Physics Exam 2019 Questions Thread (3 Viewers)

[Arrowshaft]

Alright here’s the next question:

Explain the effectiveness of the Morgan-Keenan system in stellar classification. (4 marks)

 

[Arrowshaft]

This was meant to be in our trials. What is this hahahaha

Do you have the answer to this? So I can check

 

[Anonymous54321]

Do you have the answer to this? So I can check

701.9nm

 

[Jig]

This was meant to be in our trials. What is this hahahaha

how do you even do this. Are you supposed to use snells law or something whatt...

 

[TheOnePheeph]

how do you even do this. Are you supposed to use snells law or something whatt...

Its just extremely badly worded, I believe you get the correct answer if you just say the angle is 45 degrees and it occurs at the 5th order maximum, since that way the two maxima 5 away from the center subtend an angle of 90. The diagram honestly makes the question more difficult with the weird zig zags.

 

[Arrowshaft]

Its just extremely badly worded, I believe you get the correct answer if you just say the angle is 45 degrees and it occurs at the 5th order maximum, since that way the two maxima 5 away from the center subtend an angle of 90. The diagram honestly makes the question more difficult with the weird zig zags.

Thats what i did but i didnt get 701.9 nm, rather 707.1 i believe

 

[TheOnePheeph]

Alright here’s the next question:

Explain the effectiveness of the Morgan-Keenan system in stellar classification. (4 marks)

This question seems a bit difficult to answer, but I will give it my best shot (Im assuming Morgan-Keenan refers to stellar spectral classes divided by temperature?) I would assume that you say:

-The Morgan Keenan system is extremely effective at separating stars into spectral classes based off of temperature. It allows for a separation by the colour for the star as well, since Wien's Law states that the peak wavelength is inverselvely proportional to the temperature of the star.

-When plotted on the HR diagram, the relative position of a star in the main sequence can also be determined. This allows for a determination of its absolute magnitude, which is related to luminosity and surface area, as well as the expected lifetime of the star and the predominant nuclear fusion reactions (p-p chain in smaller main sequence stars as opposed to CNO in larger main sequence). The position of a star in the main sequence can also be used to predict the formation of a black hole or neutron star in the stars future.

-Post main sequence stars can also be classified based on their colour and temperature, i.e. the position of red giants and white dwarfs. The sub classes help determine whether stars in these red and white regions are either main sequence stars or red giants/white dwarf due to their relative sizes. These also allow for a comparison of luminosities of stars, usually compared to the luminosity of our sun in practice.

Thats what i did but i didnt get 701.9 nm, rather 707.1 i believe

Yeah theres also a typo in the marking guidelines (This is fron the TEC exam). Instead of saying d*sin45 they say d+sin45. Really badly designed question lol.

 

[Arrowshaft]

Ah, but remember the MK system doesn’t just describe the usual OBAFGKM , it also has sub classes of luminosity (I-IV) (iirc) and temperature (0-9), e.g. the sun is a G2V star.

 

[TheOnePheeph]

Ah, but remember the MK system doesn’t just describe the usual OBAFGKM , it also has sub classes of luminosity (I-IV) (iirc) and temperature (0-9), e.g. the sun is a G2V star.

Lol we never learned this, sorry, I just assumed it was a name for the usual temperature classifications.

 

[Arrowshaft]

Lol we never learned this, sorry, I just assumed it was a name for the usual temperature classifications.

the atar notes topic test book covers it because there is a dot point exploring stellar classifications, but i dont think its super necessary, just an extra measure

 

[TheOnePheeph]

the atar notes topic test book covers it because there is a dot point exploring stellar classifications, but i dont think its super necessary, just an extra measure

Ok I will keep it in mind, I edited my original comment to include a bit on it, the answer feels super bad though.

 

[Arrowshaft]

That’s a good answer, you explored in detail how position affects energy production, luminosity and colour. I’d say that’s full marks

 

[Arrowshaft]

Also did you say the position helps predict the formation of a black hole or supernova? Because the black hole arises from the supernova if the mass of the star is between 8-50 solar masses, otherwise it becomes a neutron Star. Other stars die by pulsating their outer layers out to form a planetary nebula leaving behind a white dwarf

 

[TheOnePheeph]

Also did you say the position helps predict the formation of a black hole or supernova? Because the black hole arises from the supernova if the mass of the star is between 8-50 solar masses, otherwise it becomes a neutron Star. Other stars die by pulsating their outer layers out to form a planetary nebula leaving behind a white dwarf

I meant neutron star not supernova hahahha. I swear my number one dumb mistake is getting distracted and writing the wrong word down in physics.

 

[Anonymous54321]

Could someone please explain the mechanics as to why this is A? I understand it has to do with Lenz's law but not sure how to approach it.

 

[Arrowshaft]

View attachment 27536

Could someone please explain the mechanics as to why this is A? I understand it has to do with Lenz's law but not sure how to approach it.

That was a stupidly worded q, i initially thought it was induction but its just RHR

 

[Arrowshaft]

How you do it though is, since the conductor is being moved right point your thumb to the right (as the current moves right). Now the magnetic field points inward, so orient your fingers into the page. You notice now that there is a force induced up the page by using your right hand rule. This force pushes the positive charge upward hence resulting in a current flowing up the page (X to Y).