HSC Physics Question Thread

[jakesilove]

With the dotpoint about Hertzs observations with radio waves producing a photoelectric effect, would I be correct in saying that the fact that the spark stopped when UV was blocked by glass showed that it was preventing the photoelectric effect from happening because no UV light was hitting the receiver meaning it was no longer emitting electrons which therefore meant there was no spark?

Actually, the sparking wasn't stopped by the glass, it was only dampened! This showed that whatever was causing the spark (ie. UV rays) could travel through material such as glass.

[Cindy2k16]

Actually, the sparking wasn't stopped by the glass, it was only dampened! This showed that whatever was causing the spark (ie. UV rays) could travel through material such as glass.

Hi Jake I've also heard that Hertz put his receiver coil in a dark container and found that the sparks were dampened. And that when he exposed the receiver to UV light, sparks were more readily induced. Is this also correct to talk about when referring to Hertz's observation of the photoelectric effect? TIA

[MysteryMarker]

Hey for Q20 of 2006, could someone show me how to do part d?

http://prnt.sc/d0py3e

http://prnt.sc/d0pyhr

Cheers.

[Brenda0708]

Confused as to which l is which??? Doesn't length contract in the stationary observers view not for the person moving??

[jakesilove]

Hi Jake I've also heard that Hertz put his receiver coil in a dark container and found that the sparks were dampened. And that when he exposed the receiver to UV light, sparks were more readily induced. Is this also correct to talk about when referring to Hertz's observation of the photoelectric effect? TIA

Absolutely right! He did a few experiments like that, with the results you described, but with no real analysis of what he was observing

[Brenda0708]

I keep getting stuck on projectile q LOL please help for 19b) not sure where to start

[Mei2016]

Hi i can help with some of these questions (i dont do quanta to quarks)

-In 2006 paper, in Q9, the answer is B and do they mean clockwise or anticlockwise?
(is it anticlockwise because charges would move clockwise Y to X, however, it's an induced current so due to Lenz's law it would flow the opposite direction, so anticlockwise?)

It is clockwise from Y to X. When you apply the right hand palm rule, you point the thumb in the direction that the wheel is turning through the magnetic field since that is the direction of movement of positive charge. The movement of the wheel through the magnetic field is basically the movement of the 'positive charge' in the metals as they are being moved with the wheel. Does this make sense? And then you point ur fingers in the direction of the magnetic field and you will get a downwards force on the positive charges. Thus current flows down out of the wheel through Brush Y, through the external circuit, to X.

And for 2013, Q16, why is the answer D and not C?, because isn't it in metals that Valence Band and Conduction Band are both full at the same time. The question doesn't specify what band it is 'electrons in a fully filled band' so I wasn't quite sure on this one.

It isn't true that in conductors both the valence band and conduction band are both full at the same time. In fact too many electrons in the conduction band make it too crowded and in fact makes it harder for current to flow, I think. Sorry I dont know a very scientific explanation.. But in conductors the conduction band is partially filled. So its C.
Hope this helps

Hi, thanks for the help. It's just that for the 2006 question, the answers mentioned Lenz's law so I got a bit confused there.   Thanks

[Cindy2k16]

(Image removed from quote.)

I keep getting stuck on projectile q LOL please help for 19b) not sure where to start

HI I would start by calculating the acceleration the electron experiences inside the electric field. Use F=ma for this, and sub in the force you found in a and the mass of an electron to find a.
Then draw the triangle for the initial velocity to determine the initial horizontal and vertical velocity of the electron at A.
Once you have all these values, you can act as if this is a normal projectile motion question except the acceleration is not 9.8, but the 'a' you calculated.

To find the time taken to travel from A to D, that's basically working out the time of flight. Use the formula starting with 'change in y' (the triangle with 'y' next to it). Sub in the change in y as 0 (since overall, the vertical displacement of the particle from A to D is 0), sub the initial vertical velocity, the acceleration you calculated and then rearrange the equation to find t. One value of t will be 0 (since the particle has 0 vertical displacement at the beginning too) and the other value will be the time it took to reach D.

