The h Bar

12:36 AM

@JohnRennie Perhaps it's just my experience, but requiring that students know a lot of information by the end of a relatively short time often means the students have to learn at a faster pace then they'd prefer. I'd find it much easier to teach myself the material if I could read from the textbook at my own pace, such that I can truly understand the material before moving on.

University requires that you've caught up to speed with the material by the time the homework is due, and that you've had enough practice and understanding to be ready for a midterm in a few weeks. That limits how much one can ponder the concepts and gain an intuition.

DanielSank

12:49 AM

@SirCumference You have to balance between trying to understand things well and the fact that you will never understand anything the first time you learn it.

Sir Cumference

@DanielSank That's pretty frustrating when the uni ironically makes the learning more difficult. SR left a bad taste in my mouth because of the undue stress and lack of full understanding I associate with it. I should have loved that class, so forgive me if I seem like a grump.

Bernardo Meurer

1:01 AM

@SirCumference Teaching is hard

DanielSank

@SirCumference I think that's a little naive.

University does something you'd never do on your own: it forces you to learn a lot about many topics while your brain is still young enough to absorb information.

I agree that it's frustrating though. I spent my university summers reviewing what I'd learned during the academic year.

I would also say that the more advanced classes tended to go too quickly.

Sir Cumference

@DanielSank Perhaps the most frustrating thing is when tests are curved, so that even a ~75 is still a good score. If you're a professor and your students do not know a quarter of the information, then you are failing at your job at teaching

Sigh, I guess I'm finished venting...

DanielSank

2:31 AM

@SirCumference No way, dude.

If students all get 90% on an exam then you're wasting information bandwidth.

Dawood ibn Kareem

5:59 AM

A well-formulated exam question requires a higher percentage of the knowledge from the course than its contribution to the final mark. In other words, an exam might contain 10 questions, each of which requires knowledge of 40% of the course's material to complete. In such a case, you might know 90% of the material, but only be able to do 6 of the 10 questions. So a mean score of 75% of an examination in no way indicates that the average student doesn't know a quarter of the material.

2

2physics

6:21 AM

Hi, I have a couple of questions, anybody there who can help?

Dawood ibn Kareem

Possibly. What are your questions?

John Rennie

@2physics ask away!

Dawood ibn Kareem

Hello John.

2physics

@JohnRennie Assume an observer is located in a magnetic field (both of the observer and object are stationary with respect to each other, and the magnetic field is constant in time ). Suppose an object turns into the magnetic field. I believe the object should measure an electric field (Maxwell’s equations). The question is that: if the observer feels the caused electric field?@DawoodibnKareem

Shing

I am wondering if energy eigenstates are always complete given whatever potential?

John Rennie

6:30 AM

@2physics Electrodynamics isn't really my area, but I think the answer is no.

2physics

Another question is that, does the force exerted by the magnetic field (Lorentz law) directly affect the electric charge or it turn into an emf and then affect it?

John Rennie

The field is just the field. The static observer and the moving observer simply have different views of it due to their different inertial frames.

Dawood ibn Kareem

Why would the object measure an electric field, assuming it has no charge?

2physics

@DawoodibnKareem Ampere's law says that.

Dawood ibn Kareem

I really don't think it does.

Ampere's law requires a current in a loop.

2physics

6:33 AM

@DawoodibnKareem it doesn't matter if there is a charge or not, as long as we have a moving magnetic field we'll have electric field too.

Dawood ibn Kareem

OK, what do you mean by "turns into the magnetic field"? Do you mean the object is turning? Or do you mean it's moving from a place where the field is weaker to a place where the field is stronger?

2physics

@JohnRennie alright thanks..

Dawood ibn Kareem

If it's the latter, then yes, there's an electric field.

2physics

@DawoodibnKareem no no . I mean it enters into the field

@DawoodibnKareem let me make it clearer. Does a moving magnet create electric field?

Dawood ibn Kareem

OK, sure. If the object were charged, it would feel a force because of the changing intensity of the field that it's in. Therefore, even if it's not charged, it's experiencing an electric field.

John Rennie

6:37 AM

@2physics that's an interesting question and I must admit it had never occurred to me. In the rest frame of the moving charge there is an electric field present and the force on the charge may well be equal to the charge times this field. I'd have to sit down and crunch some equations to work out if this is true.

Dawood ibn Kareem

I simply misunderstood what you meant by "turns". I was thinking a rotating object.

John Rennie

It certainly seems a tempting explanation ...

dmckee

@DawoodibnKareem Many people say "Ampere's law" meaning the fourth of Maxwell's equations (i.e. Ampere's law plus the displacement current correction). Not surprising because there is no short, conventional name for the version after Maxwell fixed it.

2physics

@JohnRennie I think if the answer is yes, then moving an object around a coil which is stationary with respect to a magnet will induce an electric current in it!

@DawoodibnKareem yup. of course

John Rennie

@2physics I don't think that's the case. Moving a charge round the coil will obviously induce currents, but moving an uncharged object won't.

