HSC Physics Question Thread

[jamonwindeyer]

Hi, I'll try answer this the best I can, splitting the question into 2 parts for 4 marks each.

Changes in gravitational energy
- Gravitational potential energy is defined as -GmM/R, therefore as the rocket LEAVES Earth, it's gravitational energy increases. However, the law of conservation of energy (dictating that the total energy in a closed system is constant), suggests that energy must come from another source. This other source is the Kinetic energy / chemical energy within the fuel of the rocket. As the rocket leaves Earth, the KE of the rocket decreases (increasing GPE). This is a problem. In order to maintain the level of KE and increase the velocity of the rocket, more fuel must be burnt and used up. This involves the firing of rocket boosters to provide a constant thrust for the rocket.
- When the rocket enters the G field of Mars, the reverse situation happens. It begins to accelerate, being attracted by the gravity of the planet, towards the surface. As a result of its acceleration, it converts GPE to KE very quickly, losing altitude but gaining heat (from friction) and velocity. (Law of conservation of energy). This again is a problem as a velocity too high is difficult to control, and may be fatal for the probe/occupants inside. In order to solve this problem, rocket boosters are fired in the opposite direction to slow the descent of the probe, reducing its KE as it's GPE decreases.

High Speed Electrical charged Particles
- High speed electrically charged particles coming from the sun are often come from solar flares (where the sun emits a burst of solar radiation). They are quite problematic as they can damage sensitive equipment within the spacecraft and also penetrate through the walls of the spacecraft, damaging other parts inside and may be harmful for the occupants. Alongside causing damage, these charged particles interfere with communication (much like the situation of an ionisation blackout in re-entry), which is another problem.
- In order to solve this problem, the outer layer of the spacecraft can be coated with a material which can be electrically charged to repel these high speed charged particles. Insulating material which prevents these particles from penetrating into the spacecraft can also be applied. Furthermore, communication signals can be sent from the backend or the spacecraft (or whichever end isn't being exposed to these high speed charged particles).

I hope thats enough to get 7 marks at least haha but that's my attempt

This is a fantastic answer to what was a really difficult question!!

This would almost definitely get the full 8 marks (though it is probably too long to replicate in exam conditions), some other things you could discuss:

- The specific issue of fuel consumption when discussing the changes in GPE
- Use of magnetic fields to deflect the charged particles

With these problems (where you are asked to analyse) be sure to relate to their real world implications! Why does the Physics pose a real world issue? That is what they are looking for in these sorts of questions

PS - These sorts of questions are why I can't wait for the new HSC syllabus!

[Neutron]

This is a fantastic answer to what was a really difficult question!!

This would almost definitely get the full 8 marks (though it is probably too long to replicate in exam conditions), some other things you could discuss:

- The specific issue of fuel consumption when discussing the changes in GPE
- Use of magnetic fields to deflect the charged particles

With these problems (where you are asked to analyse) be sure to relate to their real world implications! Why does the Physics pose a real world issue? That is what they are looking for in these sorts of questions

PS - These sorts of questions are why I can't wait for the new HSC syllabus!

Thank you Spencer and Jamon! Your responses have been extremely helpful Just with the fuel consumption thing though, doesn't the mass of the rocket decrease as it increases in altitude? Meaning the acceleration should increase? Why would the overall kinetic energy be decreasing (and thus why would more fuel need to be used?) Thank you!

[jamonwindeyer]

Thank you Spencer and Jamon! Your responses have been extremely helpful Just with the fuel consumption thing though, doesn't the mass of the rocket decrease as it increases in altitude? Meaning the acceleration should increase? Why would the overall kinetic energy be decreasing (and thus why would more fuel need to be used?) Thank you!

Ohhh yep good catch Neutron! Missed that in my midnight reading, your interpretation is 100% correct! The overall kinetic energy would increase, assuming that the change in mass causes a reduction that is lower in magnitude than the increase of velocity squared! Which is almost certain

[dylan862]

What exactly do the donor and acceptor levels do in semiconductors?