Hope this helps

[Mei2016]

(Image removed from quote.)

I keep getting stuck on projectile q LOL please help for 19b) not sure where to start

Hi, how I would do this is follow Cindy2k16's way of getting the acceleration from F=ma and getting the y-component of the velocity with the triangle.
But an easier way to do the projectiles part is use v=u+at, where v=-u as this is using the symmetry of the parabolic arc and by using the a from above,
t=-2u/a  [ie t=(-2x6x10^6sin60)/(-1.7587x10^14)]

Hope this helps 

[Mei2016]

(Image removed from quote.)

Confused as to which l is which??? Doesn't length contract in the stationary observers view not for the person moving??

So for these kinds of questions, always think: who's moving at relativistic speeds? When moving at such high speeds, recall that you experience time and mass dilation and length contractions. So since you're moving in this question, the length you measure is always shorter than the real length, so we are trying to calculate Lv. (note the real length is stated as a fact in the first sentence and this is Lo).

Hope this helps. 

[Albertenouttaten]

Hi can anyone help me answer this question, I can't think of what to write:

In a particle accerlator called a synchroton, magnetic fields are used to control the motion of an electron so that it follows a circular path of fixed radius (i know this would probably require me to use either centripetal force of acceleration formula)

Describe the changes required in the magnetic field to accelerate an electron to near the speed of light. Support answer with appropriate mathematical relationships.

Thanks to anyone who is able to put any sort of input at all, any help is greatly appreciated.

[jakesilove]

Hi can anyone help me answer this question, I can't think of what to write:

In a particle accerlator called a synchroton, magnetic fields are used to control the motion of an electron so that it follows a circular path of fixed radius (i know this would probably require me to use either centripetal force of acceleration formula)

Describe the changes required in the magnetic field to accelerate an electron to near the speed of light. Support answer with appropriate mathematical relationships.

Thanks to anyone who is able to put any sort of input at all, any help is greatly appreciated.

Hey! So essentially what this question is asking is the method by which we can accelerate electrons, using a magnetic field. We know that the force due to a magnetic field is



Where B is the magnetic field strength. Obviously q is fixed (it's the charge of the electron), so we can only really vary the magnetic field strength. Now, if we apply a constant magnetic field (in terms of direction, at least), it will provide a centripetal force. So, we want the magnetic field to continuously be in the same direction, but we want to keep increasing it's strength.

How do we increase the strength of a magnetic field? Well, assuming that they are electromagnets, we would want to put more and more current through the wiring! Additionally, if we used superconductors, we would get a seriously strong field. You can really talk about whatever you want in a question like this; as you can see, I'm rambling, because I'm making up the answer as I go aha.

You could potentially discuss relativistic effects, such as mass dilation, as problematic, however that's just the icing on the cake

[Cindy2k16]

Hi for 2010 q32
"Two significant problems that will affect a manned spaceflight to Mars are:
-the changes in gravitational energy
- protecting the space vehicle from high-speed electrically charged particles from the Sun
Use your understanding of physics to analyse these problems"
8 marks

How could you answer this question? is it necessary that we know about the gravitational field/atmosphere etc of Mars?
TIA

[jakesilove]

Hi for 2010 q32
"Two significant problems that will affect a manned spaceflight to Mars are:
-the changes in gravitational energy
- protecting the space vehicle from high-speed electrically charged particles from the Sun
Use your understanding of physics to analyse these problems"
8 marks

How could you answer this question? is it necessary that we know about the gravitational field/atmosphere etc of Mars?
TIA

Is this part of an option? Unfortunately, I won't be able to help you out, but hopefully someone else can!

[JemexR]

Hi, for HSC 2005 Q20, when it mentions reliability, in what sense are we meant to respond in? Given that reliability is INCREASED by repetition, it is asking us to "describe how you assessed the reliability of the information you found"; I can only think of accurate source (reliable, i.e. from a scientific institution) at the present moment, but it's worth 6m :/