HSC Physics Marathon 2013-2015 Archive (5 Viewers)

[Drsoccerball]

re: HSC Physics Marathon Archive

Yo people I've been teaching myself From Ideas to Implementation (I am up to section 4 atm)

I just need some clarification in regards to section three and the band theory

Some textbooks say that the conduction band and valence band OVERLAP whilst others say that they are COMBINED, or say that the FORBIDDEN ENERGY GAP IS NON-EXISTENT in a conductor

(I feel as if it is the former and then I could mention it in regards to the latter - In conductors the conduction band and valence band overlap, meaning that the forbidden energy gap in non-existent, etc. The "combined" just doesn't sound right)

These seem like three completely different ways to refer to it but I was wondering if they could be used synonymously in this context?

If not, which is preferred?

Conductors valence band and conduction band are so closs that its energy gap is "practically" non existant

 

[iforgotmyname]

re: HSC Physics Marathon Archive

Yo people I've been teaching myself From Ideas to Implementation (I am up to section 4 atm)

I just need some clarification in regards to section three and the band theory

Some textbooks say that the conduction band and valence band OVERLAP whilst others say that they are COMBINED, or say that the FORBIDDEN ENERGY GAP IS NON-EXISTENT in a conductor

(I feel as if it is the former and then I could mention it in regards to the latter - In conductors the conduction band and valence band overlap, meaning that the forbidden energy gap in non-existent, etc. The "combined" just doesn't sound right)

These seem like three completely different ways to refer to it but I was wondering if they could be used synonymously in this context?

If not, which is preferred?

In a conductor, an overlap exists between the conduction and the valency band (ie some of the electrons are already in conduction), however a very small forbidden energy gap exist between conduction and valency (ie small amounts of energy is needed for more electrons to go from conduction to valency)

 

[Drsoccerball]

re: HSC Physics Marathon Archive

In a conductor, an overlap exists between the conduction and the valency band (ie some of the electrons are already in conduction), however a very small forbidden energy gap exist between conduction and valency (ie small amounts of energy is needed for more electrons to go from conduction to valency)

isnt that what i just said :l ...

 

[Crisium]

re: HSC Physics Marathon Archive

isnt that what i just said :l ...

You said that they are

so closs

but didn't refer to them overlapping

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive



The energy bands of a conductor actually depend on the number of valence electrons and assuming T > 0K.

Materials with 1 valence electron have a partially filled valence band, band gap and empty conduction band.

Materials with 2 valence electrons have an overlapping VB and CB. (This is what the HSC wants u to model)

If all metals had the same energy band levels (as HSC wants), then why are materials with a lower number of valence electrons (Gold, Platinum), better conductors than those with more valence electrons

 

[Crisium]

re: HSC Physics Marathon Archive

View attachment 32171

The energy bands of a conductor actually depend on the number of valence electrons and assuming T > 0K.

Materials with 1 valence electron have a partially filled valence band, band gap and empty conduction band.

Materials with 2 valence electrons have an overlapping VB and CB. (This is what the HSC wants u to model)

If all metals had the same energy band levels (as HSC wants), then why are materials with a lower number of valence electrons (Gold, Platinum), better conductors than those with more valence electrons

Can we relate our answer to temperature or is it assuming a specific temperature?

 

[Kaido]

re: HSC Physics Marathon Archive

I'm getting owned by those really tricky (imo) Lenz's law questions in the MC section, anyone got some tips or can help me out with my stash of Qs?

 

[rand_althor]

re: HSC Physics Marathon Archive

I'm getting owned by those really tricky (imo) Lenz's law questions in the MC section, anyone got some tips or can help me out with my stash of Qs?

Could you please post some of the questions up?

 

[Kaido]

re: HSC Physics Marathon Archive

Uhh I won't post it as I'm not sure how lol, but if you look through the HSC papers you can find it in the MC section
Unfortunately, no worked solutions to the MC

 

[Drsoccerball]

re: HSC Physics Marathon Archive

Uhh I won't post it as I'm not sure how lol, but if you look through the HSC papers you can find it in the MC section
Unfortunately, no worked solutions to the MC

brother you can message me and we can go through it

 

[leehuan]

re: HSC Physics Marathon Archive

(a) Today, we know that cathode rays are negatively charged particles called electrons. Explain why historically there was debate against the nature of cathode rays. (4)
(b) Outline the experiment J. J. Thomson performed to effectively settle this debate. (5)
(c) J. J. Thomson's experiment also lead to some consequences. Outline one consequence. (1)

 

[rand_althor]

re: HSC Physics Marathon Archive

(a) Today, we know that cathode rays are negatively charged particles called electrons. Explain why historically there was debate against the nature of cathode rays. (4)
(b) Outline the experiment J. J. Thomson performed to effectively settle this debate. (5)
(c) J. J. Thomson's experiment also lead to some consequences. Outline one consequence. (1)

(a) There has been a significant debate regarding whether cathode rays were either waves or composed of particles. Various experiments were performed to provide evidence for either side. The Maltese cross experiment showed that cathode rays cast a shadow over glass, suggesting they travel in straight lines, a property of waves. However, the Paddle wheel experiment showed that cathode rays are able to cause a paddle wheel to move, suggesting they have momentum, and consequently mass. As a result, they were classified as waves. The debate was soon over following Thomson's experiment which showed that cathode rays were negatively charged particles.

Is this enough for 4 marks? Should I include an outline of another experiment?

