Problem Solving Strategies

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3:58 AM

@JohnRennie Sir if a superconducting loop is passing thorough a region of magnetic field (partial loop is in MF region and partly not), what would happen ? does current flow ? Is electric field inside the wire would be zero ? is total flux would be zero ?

@RishiNandhaVanchi this metal S in jee advance 2021 is not superconductor ??

John Rennie

4:12 AM

@YashAgrawal There is little difference between a superconducting wire and a regular low resistance wire like very pure copper. In both cases Faraday's law applies.

Yash Agrawal

yes sir

Sir is electric field always zero in superconducting wire ?

John Rennie

Yes

Yash Agrawal

then how current flow ?

some sources seem to claim , that current flows on the surface but this doesn't seems to make sense for me

John Rennie

Suppose you throw a ball. To make it accelerate you have to apply a force to it. Yes?

Yash Agrawal

yes

John Rennie

4:20 AM

Once the ball has left your hand your hand there is no longer any force on it (assume we're in space away from gravity) but the ball doesn't stop moving. Yes?

Yash Agrawal

Yes

John Rennie

Well the electrons in a wire are like little balls. It takes a force (i.e. an electric field) to get them moving, but once they are moving they will continue to move unless some opposing force stops them.

Yash Agrawal

who gives the initial force then ?

John Rennie

In a regular wire that force is resistance, and it slows the electrons to a halt so the current stops.

Yash Agrawal

yes

John Rennie

4:23 AM

But in a superconductor there is no resistance, so a current will just keep flowing even when no EMF is applied.

But as you say, it's getting the current started that's hard in a superconductor.

Yash Agrawal

yes sir , how would've do that ?

John Rennie

Typically if you have a loop of superconducting wire the way you get a current started is by induction using a magnetic field.

Yash Agrawal

I guess both electric and magnetic fields are always zero inside a superconducting material

RishiNandha Vanchi

@YashAgrawal idk lol

my point is, if you are asking for adv you dont need to know

Yash Agrawal

@JohnRennie So , that's why current couldn't decay in superconductor

John Rennie

4:27 AM

@RishiNandhaVanchi You can be interested in things that aren't part of the JEE :-)

Yash Agrawal

@RishiNandhaVanchi Yes , I guess so , I was able to solve the question without knowing about superconductors :)

Lalit Tolani

@JohnRennie what about collisions with ions, i.e. when we leave those little balls wouldn't they loose their energy to other much bigger balls?

Yash Agrawal

but still superconductors seems interesting

RishiNandha Vanchi

@JohnRennie Definitely!! That's I wanted to attach the "if you are asking for adv" XD

John Rennie

@YashAgrawal There are other effects you need to consider. For example a superconducting loop has an inductance i.e. when a current flows in superconducting loop it creates a magnetic field and that requires energy.

So if you applied an EMF E to a superconducting wire you don't get a current I = E/0 = ∞

Plus electrons do have a mass, even though it's small, so you have to supply them with kinetic energy to make them move.

Yash Agrawal

4:29 AM

yes but as you said inertia of electrons keep them moving , so source of magnetic energy seems mysterious

John Rennie

@YashAgrawal It's when you accelerate the electrons to raise the current from zero that you have to supply the energy.

Once you have the electrons moving no energy is needed to keep them moving at constant speed.

Yash Agrawal

My question is how would be accelerate electrons at the first place ?

John Rennie

@LalitTolani This is the key property of superconductors.

Suppose you try to add 1 eV to the electron in a hydrogen atom then you'll find you can't do it. Yes?

The minimum amount of energy you can add is 10.4 eV i.e. the 1s ⟶ 2p energy.

Yash Agrawal

yes , knew that

John Rennie

Because in the H atom the electron occupies specific energy levels and there is a gap between the ground and first excited states.

Lalit Tolani

4:34 AM

@JohnRennie yes

John Rennie

Well in a superconductor the same thing happens to the conduction electrons. A gap opens up between the ground state and the first excited state.

