# General Thoughts: Physics (1 Viewer)

#### RealiseNothing

##### what is that?It is Cowpea
The thing is marking guidelines are actually made AFTER the markers go through a few exams. So they might go through them, see that a large amount of students talked about the solar cell, and it might be accepted in the marking guidelines.

This is what I've been told, fizzy can confirm this or not.

#### RealiseNothing

##### what is that?It is Cowpea
What was the explanation for the braking of the vehicle from the 'motor acting as a generator'?
I talked about back EMF. Not sure if this is right though, but I did a little google search yesterday and the information I found seemed to agree with me. Hoping it's right lol.

##### New Member
The thing is marking guidelines are actually made AFTER the markers go through a few exams. So they might go through them, see that a large amount of students talked about the solar cell, and it might be accepted in the marking guidelines.

This is what I've been told, fizzy can confirm this or not.
Yea i heard from a hsc marker that they only decide marking guidelines and decide what constitutes a good answer after going through lots of papers. Fingers crossed for the photocell but i honestly think we may end up losing at least 3-4 marks

#### panda15

##### Alligator Navigator
BTW. Sorry I haven't read all the replies here but from what I've read it looks like no one has put this answer into units of *rotational* speed (eg radians per second, revs per second, revs per minute etc).

The correct linear speed at the circumference is 51.91 m/s, but shouldn't you divide this by (2 pi r) and give the results in revolutions per second or similar units? Or is circumferential speed considered a good enough answer for "rotational speed" in this course. (This is genuine question BTW, not rhetorical. I'm not doing the course but was helping someone else with it).
Technically you're right, but that answer style is only taught in Ext. 2 maths, not in physics, so marking people down for writing their answer in m/s would be extremely unfair towards the people not doing ext. 2 maths.

#### Fizzy_Cyst

##### Well-Known Member
Well she had a look at the paper months before it was released, and thus knew the marking guidelines beforehand. I called her and she said solar cells are fine to use and I won't get penalised for it
Dude, marking guidelines aren't decided upon until AFTER the examination, when they have gone through a few papers (I have never marked HSC, but this is what others have told me!)

#### uart

##### Member
I talked about back EMF. Not sure if this is right though, but I did a little google search yesterday and the information I found seemed to agree with me. Hoping it's right lol.
Yes it's definitely related to back EMF. The key thing that they would have wanted here is to recognize the similarity of the DC motor and the DC generator (of course at the level expected here they are identical). So the one machine shifts seamlessly between being a motor and a generator, with the mode of operation depending only upon the relationship between the externally applied voltage and the back EMF.

When the externally applied voltage is greater than the internally generated voltage (aka the back EMF) then the direction of current *and power* is into the DC machine, so it operates as a motor. When the applied voltage is less than the back EMF however, then the direction of current and power is reversed, meaning that the machine is operating as a generator.

It would probably also be good to mention that when acting as a generator, that the direction of power is reversed and therefore that the direction of torque at the wheels must also be reverse. Hence the braking effort.

#### anomalousdecay

It doesn't.
Now I'm seriously confused. Half say it does, half say it doesn't.

Isn't it Meissner, but an initial thrust is given, propelling it along the track?

There's EDS and EMS. One doesn't use superconductors, just normal magnetic fields, propelling due to Lenz's Law right?

#### anomalousdecay

Yes it's definitely related to back EMF. The key thing that they would have wanted here is to recognize the similarity of the DC motor and the DC generator (of course at the level expected here they are identical). So the one machine shifts seamlessly between being a motor and a generator, with the mode of operation depending only upon the relationship between the externally applied voltage and the back EMF.

When the externally applied voltage is greater than the internally generated voltage (aka the back EMF) then the direction of current *and power* is into the DC machine, so it operates as a motor. When the applied voltage is less than the back EMF however, then the direction of current and power is reversed, meaning that the machine is operating as a generator.

It would probably also be good to mention that when acting as a generator, that the direction of power is reversed and therefore that the direction of torque at the wheels must also be reverse. Hence the braking effort.
I wrote something like this:

"when braking, the magnetic fields lock up and the back emf if zero. As a result, the coil will stop rotating."

Then I explained it to the rest of the question.

0, 1 or 2 marks?

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
Now I'm seriously confused. Half say it does, half say it doesn't.

Isn't it Meissner, but an initial thrust is given, propelling it along the track?

There's EDS and EMS. One doesn't use superconductors, just normal magnetic fields, propelling due to Lenz's Law right?
It's like-pole repulsion or something. I'm pretty sure saying Meissner effect is wrong (that's what my teacher told me).

#### anomalousdecay

It's like-pole repulsion or something. I'm pretty sure saying Meissner effect is wrong (that's what my teacher told me).
It levitates due to Meissner, but then the pole repulsion propels it.
Jacaranda said there are two different systems: One that uses superconductors and one that doesn't.

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
It levitates due to Meissner, but then the pole repulsion propels it.
Jacaranda said there are two different systems: One that uses superconductors and one that doesn't.
"In electrodynamic suspension (EDS), both the guideway and the train exert a magnetic field, and the train is levitated by the repulsive and attractive force between these magnetic fields"
From Wikipedia

Other is EMS which doesn't use superconductors.

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
So with EDS, where do the superconductors come into play?
I guess they create stronger repulsive/attractive forces?

#### anomalousdecay

Meissner effect
1.
the expulsion of magnetic flux when a material becomes superconducting in a magnetic field.

Ok, so I'm guessing the reason why its not the Meissner effect is because the superconductor exerts a magnetic field as well.

But still, the superconductor will oppose the change in flux from the electromagnets on the track, propelling it right?

#### Jimmy2064

##### Member
It's not like you're going to lose many marks for stating the Meissner effect though, I mean if you included an understanding of superconductor's and their electrical properties, then used Maglev as an example of one, and referenced it's effect on society.. that's most of the question answered. All you've missed is the specific electrical properties of the Maglev train example, not superconductors as a whole, so my bet is that's probably 1 mark lost at best. How many marks was that question worth again?

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
It's not like you're going to lose many marks for stating the Meissner effect though, I mean if you included an understanding of superconductor's and their electrical properties, then used Maglev as an example of one, and referenced it's effect on society.. that's most of the question answered. All you've missed is the specific electrical properties of the Maglev train example, not superconductors as a whole, so my bet is that's probably 1 mark lost at best. How many marks was that question worth again?
You wouldn't lose any marks for mentioning the Meissner effect incorrectly

#### anomalousdecay

You wouldn't lose any marks for mentioning the Meissner effect incorrectly
I'd only lose a mark if I did not have a sufficient reason, but I did have a reason (EDS --> less travel times--> society has more leisure time and can increase living standards efficiently).

And it was 7 marks. Should get a 6 at least. Should.

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
I'd only lose a mark if I did not have a sufficient reason, but I did have a reason (EDS --> less travel times--> society has more leisure time and can increase living standards efficiently).

And it was 7 marks. Should get a 6 at least. Should.
Sounds good