(b) Thomson set up a cathode ray tube. He turned on an electric field and magnetic field such that the cathode rays passed through the cathode ray tube undeflected. As this point, the force due to the magnetic field was equivalent to the force due to the electric field. As Thomson knew the strengths of both these fields, he mathematically calculated the velocity of the cathode rays (F_E=F_B). Thomson then turned off the electric field, so that the only force acting on the cathode rays was due to the magnetic field. At this point, the centripetal force acting on the cathode rays was equal to the force due to the magnetic field. Thomson measured the radius of curvature of the cathode rays, and as he had previously calculated velocity, he was able to mathematically determine the charge:mass ratio of the cathode rays (F_B=F_C). In doing so, Thomson proved that cathode rays were in fact particles, ending the debate.

(c) Cathode rays have mass. They are negatively charged particles, as seen from their deflection in an electric field.

 

[leehuan]

re: HSC Physics Marathon Archive

For the 4 marks you should've tried to include two for each side.

You can argue that how they cause objects to fluoresce is also evidence for the wave theory and the deflection in the magnetic field is evidence for the particle theory.

I like (b). (c) That is correct, but I was actually looking for something like this: This particle has quite a large charge for such a miniature mass. There were also other particles besides ions that had charge as a result of this. But I'm not sure if they're in the syllabus so I'll take your answer.

 

[rand_althor]

re: HSC Physics Marathon Archive

For the 4 marks you should've tried to include two for each side.

You can argue that how they cause objects to fluoresce is also evidence for the wave theory and the deflection in the magnetic field is evidence for the particle theory.

I like (b). (c) That is correct, but I was actually looking for something like this: This particle has quite a large charge for such a miniature mass. There were also other particles besides ions that had charge as a result of this. But I'm not sure if they're in the syllabus so I'll take your answer.

Thanks for the feedback.

Next question: Explain why the wires of the secondary coil in a step up transformer are a different thickness to the wires in the primary coil. (3 marks)

 

[Drsoccerball]

re: HSC Physics Marathon Archive

(a) There has been a significant debate regarding whether cathode rays were either waves or composed of particles. Various experiments were performed to provide evidence for either side. The Maltese cross experiment showed that cathode rays cast a shadow over glass, suggesting they travel in straight lines, a property of waves. However, the Paddle wheel experiment showed that cathode rays are able to cause a paddle wheel to move, suggesting they have momentum, and consequently mass. As a result, they were classified as waves. The debate was soon over following Thomson's experiment which showed that cathode rays were negatively charged particles.

Is this enough for 4 marks? Should I include an outline of another experiment?

(b) Thomson set up a cathode ray tube. He turned on an electric field and magnetic field such that the cathode rays passed through the cathode ray tube undeflected. As this point, the force due to the magnetic field was equivalent to the force due to the electric field. As Thomson knew the strengths of both these fields, he mathematically calculated the velocity of the cathode rays (F_E=F_B). Thomson then turned off the electric field, so that the only force acting on the cathode rays was due to the magnetic field. At this point, the centripetal force acting on the cathode rays was equal to the force due to the magnetic field. Thomson measured the radius of curvature of the cathode rays, and as he had previously calculated velocity, he was able to mathematically determine the charge:mass ratio of the cathode rays (F_B=F_C). In doing so, Thomson proved that cathode rays were in fact particles, ending the debate.

(c) Cathode rays have mass. They are negatively charged particles, as seen from their deflection in an electric field.

I'd definitely mention the deflection by magnetic fields...By the way apparently both the propositions were right! A cathod ray is both a particle and wave (not required in HSC)

 

[leehuan]

re: HSC Physics Marathon Archive

I'd definitely mention the deflection by magnetic fields...By the way apparently both the propositions were right! A cathod ray is both a particle and wave (not required in HSC)

Oh gees more particle-wave duality?

And can someone please answer rand_althor's question because that is something I need to learn.

 

[Drsoccerball]

re: HSC Physics Marathon Archive

Thanks for the feedback.

Next question: Explain why the wires of the secondary coil in a step up transformer are a different thickness to the wires in the primary coil. (3 marks)

Just going to give a dodge answer but :
Step up transformers require the amount of coils in the secondary coil to be more than the primary coil and the amount of current in the primary to be less than the secondary. This then means that more current passes through the secondary coil. This provides less resistance for the secondary wire thus not loosing energy in its stepping up process. P=I^2R The lower the resistance the less energy lost. Not sure if this correct

 

[rand_althor]

re: HSC Physics Marathon Archive

Just going to give a dodge answer but :
Step up transformers require the amount of coils in the secondary coil to be more than the primary coil and the amount of current in the primary to be less than the secondary. This then means that more current passes through the secondary coil. This provides less resistance for the secondary wire thus not loosing energy in its stepping up process. P=I^2R The lower the resistance the less energy lost. Not sure if this correct

It isn't correct. In a step-up transformer, there is a greater voltage in the secondary coil compared to the primary coil, and a greater current in the primary coil relative to the secondary coil. You can confirm this for yourself by checking . Consider the effects of a lower current in the secondary coil on resistance in the secondary coil. Also, what is the link between resistance and thickness of wiring?

 

[Drsoccerball]

re: HSC Physics Marathon Archive

It isn't correct. In a step-up transformer, there is a greater voltage in the secondary coil compared to the primary coil, and a greater current in the primary coil relative to the secondary coil. You can confirm this for yourself by checking . Consider the effects of a lower current in the secondary coil on resistance in the secondary coil. Also, what is the link between resistance and thickness of wiring?

Yeah i was thinking there was something wrong with my explanation... Resistance is inversely proportional to wire thickness

 

[Crisium]

re: HSC Physics Marathon Archive

Yeah i was thinking there was something wrong with my explanation... Resistance is inversely proportional to wire thickness

Cross-sectional area?