HSC Physics Marathon 2013-2015 Archive (2 Viewers)

[Crisium]

re: HSC Physics Marathon Archive

- 3 phase AC into the stators that creates a rotating magnetic field
- By f-law, the changing mag flux due to the rotating mag field induces a current in the rotor coils
- By l-law, the current is in a direction such that the mag field it induces opposes the original change in flux the created it
- thus, the induced mag field interacts with the rotating mag-field and the rotor begins to spin as it 'chases' the rotating mag field

how much would this score if i wrote this as it is

3

You're missing a few things

 

[Kaido]

re: HSC Physics Marathon Archive

3

You're missing a few things

Ok, should i elaborate as to why the rotor chases the field
And also mention what happens if a load is applied?
Perhaps even go into detail about the end rings and the peaking of each phase?

If I'm not exactly sure as to what will score the 5 marks, should i just spend time and dump everything lol

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

- 3 phase AC into the stators that creates a rotating magnetic field
- By f-law, the changing mag flux due to the rotating mag field induces a current in the rotor coils
- By l-law, the current is in a direction such that the mag field it induces opposes the original change in flux the created it
- thus, the induced mag field interacts with the rotating mag-field and the rotor begins to spin as it 'chases' the rotating mag field

how much would this score if i wrote this as it is

I would give this A 3 or 4, probably a 4. A few minor (but key) points missing
- no description of AC induction motor (stator consists of 3 pairs of opposing electromagnets each connected to a different phase of 3 phase AC supply, rotor consists of bars connected by end plates)
- then go on to description of APPARENT rotating field caused by the three phase AC supply
- don't say rotor coils, rather rotor bars
- I like most of your description of Faraday and Lenz, but expand a little bit and talk about the force / torque, perhaps also mention that the induced magnetic field creates an OPPOSING pole -- use that to explain how it chases the apparent rotating field. Also link the rotation in the same direction to the desire of the rotor to minimise change in flux.

Apart from that , I thought your response was quite well done. Just make sure that you explicitly show your cause and effect statements. Use those linking words 'which results in', 'which causes', 'leading to', 'consequently' etc..

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

New Question:

Assess the validity of the transformer equation Vp/Vs = np/ns (3 marks)

 

[Drsoccerball]

re: HSC Physics Marathon Archive

New Question:

Assess the validity of the transformer equation Vp/Vs = np/ns (3 marks)

How do you assess a formula :L...
The ratio of voltage of primary to number of coils in primary is eqaul to the rati of voltage of secondary to number of secondary coils..But doesn't take into external factors such as resistance
Therefore not really valid :L

 

[Thunderstorm]

re: HSC Physics Marathon Archive

How do you assess a formula :L...
The ratio of voltage of primary to number of coils in primary is eqaul to the rati of voltage of secondary to number of secondary coils..But doesn't take into external factors such as resistance
Therefore not really valid :L

Exactly. You'd have to explain the energy losses that occur in reality through resistance heating, etc while the formula only accounts for complete, perfect, energy transformation.

 

[Kaido]

re: HSC Physics Marathon Archive

What if we used superconducting coils, is the equation then valid?

 

[Drsoccerball]

re: HSC Physics Marathon Archive

What if we used superconducting coils, is the equation then valid?

The degree of error would be less
y u no go on maths marathons anymore
EDIT: energy is not only lost through heat

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

You need to understand where the equation arises from.

Energy losses will not affect this particular equation.

 

[InteGrand]

re: HSC Physics Marathon Archive

Energy losses will not affect this particular equation.

They will affect the equation, won't they? Because we're saying and similarly for , and then dividing these equations gives the equation .

But terminal voltages V_p and V_s can only be assumed to equal their respective emf's if we assume the internal resistance is 0. The real equation should be: . So we do need to assume there's no energy losses by assuming no internal resistance, e.g. from the wires etc.

(But then again, I don't think internal resistance is dealt with in the HSC, and we just assume V = emf.)

 

[Kaido]

re: HSC Physics Marathon Archive

2005HSC Q24
marking criteria insists that we identify the role oh phonons
i'm interested in your answers

and will i get penalised (or not given marks) if i recall information that is beyond the syllabus?

 

[InteGrand]

re: HSC Physics Marathon Archive

(In other words, that equation refers to an ideal transformer.)

 

[PhysicsMaths]

re: HSC Physics Marathon Archive

New Question:

Assess the validity of the transformer equation Vp/Vs = np/ns (3 marks)

This equation is not entirely valid. It assumes that the transformer is 100% efficient in transforming power, as input power = output power. According to the L.O.C.O.E, energy is never destroyed, but transformed into other forms. In a real life scenario, some of the energy is lost as heat (caused by eddy currents and magnetic hysteresis loss), and therefore, the input power must always be greater than the output power.

 

[Kaido]

re: HSC Physics Marathon Archive

The degree of error would be less
y u no go on maths marathons anymore
EDIT: energy is not only lost through heat

I do, but either the question is out of my league, or one of you guys has already answered it LOL

 

[Kaido]

re: HSC Physics Marathon Archive

For the equation, perhaps something to do with the flux hinders the validity?

I'm assuming the non-uniformity of magnetic fields as a result of (real life) current as well as the linkage of flux between the two coils may possibly affect this

 

[InteGrand]

re: HSC Physics Marathon Archive

For the equation, perhaps something to do with the flux hinders the validity?

Not really I think, because the core of the transformer has a high magnetic permeability, so almost no field lines are "lost" from the core.

 

[PhysicsMaths]

re: HSC Physics Marathon Archive

Not really I think, because the core of the transformer has a high magnetic permeability, so almost no field lines are "lost" from the core.

yes, which is another reason why some of the power is lost from the system, and Vp > Vs

 

[Kaido]

re: HSC Physics Marathon Archive

2005HSC Q24
marking criteria insists that we identify the role oh phonons
i'm interested in your answers

and will i get penalised (or not given marks) if i recall information that is beyond the syllabus?

In the meantime, this^

 

[InteGrand]

re: HSC Physics Marathon Archive

yes, which is another reason why some of the power is lost from the system, and Vp > Vs

As in, it doesn't make THAT much difference.

 

[Kaido]

re: HSC Physics Marathon Archive

As in, it doesn't make THAT much difference.

but it DOES make a difference

honestly not sure how to tackle the question, considering the many possiblities