

That would be correct even for non-constant speed. The only way that the velocity would matter would be if there was acceleration out of the xy plane.

no i think he's saying there must be acceleration under any circumstance for flux to change.

i don't think he is appreciating the difference between this image and the original.

 

[MrNeuro]

nvm

 

[milski]

no i think he's saying there must be acceleration under any circumstance for flux to change.

i don't think he is appreciating the difference between this image and the original.

We're discussing it in messages. I think the setup where the speed matters is slightly different.