[jakesilove]

What exactly do the donor and acceptor levels do in semiconductors?

All you need to know is where they are in the conduction/valence band structure, which type is which (ie. which type of semi-conductor goes with each band) and be able to draw them. They basically make it 'easier' for electrons to jump from valence to conduction bands!

That's really the greatest complexity at which you need to understand this section.

Jake

[Neutron]

Yep that definitely makes sense but I think all this fatigue has gotten to my head. So with the particle moving, isn't it only in its reference frame where time dilates? So I'm using the example of like a spacecraft flying past the Earth, only the time on the spacecraft has dilated, the time on Earth is still proper. So using that logic, wouldn't the scientist's time be proper? Sorry if this is hella dumb :/

Neutron

I think this got lost in the jumble but could someone please explain this to me D: sorry

[jamonwindeyer]

I think this got lost in the jumble but could someone please explain this to me D: sorry

Sorry Neutron! So you are sort of right. Time dilates for the particles reference frame, but only with respect to another reference frame. If we were sitting on the particle we'd notice nothing at all, that is the tricky bit to understand. When we talk about time dilation, we talk about time in one reference frame moving more slowly than another. So when we measure the life span of the particle, it lives longer in our reference frame. If we measured the life span of the particle, from the reference frame of the particle, it would be the normal value at rest! That's why we say that our time is screwed up, and then when we bring the particle to rest, our two reference frames sync up again.

Does that sort of make sense? Might have to read it twice

[Neutron]

Sorry Neutron! So you are sort of right. Time dilates for the particles reference frame, but only with respect to another reference frame. If we were sitting on the particle we'd notice nothing at all, that is the tricky bit to understand. When we talk about time dilation, we talk about time in one reference frame moving more slowly than another. So when we measure the life span of the particle, it lives longer in our reference frame. If we measured the life span of the particle, from the reference frame of the particle, it would be the normal value at rest! That's why we say that our time is screwed up, and then when we bring the particle to rest, our two reference frames sync up again.

Does that sort of make sense? Might have to read it twice

So how come in this case, it isn't the reference frame of us sitting on the particle moving more slowly than the scientist observing the particle? I just keep envisioning the particle as a spacecraft instead (just because it's slightly easier to grasp) and isn't it normally the other way around? i.e. the thing that is moving has time travelling more slowly and the thing in the external frame of reference has the 'faster' time? Ah sorry Jamon

[jamonwindeyer]

So how come in this case, it isn't the reference frame of us sitting on the particle moving more slowly than the scientist observing the particle? I just keep envisioning the particle as a spacecraft instead (just because it's slightly easier to grasp) and isn't it normally the other way around? i.e. the thing that is moving has time travelling more slowly and the thing in the external frame of reference has the 'faster' time? Ah sorry Jamon

Okay let's spaceship it! So the spaceship is travelling at speed, and let's say we are watching a clock tick inside the spaceship (as a substitute for measuring the lifespan). So, we measure the clock on the spaceship from our frame of reference. Remember, there is no absolute reference frame for time, so we have to just compare it to our own. The clock on the ship will be slower than ours. Why? Because time slows down at speed. So when we look at the spaceships clock, it has slowed down, because it is moving at relativistic speed.

Keep in mind that on the spaceship, we could watch a clock on earth. If we are in the spaceship, the clock on Earth will be slower than our clock on the ship. What the hell is that about, it's backwards to what we just said?

That's relativity. Because in our frame of reference, the spaceship is moving at speed. In the spaceships frame of reference, the spaceship isn't moving at all, the Earth is! And thus, we see the clock moving at speed and thus, the clock moving slower.

So we will always view a moving reference frame as having slower time than ours, because the frame of reference we are in is always considered as stationary, and we compare to it.

So when the spaceship comes back to earth, its clock is no longer moving slower. So, it will tick through a minute faster, and so the lifetime (as equivalent) will be shorter.

