HSC Physics Question Thread

[Cindy2k16]

Not sure where to put this question but I was just wondering approximately what raw marks you need to get in the HSC exam for a band 5 or a band 6 in physics?

(Also thanks for the replies to the photoelectric question!)

[RuiAce]

Not sure where to put this question but I was just wondering approximately what raw marks you need to get in the HSC exam for a band 5 or a band 6 in physics?

(Also thanks for the replies to the photoelectric question!)

Consider the raw marks database

[Cindy2k16]

When plotting a graph in the exam, if the points provided don't include ones close to/at the x/y axis, do we extend our line down so that it does cut the x/y axis or only draw the line within the points? I've heard conflicting opinions about this from people of different schools so I'm not sure what to think. (also does the same apply in chemistry? i dont want to post the same question again in chem)

[jakesilove]

When plotting a graph in the exam, if the points provided don't include ones close to/at the x/y axis, do we extend our line down so that it does cut the x/y axis or only draw the line within the points? I've heard conflicting opinions about this from people of different schools so I'm not sure what to think. (also does the same apply in chemistry? i dont want to post the same question again in chem)

Hey! It genuinely doesn't matter. Personally, I would only ever plot within the range of the given values. If you're asked to estimate based on the plot, it will either only ask you for a point within that range, or explicitly tell you to extend your graph to estimate a higher/lower value. This applies to Chem and Physics; it just doesn't matter! I think it's 'better science' not to extend your graph beyond the data points you're given, though, unless they tell you to.

[RuiAce]

When plotting a graph in the exam, if the points provided don't include ones close to/at the x/y axis, do we extend our line down so that it does cut the x/y axis or only draw the line within the points? I've heard conflicting opinions about this from people of different schools so I'm not sure what to think. (also does the same apply in chemistry? i dont want to post the same question again in chem)

I'm interested in what Jamon and Jake have to say as well so I won't give a concrete answer. I only say yes to within the boundaries of the HSC as if your point extends too far, you can't do the question otherwise. Fortunately, the likelihood of being given a point out of the domain is unlikely in HSC physics and chemistry due to realistic absurdity.

I want Jamon/Jake to read this to input opinions but everyone else up to you

The absurdity arises from something that I only learnt recently. It is the difference between interpolation and extrapolation.

Given a set of data, we want to fit a polynomial of degree ?? to satisfy as many points as possible. In general, for the sake of the HSC course we're interested in fitting a linear function (a degree 1 polynomial, I.e. Straight line) through the points.

The line that we fit is, in general, good for points within the domain. If our data ranges from, say, a range of currents from 1 amp to 10 amps, then it's good to find something for 3 amps. This is interpolation - taking something out from within the boundaries.

Extrapolation is different and is what's considered here. Extrapolation is the act of relying on your polynomial to determine something OUTSIDE the boundaries.

In general, this causes havoc. Because the behaviour of a substance might change outside of reasonable boundaries and you actually need a completely new model for it. An example is in how germanium starts becoming a conductor and not a semiconductor.

That being said, the questions in the HSC are usually designed so that the region of extrapolation is not massive enough to cause problems. These issues would've been monitored in advance.

[jakesilove]

I'm interested in what Jamon and Jake have to say as well so I won't give a concrete answer. I only say yes to within the boundaries of the HSC as if your point extends too far, you can't do the question otherwise. Fortunately, the likelihood of being given a point out of the domain is unlikely in HSC physics and chemistry due to realistic absurdity.

I want Jamon/Jake to read this to input opinions but everyone else up to you

The absurdity arises from something that I only learnt recently. It is the difference between interpolation and extrapolation.

Given a set of data, we want to fit a polynomial of degree ?? to satisfy as many points as possible. In general, for the sake of the HSC course we're interested in fitting a linear function (a degree 1 polynomial, I.e. Straight line) through the points.

The line that we fit is, in general, good for points within the domain. If our data ranges from, say, a range of currents from 1 amp to 10 amps, then it's good to find something for 3 amps. This is interpolation - taking something out from within the boundaries.

Extrapolation is different and is what's considered here. Extrapolation is the act of relying on your polynomial to determine something OUTSIDE the boundaries.

In general, this causes havoc. Because the behaviour of a substance might change outside of reasonable boundaries and you actually need a completely new model for it. An example is in how germanium starts becoming a conductor and not a semiconductor.

That being said, the questions in the HSC are usually designed so that the region of extrapolation is not massive enough to cause problems. These issues would've been monitored in advance.

