HSC Physics Question Thread

[Cindy2k16]

What did you use for your peak wavelength? If you use something just a TOUCH lower, then you will get to the value required for B. To effectively detect human radiation, we'd want an energy gap close to the energy in the typical human-emitted photon. The value you calculated is closest to B (even if it is slightly below), so we'd prefer B over A in that circumstance. A's band gap is too low; so we'd presumably pick up radiation from other stuff!

I used 8micrometres for the peak wavelength. But I see why the answer is B. Thank you!

[Cindy2k16]

Hi for this question I used the formula starting with v_y² and presumed that it would hit the base of the house at max height (so i didn't work out the time) I got the required answer of 30m/s. but in the answers they found the time it would take and subbed it into the other formula. So would the way I worked it out be invalid since I had assumed it would hit at max height even though the questions didn't say it did?

[imtrying]

For the dot point 'discuss the concept that length standards are defined in terms of time in contrast to then original metre standard" what is it that we discuss? I understand that it was a change from 1 ten millionth of the distance between the equator and north pole  along the meridian to the distance travelled by light in a vacuum in a fraction of a second, but how exactly does this relate to relativity?

[jamonwindeyer]

For the dot point 'discuss the concept that length standards are defined in terms of time in contrast to then original metre standard" what is it that we discuss? I understand that it was a change from 1 ten millionth of the distance between the equator along the meridian to the distance travelled by light in a vacuum in a fraction of a second, but how exactly does this relate to relativity?

Hey! Well basically the only thing to discuss is that we are now defining the metre based on a more absolute standard. The speed of light is constant! Length is not constant according to special relativity. Therefore, if we use light to define the metre, then we are more accurate!

[jamonwindeyer]

Hi for this question I used the formula starting with v_y² and presumed that it would hit the base of the house at max height (so i didn't work out the time) I got the required answer of 30m/s. but in the answers they found the time it would take and subbed it into the other formula. So would the way I worked it out be invalid since I had assumed it would hit at max height even though the questions didn't say it did?

Hey Cindy! Unfortunately it would be; unless you could somehow proved that it had to strike at the point of maximum height! you'd probably just lose a mark

[znaser]

Hi. So this is a 7 marker from the 2007 hsc: Analyse how de Broglie's proposal and supporting experimental evidence led to the move from classical physics to quantum physics.
After outlining his proposal and Davission and Germer's nickel experiment, is it enough to say that the the treatment of electrons as waves resulted in the explanation of the shortcomings of Bohr's model (stability of electron orbits) and hence, the move from classical to quantum physics, or is there more to it? Thanks

[bethjomay]

Help! Is this a syllabus point I'm missing or are we meant to just figure it out?

[Goodwil]

Hi there. Why is the answer B and not C in this question?

[jakesilove]

Hi. So this is a 7 marker from the 2007 hsc: Analyse how de Broglie's proposal and supporting experimental evidence led to the move from classical physics to quantum physics.
After outlining his proposal and Davission and Germer's nickel experiment, is it enough to say that the the treatment of electrons as waves resulted in the explanation of the shortcomings of Bohr's model (stability of electron orbits) and hence, the move from classical to quantum physics, or is there more to it? Thanks

I think that's a completely sufficient answer! Go into a bit of depth with each of the points you've outlined, but I wouldn't recommend more

[jakesilove]

(Image removed from quote.)

Help! Is this a syllabus point I'm missing or are we meant to just figure it out?

Hey! This is a really tough question, really really tough. The motor part is just like usual; when we want to car to go forward, we pump energy into a motor which causes shit to rotate etc. Standard stuff. The important part of this question is to relate the fact that a motor is a generator, but in reverse. So, the car can convert electrical energy into kinetic energy, but it can also convert kinetic energy into electrical energy! By making the generator turn, energy is being sucked away, thus decreasing the overall speed of the vehicle. Throw in the term 'Lenz's law' etc., and describe how a motor/generator works, and you should get full marks

[jakesilove]

Hi there. Why is the answer B and not C in this question?

Because we're talking about WAVELENGTH, not frequency! Recall that when determining the maximum kinetic energy of an electron, it will have to do with the frequency of the photon that is incident. Frequency is proportional to 1/wavelength, so the function should look hyperbolic

[bethjomay]

Hey! This is a really tough question, really really tough. The motor part is just like usual; when we want to car to go forward, we pump energy into a motor which causes shit to rotate etc. Standard stuff. The important part of this question is to relate the fact that a motor is a generator, but in reverse. So, the car can convert electrical energy into kinetic energy, but it can also convert kinetic energy into electrical energy! By making the generator turn, energy is being sucked away, thus decreasing the overall speed of the vehicle. Throw in the term 'Lenz's law' etc., and describe how a motor/generator works, and you should get full marks

Ok, thank you! So the generator 'effect' is happening in the original electric motor, simultaneously? Both are happening at once?

[jakesilove]

Ok, thank you! So the generator 'effect' is happening in the original electric motor, simultaneously? Both are happening at once?

No, not quite; when the car 'speeds up', the apparatus will act as a motor. Electrical energy will be converted into kinetic energy, propelling the car forward. However, when the car needs to 'slow down', the apparatus begins to work as a generator. It will draw kinetic energy away, converting it back to electrical energy! So, whether it acts as a generator or a motor will depend on what actually needs to happen/what sort of outcome we want

[FallonXay]

Hi!

How would you do the calculations for the graph in part b?

Thanks~

[jakesilove]

Hi!

How would you do the calculations for the graph in part b?

Thanks~

Hey! Such a weird question, hey? It was from my paper, so I know it well You just need to make sure that the horizontal displacement of Ball Q stays consistent (ie 3 blocks). Then, you know that acceleration due to gravity adds a consistent amount of velocity per second. It starts by 'moving down' 3 blocks, and then 9 blocks, which means each second ADDS 6 blocks worth of down. So, the next vertical position should be 9+6=15 blocks further down! Absolutely stupid question, but it does make you think about the actual physics behind projectile motion You can also check the 'actual' maths on the BOSTES answer page, here