Yash Agrawal

John Rennie

If you scatter an electron, off an ion or anything else, this changes its energy.

Yash Agrawal

yes

John Rennie

But the energy cannot change by a small amount because the energy gap exists. In superconductor you cannot scatter the electrons a bit - you can only scatter them a lot to get them past the gap.

Yash Agrawal

4:36 AM

yes got it

John Rennie

And this does happen. Try to flow too much current through a supercondutor and the superconductivity breaks down because the scattering becomes large.

But for smaller currents the scattering simply cannot occur because of the gap, and therefore there cannot be any resistance.

Yash Agrawal

yes understood

but how it relates to my question

John Rennie

A: Intuitive reasons for superconductivity

When you scatter an electron you change it's energy. So if it wasn't possible to change the energy of an electron you couldn't scatter it. This is basically what happens in superconductors. In a metal at room temperature the electrons have a continuous range of energies. This means if I want to ...

A: Why do electrons not bump into impurities in a superconductor?

When the electrons pair up this opens an energy gap between the energy of the Cooper pairs and the energy of the lowest quasiparticle excitation. There is a nice article discussing this effect here (NB it's a PDF). The gap means that you cannot scatter a Cooper pair by an arbitrarily small energ...

@YashAgrawal Sorry, I was answering @LalitTolani rather than you.

Yash Agrawal

@JohnRennie ok sir xD

John Rennie

@YashAgrawal I'm not sure exactly how the electrons get accelerated when you use induction to create a current. I think the magnetic field must penetrate the surface of the conductor, even if by only a small distance, so it can exert a Lorentz force on the electrons at the surface.

Yash Agrawal

4:41 AM

@JohnRennie Ok sir , so current flows just on the surface ?

John Rennie

I don't know to be honest.

Yash Agrawal

and sir one last question , can current decay or not in superconductor ?

John Rennie

I didn't do a course on superconductivity at university so I only know what I've read in my casual reading about physics.

@YashAgrawal Not unless there is some mechanism to remove the energy.

For example you could use an external magnetic field to apply an EMF in the opposite direction.

Yash Agrawal

yes

@JohnRennie ok sir , thanks very much for helping :)

John Rennie

But unless you apply some external force like this the current will just keep flowing forever.

Yash Agrawal

4:44 AM

yes that's what I wanted to confirm

8 hours later…

hansika

12:38 PM

@JohnRennie Sir, can you please have a look at this question? chat.stackexchange.com/transcript/message/59401104#59401104

4 hours later…

Ashish Ahuja

4:55 PM

yesterday, by hansika

https://imgur.com/a/cUsG89e
The question asks "frequency at the instant", at that particular instant it would have received a frequency which train S1 would have emitted at time (say t) ago, and in this time t the train would have covered a distance 30t and would have reached at the the particular instant which question is talking about, so technically shouldn't we have been applying doppler's effect for train S1 t seconds ago?

@hansika yes, technically what you're saying is right. For the emitter you need to take the component of it's velocity when it emits the sound wave which eventually reaches the detector after some time $t$. But if you actually try calculating this time $t$ accurately you're going to soon run into problems.. so they've taken it as an approximation.

Time $t$ is given by $600^2 + (800 + 30t^2) = (330t^2)$ and it's solution is about 3.27 seconds; it's impossible to solve it by hand, then use whatever angle you get to calculate the component of velocity etc etc. so the approximation the question has used is the only way.

although 3.27 means that the emitter was nearly at a distance of 900 m instead of the 800 m so it isn't a very good approximation. Maybe the question should've chosen some other numbers to at least keep the approximation close.

Samyak Marathe

5:11 PM

Greetings

I have a question to ask

If anyone coould help

I m very much interested in doing physics experiments, about electricity, magnetism, optics, and literally everything. But I don't know where to find the right material for it. Is there a place where I could buy custom physics experiment kit

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Problem Solving Strategies

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