HSC Physics Question Thread

[jamonwindeyer]

Hi,

Does anyone know what the voltage time graph looks like for an AC generator which is turned twice as fast?

Thanks.

Hey! You'll get a similar looking (co)sine wave, but the period will be halved (because it is spun twice as quickly), and the amplitude will double, because the rate of change of magnetic flux is also doubled

[scienceislife]

What were some of the errors present in Hertz's experiment and how did he reduce them?

[beau77bro]

Exactly correct on all counts

Hmm, that's an interesting question! I believe that using a superconductor for the coil wouldn't actually achieve the desired effect, because superconductors exclude magnetic flux. This means that you don't get forces on current carrying wires in a magnetic field - Because the current doesn't interact with the magnetic field. I could be wrong there - Perhaps the Meisner effect would manifest in such a way as to create the force by some other means. I'm honestly not sure!

What we can (and do as of fairly recently I believe) use superconductors for in motors is to generate the magnetic fields. Superconductors, having zero resistance, are an extremely efficient way to generate powerful magnetic fields - Because they can carry HUGE amounts of current and that is with, again according to the formula, no power loss!!

I wrote a guide on Superconductors here - The BCS theory is really tough to understand just from reading though. Not sure of any good video explanations - I reckon some of the Notes in the free notes section would have good diagrammatical ones! Otherwise, a handy Youtube search will set you straight I am sure

Edit: Rathin made some recommendations here

OMG Im so sorry i missed this, thankyou sooo much jamon awesome explanation and im gonna look at the link right now thankyou soooo much

[bsdfjnlkasn]

Hey sorry if this is really silly, but will a moving charge in a changing electric field experience a force?
I'm just trying to think of an explanation (since I can't find one) as to why electrons in a discharge tube accelerate/decelerate according to an increasing/decreasing electric field around them. Is this because of a concept similar to electric flux? Or should I just accept that this is how striations are explained in the cathode ray experiment.

Any help would be super appreciated, thank you!!

[jamonwindeyer]

Hey sorry if this is really silly, but will a moving charge in a changing electric field experience a force?
I'm just trying to think of an explanation (since I can't find one) as to why electrons in a discharge tube accelerate/decelerate according to an increasing/decreasing electric field around them. Is this because of a concept similar to electric flux? Or should I just accept that this is how striations are explained in the cathode ray experiment.

Any help would be super appreciated, thank you!!

Hey! Never a silly question

Moving charges experience a force due to an electric field just the same as they would if stationary. This also goes for if the field is changing. So, according to \(F=Eq\), if the field doubles in strength, the force doubles in strength. It is a direct relationship, it's different to something like induction where the change itself (or the rate of change) is what causes the forces involved. Here, it is just the electric field interacting with charge (be it stationary or moving). If field gets bigger, force gets bigger

Exactly what scenario in the cathode ray section is confusing you? Is it the deflection plates/coils?

[jamonwindeyer]

What were some of the errors present in Hertz's experiment and how did he reduce them?

Hey! I don't think I'm aware of any errors in the experiment besides inaccuracy of measuring equipment and basic human error, both of which you resolve just by being careful

That question (if it is a question from an exam) could be indirectly asking about the calculation of the velocity of the radio waves. To be most accurate, Hertz set up a reflected standing wave and measured the distance between subsequent nodes/anti-nodes (essentially, measured the wavelength of the wave). He could use this, combined with the known frequency of his transmitter, to determine velocity with the wave formula \(v=\lambda f\), and this is much more accurate than measuring with the tools that he had access to

Would love for others to chime in if they have more

[beau77bro]

We just had this question in the half yearly. What is the answer and why Idgi. Thankyou very muchhhh

[bsdfjnlkasn]

(Image removed from quote.)