Rui, your explanation is essentially why I said it isn't 'good science' to continue your sketch beyond the data points given. Consider Hooke's law, which looks like this over large enough delta x.



Most experiments only let you take measurements in the first region. So, should you draw a straight line into infinity? Obviously not; that doesn't represent the physical phenomena. This occurs in probably every non-electrodynamic experiment (I believe) due to relativistic effects. So, if it's not a good idea some of the time, it isn't a good idea any of the time. Obviously this strays beyond HSC physics, but does explain why I wouldn't recommend extrapolating unless you're asked to in an exam.

[RuiAce]

Rui, your explanation is essentially why I said it isn't 'good science' to continue your sketch beyond the data points given. Consider Hooke's law, which looks like this over large enough delta x.

(Image removed from quote.)

Most experiments only let you take measurements in the first region. So, should you draw a straight line into infinity? Obviously not; that doesn't represent the physical phenomena. This occurs in probably every non-electrodynamic experiment (I believe) due to relativistic effects. So, if it's not a good idea some of the time, it isn't a good idea any of the time. Obviously this strays beyond HSC physics, but does explain why I wouldn't recommend extrapolating unless you're asked to in an exam.

They do say that in the real world extrapolation is one of the most dangerous things you can do.

[jakesilove]

They do say that in the real world extrapolation is one of the most dangerous things you can do.

They don't call it extrapolation for nothing

[Cindy2k16]

Thanks for the replies! It occurred to me because in the biology exam they didn't give us plots near the x/y axis and I only plotted the line within the points but it suddenly occurred to me that what if i should've extended it? I've heard teachers say extending it isnt good science but some teachers say that extending it is what the HSC markers want, even though it's not necessarily 'good science'. I've heard teachers who do HSC marking both say not to extend and also the opposite :/

[jakesilove]

Thanks for the replies! It occurred to me because in the biology exam they didn't give us plots near the x/y axis and I only plotted the line within the points but it suddenly occurred to me that what if i should've extended it? I've heard teachers say extending it isnt good science but some teachers say that extending it is what the HSC markers want, even though it's not necessarily 'good science'. I've heard teachers who do HSC marking both say not to extend and also the opposite :/

Yeah look, you're absolutely right, and different teachers have different preferences. The fact is that you'll get full marks no matter what you do, because teachers teach differently.

[Happy Physics Land]

Rui, your explanation is essentially why I said it isn't 'good science' to continue your sketch beyond the data points given. Consider Hooke's law, which looks like this over large enough delta x.

(Image removed from quote.)

Most experiments only let you take measurements in the first region. So, should you draw a straight line into infinity? Obviously not; that doesn't represent the physical phenomena. This occurs in probably every non-electrodynamic experiment (I believe) due to relativistic effects. So, if it's not a good idea some of the time, it isn't a good idea any of the time. Obviously this strays beyond HSC physics, but does explain why I wouldn't recommend extrapolating unless you're asked to in an exam.

Oh no material testing experiments and Hooke's law ... XD where are my yield stress maximum tensile stress and necking point??

[jakesilove]

Oh no material testing experiments and Hooke's law ... XD where are my yield stress maximum tensile stress and necking point??

.........

[nay103]

Hey quanta peeps,
I was doing 2009 HSC and came across this question:

31) a) ii) Describe TWO problems associated with Rutherford’s model and how these were explained by Bohr’s model of the hydrogen atom. (4)

I could only think of the fact that Rutherford's model didn't configure electrons into orbits, so I looked at the textbook answer. It said that Rutherford didn't explain how negative electrons weren't pulled into the positive nucleus, and Bohr solved this by saying that an electron can orbit a nucleus like a planet around the sun, where angular momentum is quantised. But isn't this wrong, since electrons are actually just standing waves?

How else would I be able to answer this question?

Thanks!

[Cindy2k16]

Hi could someone clarify how to ensure/determine the reliability, accuracy and validity of secondary sources? I think reliability is consistency in information across multiple reputable sources but I'm less sure about the difference between accuracy and validity for secondary sources.

[Cindy2k16]

For this question I wrote that the rotation of the space station would provide a centripetal force that acts on the astronaut causing the astronaut to accelerate towards the centre and thus stimulates gravity. Is that enough to answer the question?
In the sample answer it mentions Newton's 3rd Law- is that necessary to mention? If so, how does the reaction force show that gravity is stimulated because I don't really understand that.
Thanks in advance