HSC Physics Question Thread

[jamonwindeyer]

Just to clarify, for 15, the holes move in same way as electrons because they are relatively positive but not considered positively charged? And they wont speed up or slow down since to do so, the force must be in the same direction of motion? In this case it's down?

Close - They are relatively positive so we do consider them as positive charges. Normally, the force on an electron and hole would be in opposite directions - If they were stationary. Here, they are moving in opposite directions, so the fact they are moving in opposite directions and opposite in charge cancels out, and they move in the same direction. You can deduce all this with the right hand slap rule, but it is faster to just use intuition and yes, to speed them up or slow them down you would need forces parallel to motion - It doesn't really make sense as an answer in this context

[katnisschung]

hi
question for quanta to quarks regarding nuclear decay/radiation

how do i determine what type of radiation is emitted by the element given its altered atomic weight.

i.e. potassium-40
thorium-232
radium-226
iodine-131

so in my notes i have written the rules to determine what type of radiation emitted to be the following:
alpha decay--> occurs for elements too big, atomic number greater than 83

beta minus decay--> occurs for elements where there are too many neutrons compared to protons, in larger
atoms where neutron:proton ratio is 1.5:1

so i did the calculations and the rules all work as i checked my answers but radium-40 undergoes beta minus decay according to the answers but the ratio is no where near 1.5:1 its more 1.1....... could someone clarify thanks

[kiwiberry]

hi
question for quanta to quarks regarding nuclear decay/radiation

how do i determine what type of radiation is emitted by the element given its altered atomic weight.

i.e. potassium-40
thorium-232
radium-226
iodine-131

so in my notes i have written the rules to determine what type of radiation emitted to be the following:
alpha decay--> occurs for elements too big, atomic number greater than 83

beta minus decay--> occurs for elements where there are too many neutrons compared to protons, in larger
atoms where neutron:proton ratio is 1.5:1

so i did the calculations and the rules all work as i checked my answers but radium-40 undergoes beta minus decay according to the answers but the ratio is no where near 1.5:1 its more 1.1....... could someone clarify thanks

Hiya!
You're right about alpha decay. Beta minus decay does occur when the n:p ratio is too high, however there is a different limit to how high this can get depending on the size of the atom

From this graph, an atom will be unstable if its n:p ratio is above about:
- 1:1 for atomic number<20
- 1.3:1 for atomic number\(\approx \)50
- 1.5:1 for atomic number\(\approx \)80

Did you mean potassium-40? Assuming you did, potassium has an atomic number of 19, so it will undergo beta decay with an n:p ratio of above 1:1

Hope this helps!

[austv99]

What's the best appraoch for this question? And in general? Excluding the example.

[jamonwindeyer]

What's the best appraoch for this question? And in general? Excluding the example.

You could discuss pretty much any of the experiments/scientists you've studied that lead to changes in our understanding of how things work. Particularly:

- Einstein's Special Theory of Relativity (particularly this one)
- Michelson and Morley
- Thompson's Experiment with Cathode Rays

You could discuss those, explaining what the model was, how the experiment was tested and how it validated/altered the model

[pikachu975]

What's the best appraoch for this question? And in general? Excluding the example.

This is how they validate models:

Observation -> Problem Raised -> New hypothesis -> Experiment -> Collection of data -> Analysis of results -> Results support hypothesis (if not then back to new hypothesis) -> New theory or law

from 2007 HSC

[austv99]

Would appreciate help with 26c
With 27a, would i talk about the uv catastrophe and explain the existence of the peak and the graph going towards zero?

[kiwiberry]

Would appreciate help with 26c
With 27a, would i talk about the uv catastrophe and explain the existence of the peak and the graph going towards zero?

26) The power loss will be the difference between the input power and the output power:
Watts=joules/s, so the energy lost in 8 hours will be

27) Yep! Make sure to mention Planck's hypothesis in there somewhere

[pikachu975]

Would appreciate help with 26c
With 27a, would i talk about the uv catastrophe and explain the existence of the peak and the graph going towards zero?

Yep for 27 first speak about what classical thought, basically the UV catastrophe how at shorter wavelengths there would be faster oscillations hence infinite energy which wasn't possible according to conservation of energy. Then introduce Planck's quantisation of energy emitted by a black body cavity as E = hf and explain how experiments showed definite peaks, which was due to some transitions being more probable than others. Speak about how he explained the UV catastrophe because there is a maximum amount of shells an atom can jump (quantised energy levels).

[beau77bro]

hey so in medical, it talks about x, y, z axes. the z axis has a gradient magnetic field which alters larmor frequency, but the x-axis changes the precession frequency. what is the difference between the larmor frequency and precession frequency?

[jamonwindeyer]

hey so in medical, it talks about x, y, z axes. the z axis has a gradient magnetic field which alters larmor frequency, but the x-axis changes the precession frequency. what is the difference between the larmor frequency and precession frequency?

Hey! So I wouldn't worry about this in toooo much detail, it's a complicated bit of Physics we're dealing with here. Basically, the magnetic fields in the MRI make it so that every single position in the body is subjected to a magnetic field that is slightly different in terms of either strength or direction - This will slightly change the frequency and phase of radio waves emitted by the resonating hydrogen nuclei. This is what they mean by frequency (I believe, my understanding here isn't much greater than yours!). Incidentally, the y-axis is used to control phase

Probably not the best answer, and someone else might have done wider reading than I and be able to help more!! But understanding broadly was always enough for me and will absolutely be enough to get the marks in an MRI question

[katnisschung]

hi
I forgot the rule for how many significant figures u need to take in your answer??
was it sth like u take the most in your answer as the most in the question?

[Natasha.97]

hi
I forgot the rule for how many significant figures u need to take in your answer??
was it sth like u take the most in your answer as the most in the question?

Yep! You take the least accurate value given in the question (e.g. if given 0.3836 and 0.40, do 2 s.f.)

[katnisschung]

what is the speed for matter waves?

I got this rather simple question wrong when revising content (probably becos i don't know quanta to quarks well enough)
calculate the frequency of the matter wave when a neutron is made to move at 53.6x10^3 m/s
so why is it that they take 'c' as the speed. do all matter waves travel at c

[austv99]

Would appreciate help with this question
2013 HSC 27b