Shing

6:40 AM

I am watching Nima Arkani-Hamed's course on PSI.

didn't know he is the director of the China's coming accelerator

2physics

@JohnRennie I know. that's why I asked the question. suppose the observer is the charge which is on the coil.

if the entering object causes an electric field which is measurable by the observer(stationary charge for example) then it should have some effect on the observer too.

shouldn't it?

John Rennie

@2physics The entering object doesn't cause an electric field.

The field is an electromagnetic field. Some observers will observe a purely magnetic field while some will observe a mixture of magnetic and electric fields.

The fact a moving observer detects an electric field does not mean the field has changed. The field is constant but the different observers have different views of it.

2physics

@JohnRennie which observers observe what? lol

John Rennie

To compare what different observers see we use a Lorentz transformation.

2physics

the observer which is stationary with respect to the magnetic field. doesn't it measure any electric or EM field?

John Rennie

6:51 AM

If you start with a static observer in a purely magnetic field, then to find out what the field looks like for an observer moving with some velocity $v$ you just use a Lorentz transformation.

The result will be a mixture of magnetic and electric fields.

I think you're imagining this is somehow spookier than it is. There is an observer independent description of the electromagnetic field called the four-potential, or equivalently the field strength tensor.

2physics

Alright then from what that static observer observes we have an electric field added by the entering object to the system. right?

John Rennie

Nothing is changing. The EM field doesn't change and what the static observer sees doesn't change.

The fact some other observer is moving doesn't change the field, it just means the moving observer is in effect seeing the EM field from a different perspective.

At the risk of using a slightly dodgy analogy, the EM field is a 4D object living in 4D spacetime.

But we humans live in 3D so we see a 3D view of it.

If you have two observers, one moving and one stationary, then those two observers have different 3D views of the 4D object - kind of as if they're seeing it from different angles.

So the two observers see different things, but they are just seeing the same thing from different angles.

2physics

"The fact some other observer is moving doesn't change the field" , doen't change the field which static observer measures you mean? but moving observer measures produced EM fields?

:42594438 but according maxwell's equations a moving magnetic field creates electric fields

I'm confused

John Rennie

@2physics have a quick look at the Wikipedia article on the field strength tensor - just a quick look, don't worry about the detail.

The field strength tensor, $F$, is a Lorentz covariant description of the electromagnetic field. Note that it has components that look like an electric field, $E$, and components that look lie a magnetic field, $B$.

2physics

7:07 AM

and?

John Rennie

When you change speed you are changing the coordinate system you use to write down this tensor, and as a result the individual entries in the matrix change.

The field strength isn't changing, but your view of it is changing.

There may be a velocity that makes all the $E$ components zero, and for an observer moving at this velocity it will appear as if only a magnetic field is present.

Likewise there may be a velocity that makes all the $B$ components zero, and for an observer moving at this velocity it will appear as if only an electric field is present.

In general both the $E$ and $B$ components will be non-zero and the field appears as a mixture of electric and magnetic fields.

But it's not the field changing. It's the view of the field for the observer that is changing.

If it's any consolation this is pretty advanced stuff - I don't really understand it fully myself!

2physics

well (although I don't understand the notation! lol)

I still have some problems with this

John Rennie

@2physics I'm not surprised. It's flipping hard!

F is a tensor and you need to understand the maths of tensors and coordinate transformations to really get what is going on. That's stuff you won't do until the final years of a physics degree.

2physics

and according this, you mean I don't interpret maxwell's equations correctly? curl of H equals J and a time varying D. a moving H field can't create a non-zero Curl?

John Rennie

When you use Maxwell's equations you use various operators like curl. And these depend on your choice of coordinates because the operators involve $d/dx$, $d/dy$, $d/dz$ and $d/dt$.

2physics

7:17 AM

yes

John Rennie

So you need to have chosen your x, y, z and time axes to do the calculations.

But if you now start moving your axes are different to those of a stationary observer, so all your operators are different.

2physics

you mean it depends on the frame reference that a moving magnet causes an E fields or not?

John Rennie

You need to be a little cautious about statements like a moving magnet causes an E field

2physics

why?

John Rennie

If the magnet is moving relative to you then you are moving relative to the magnet.

So it's the same as a static magnetic field with you moving through it. Yes?

2physics

7:22 AM

yes. therefore it depends on where we choose our reference frame to be?

John Rennie

Yes. We decided a while back that if you're moving through a static magnetic field then you see an electric field.

So does the moving magnet cause an electric field, or does your motion past a static magnet cause an electric field?

And the answer is kind of both and neither

2physics

well you mean the static observer decides that there wouldn't be any electric field where the moving object enters into the Magnetic field. true?

John Rennie

Yes

2physics

But suppose the moving object is a coil(say a galvanometer). How does the static observer interpret the current that it shows?

John Rennie

I have to work now for a bit. Back in an hour or so.

2physics

7:37 AM

@JohnRennie sure thanks. I'll ask for other opinions on it too. thank you for your help.

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