HSC Physics Question Thread

[jakesilove]

Thankyou!! I can't ever recall learning about donor/acceptance will definitely add it to my notes

Yep, that's not uncommon; most teachers/tutors/notes leave it out, because its rarely assessed and generally doesn't make much sense (at least at the level of physics taught in the HSC!). Definitely worth knowing though, so great job finding a question like that whilst doing past papers! It's the weird, auxiliary sections that catch students off guard

[Adriaclya]

Hi!
Is there an explanation as to why if an electric field exists, so too a magnetic field?  Is there a reason why electricity and magnetism is strongly linked?
Thanks!

[jakesilove]

Hi!
Is there an explanation as to why if an electric field exists, so too a magnetic field?  Is there a reason why electricity and magnetism is strongly linked?
Thanks!

Hey!

Look, to answer your question properly would take a semester of University, a hell of a lot of maths and some history lessons at a bare minimum. Essentially, though, the answer to your question is simple; Electricity and Magnetism are the same force. Up until 1873, Electricity and Magnetism were studied completely separately, and thought of as two of the fundamental universal forces. In steps Maxwell, a brilliant mathematician and Physicist, who essentially united the two forces into electromagnetism, formalised in Maxwell's equations. Electricity and Magnetism are two sides to the same coin; one necessarily implies the other. It just took us a while to realise it!

This isn't the most satisfying answer out there (if you want a better answer, try an electromag course at University!), but it is an answer. If an electric field exists, a magnetic fields exists, because it does, and that's how the universe works. It's almost like asking why the speed of light is 2.998*10^8; I can explain why it's a constant, I can explain what it implies, I can explain how it impacts us and I can use it to my advantage. I CAN'T explain why it is that number in particular (at least, not with the physics I know right now!). The universe is a funky place, with some fundamental laws. You've stumbled across one of them, and if the year you graduate in is correct, you've stumbled upon it really really early in your Physics career. So, congratulations, and good luck coming to terms with some of Physics' fundamental truths.

Jake

[Meckenza]

Hi,

I was  just wondering - why does a graph plotting g-forces against time in a spacecraft have a non-linear relationship?

Thanks.

[Adriaclya]

Thank jake!
Whoops. Grad year 2017....

Also, is this correct:
Wind turns wind turbines = turns shaft= turns magnetic coil=generates current?
And for solar panels:
Light photons=knock electrons of silicon layer which has been doped in phosphorus=negative terminal=electrons carried through the silicon conductor to wires for use=comes back to silicon layer (that has been doped with boron)=electrons reset for use again??
Thanks once again!

[jakesilove]

Hi,

I was  just wondering - why does a graph plotting g-forces against time in a spacecraft have a non-linear relationship?

Thanks.

Hey!

So, when plotting g-force against time, we have to think about what is actually happening to the shuttle. So, first let's look at the formula for g-force.



Where g is the force due to gravity, and a is the acceleration of the rocket.

So, since g is approximately constant in the initial stages of the launch (it will get weaker as the rocket moves away from the earth, but let's ignore this as it will hardly contribute to the graph, at least for small t), what we really want to know is the values for a. If a increases linearly, so will the g-force. If a decreases linearly, so will the g-force. And, if a increases non-linearly (as we will soon find that it does), so will the g-force.

Now, we know from Newton's laws that



This is how we figure out what the acceleration of the rocket is. The burning of fuel creates a fairly constant force downwards and, by principles of conservation of momentum, the rocket is therefore propelled upwards. However, let's think about what's actually happening. As the fuel is being burnt, resulting in a constant force (ie. the left hand side of the Newton's equation is constant), the mass of the rocket is decreasing by quite a lot. This is because most of the mass of most rockets is made up of fuel! So, over time, the mass is decreasing. However, if the left hand side of the equation is constant over time, the right hand side must be as well! So, if the mass is going down, the acceleration must be going up. In other words, acceleration increases over time.

Huh. Except, when you reach this point, you expect acceleration to increase linearly over time. Which means, as I've described above, that the g-force should increase linearly over time.

Damn.

Okay, so turns out I was wrong, but I'm going to leave my answer above for completeness, and to show the logical steps you should be taking (and, presumably, you did take, in order to reach the linear conclusion).

In that case, it is likely because g decreases as t increases (the rocket gets further from the earth). This will result in a non-linear relationship. Also turns out to be a way easier solution! Sorry for the rambliness but hopefully this all made sense; really great question!

Jake

[jakesilove]

Thank jake!
Whoops. Grad year 2017....

Also, is this correct:
Wind turns wind turbines = turns shaft= turns magnetic coil=generates current?
And for solar panels:
Light photons=knock electrons of silicon layer which has been doped in phosphorus=negative terminal=electrons carried through the silicon conductor to wires for use=comes back to silicon layer (that has been doped with boron)=electrons reset for use again??
Thanks once again!

Totally correct! Are you doing accelerated Physics or something? That's definitely not in the Prelim curriculum...

[Adriaclya]

Thanks jake.
Nope, just further reading haha!

[jakesilove]

Thanks jake.
Nope, just further reading haha!

That's seriously cool (I'm a nerd, and Physics is cool. Hopefully you agree!). Feel free to ask me anything, anytime, whether it's part of your/a curriculum or not Glad to see your enthusiasm!

[Swagadaktal]

Thanks jake.
Nope, just further reading haha!

Yo when you're free hmu with some of that motivation/enthusiasm. Need some in yr 12 tbh

[jamonwindeyer]

So, to your actual question. Based on the location of the additional band, we're expecting a P-type semiconductor. This means it has been doped with a Group 5 element. The answer, therefore, is Boron.
Jake

Hey guys! Answer doesn't change for above, but in case it causes confusion later, this should read "Group 3 element"

[RuiAce]

Hey guys! Answer doesn't change for above, but in case it causes confusion later, this should read "Group 3 element"

I thought you were the chemist Jake
_______________________

I remember when I did further reading for maths... but now it's like stick to what you've learnt

[Spencerr]

Hi there!

Could someone please explain to me the concept of mass defect and binding energy in the context of nuclear fission. E.g. If a Uranium nuclei splits into two daughter nuclei and releases a large amount of energy where does that energy come from? and does nuclear fusion also release energy?

[jakesilove]

Hi there!

Could someone please explain to me the concept of mass defect and binding energy in the context of nuclear fission. E.g. If a Uranium nuclei splits into two daughter nuclei and releases a large amount of energy where does that energy come from? and does nuclear fusion also release energy?

Hey!

I can't answer much of your question, because I didn't do this option. I can briefly explain the whole 'mass defect' idea, I think, based on my knowledge of Physics.

When a nuclei splits, and releases energy, the energy itself actually comes from the mass loss. So, the 'mass defect' and energy release is the same! For instance, say the original nuclei weighed 1g (which is ridiculous, just using it for ease but obviously they weigh a lot less) and the two daughter nuclei weigh 0.4g each. Clearly, if you add the daughter nuclei up, you've lost 0.2g somewhere! In comes Einstein, with the most equation in Physics



We can sub the 0.2g of lost matter into this formula, and out pops a huge amount of energy! And that's where the energy release comes from

Hopefully someone else on the forum can help you with the rest!

[conic curve]

Hi there!

Could someone please explain to me the concept of mass defect and binding energy in the context of nuclear fission. E.g. If a Uranium nuclei splits into two daughter nuclei and releases a large amount of energy where does that energy come from? and does nuclear fusion also release energy?

Is this Quanta to Quarks?