I kind of feel like you understand this Neutron! Because the expression:

i.e. the thing that is moving has time travelling more slowly and the thing in the external frame of reference has the 'faster' time?

That is correct, the thing we observe has time travelling more slowly. So you get this. I think there is some subtle thing that's throwing you off (which is common with relativity). Read this a few times, does it help? I'm waiting for the "aha" moment

[Neutron]

Okay let's spaceship it! So the spaceship is travelling at speed, and let's say we are watching a clock tick inside the spaceship (as a substitute for measuring the lifespan). So, we measure the clock on the spaceship from our frame of reference. Remember, there is no absolute reference frame for time, so we have to just compare it to our own. The clock on the ship will be slower than ours. Why? Because time slows down at speed. So when we look at the spaceships clock, it has slowed down, because it is moving at relativistic speed.

Keep in mind that on the spaceship, we could watch a clock on earth. If we are in the spaceship, the clock on Earth will be slower than our clock on the ship. What the hell is that about, it's backwards to what we just said?

That's relativity. Because in our frame of reference, the spaceship is moving at speed. In the spaceships frame of reference, the spaceship isn't moving at all, the Earth is! And thus, we see the clock moving at speed and thus, the clock moving slower.

So we will always view a moving reference frame as having slower time than ours, because the frame of reference we are in is always considered as stationary, and we compare to it.

So when the spaceship comes back to earth, its clock is no longer moving slower. So, it will tick through a minute faster, and so the lifetime (as equivalent) will be shorter.

I kind of feel like you understand this Neutron! Because the expression:

i.e. the thing that is moving has time travelling more slowly and the thing in the external frame of reference has the 'faster' time?

That is correct, the thing we observe has time travelling more slowly. So you get this. I think there is some subtle thing that's throwing you off (which is common with relativity). Read this a few times, does it help? I'm waiting for the "aha" moment

Oh my god I get it :O Aha! hahaha i think it was the 'lifespan' bit which threw me off completely cause now that i re-read the question, it's quite obvious that the time given was the dilated one! I feel kinda dumb now cause lifespan is kinda like a clock ticking aye hahah thank you Jamon!!

[jamonwindeyer]

Oh my god I get it :O Aha! hahaha i think it was the 'lifespan' bit which threw me off completely cause now that i re-read the question, it's quite obvious that the time given was the dilated one! I feel kinda dumb now cause lifespan is kinda like a clock ticking aye hahah thank you Jamon!!

Ayyy I knew we'd get there! Yep I totally get it, definitely don't feel dumb, it took one of the greatest Physicists of all time to understand this at first you are most welcome!

[GeekModeEngaged]

Could you please check if I got this wrong or not? I though it was a relatively simple question and then BOSTES said I got it wrong. #storyofmylife
The question should be there via attachment. Any help will be greatly appreciated!

[jakesilove]

Could you please check if I got this wrong or not? I though it was a relatively simple question and then BOSTES said I got it wrong. #storyofmylife
The question should be there via attachment. Any help will be greatly appreciated!

Hey! So for this question, you basically just need to use the right hand rule. The thing is moving down, so by lenz's law the appropriate force will push it BACK UP to counteract the change. So, force is up. Then, the field is into the page. By the right hand rule, the current should thus be flowing to the right. However, remember that electrons move in the opposite direction to current! Therefore, they will build up on the left hand side, making the answer A

[RuiAce]

Lol that one was dodgy. Like Jake said, the answer is A.

You need to make sure that you've incorporated both Lenz's law, AND the fact we're interested in the flow of electrons. It becomes A because we double reversed the right-hand push rule

[GeekModeEngaged]

Another Question: not sure how to approach this one. I tried using Keplars' Law but it didn't get me anywhere because I don't know the mass of the star.
Sorry, but you're probably about to get a bunch of questions from me that I should probably find easy! I hope you don't mind too much as I am very grateful Thanks Again!