We just had this question in the half yearly. What is the answer and why Idgi. Thankyou very muchhhh

I think the answer might be A) as it's the only one which obeys the law of conservation of energy. Considering that you can't create nor destroy energy, you know that that as one form increases another must decrease to ensure that the total energy in a system stays the same. So let's apply this to the problem. When the rocket is sitting on the launch pad, it's KE = 0 as it's not moving. We're told that the rocket rises with constant thrust so we we're looking for linear relationships. The faster the rocket goes, the more kinetic energy it has meaning that the potential energy (gravitational in this case) will have to decrease as the total energy in the system has to remain constant. You can show this with the first graph by superimposing the two lines. You will end up with a horizontal line at the height of KE @ t=0 i.e. a constant amount of energy. This obeys the law of conservation of energy unlike any of the others which means a) is our answer.

Hopefully this helped!

[bsdfjnlkasn]

Hey, can I please get some help with the following question 

Describe what happens when electrons hit the screen of a CT. What then happens to the electrons?

I can't seem to find any information on this in my textbook so was wondering if I could get an explanation. My first answer was that the electrons would hit the fluorescent screen and as a result of the collision emit light (energy transfer: E_k to heat and light). This also sort of explains why increasing the frequency of admitted electrons corresponds to a brighter on-screen display.

And now, with following the electrons after they've hit the screen, do they just you know disappear? I suspect the energy used up in the collision is the same amount as their kinetic energy and I might be missing a link here, but are the electrons effectively reduced to this light energy? I just don't know where they'd go haha, like collect at the bottom of the fluorescent screen? Idk it just seems silly thinking about it that way.

Any input would really help me clarify this problem and if someone could advise me how important this detail is, that'd be a good way to put things into perspective as I don't really see this as being relevant to I2I but 

Thank you!!

[bsdfjnlkasn]

Hey! Never a silly question

Moving charges experience a force due to an electric field just the same as they would if stationary. This also goes for if the field is changing. So, according to \(F=Eq\), if the field doubles in strength, the force doubles in strength. It is a direct relationship, it's different to something like induction where the change itself (or the rate of change) is what causes the forces involved. Here, it is just the electric field interacting with charge (be it stationary or moving). If field gets bigger, force gets bigger

Exactly what scenario in the cathode ray section is confusing you? Is it the deflection plates/coils?

Hey Jamon, thank you! That makes things so much simpler than M&G

We recently covered the discharge tubes prac and had to observe how the different gas pressures influenced the ray produced. One of the features we looked at was striations which appear as pressure is reduced - this made me wonder if we had to know specific explanations for why these occurred. Looking online, I found that they were due to changing electric field strengths (which I now understand to be the result of varying charge concentrations around the discharge tube), but if this isn't relevant to the syllabus then don't worry about my side comment from the previous post haha.

But as you mentioned deflection plates/coils, will we need to know how changing the voltage on the X and Y plates will influence the degree of deflection? I haven't heard of the use of coils in CRTs yet, so if you're free to, could you please offer some resources that do a good job of explaining how coils are involved (or if you have time, possible write something short up - whatever suits, either is awesome ).

Thanks again for all your help so far!

[beau77bro]

I think the answer might be A) as it's the only one which obeys the law of conservation of energy. Considering that you can't create nor destroy energy, you know that that as one form increases another must decrease to ensure that the total energy in a system stays the same. So let's apply this to the problem. When the rocket is sitting on the launch pad, it's KE = 0 as it's not moving. We're told that the rocket rises with constant thrust so we we're looking for linear relationships. The faster the rocket goes, the more kinetic energy it has meaning that the potential energy (gravitational in this case) will have to decrease as the total energy in the system has to remain constant. You can show this with the first graph by superimposing the two lines. You will end up with a horizontal line at the height of KE @ t=0 i.e. a constant amount of energy. This obeys the law of conservation of energy unlike any of the others which means a) is our answer.

Hopefully this helped!

hmmmm the answer is D in this book, but thankyou bsdfjnlkasn i get where you coming from. ok so i had this in my half yearly, and the answer was B, but in this textbook the answer is D, so which is it and why? -- there is a lot of dispute in my class and im arguing for D, because it escapes so it's Ep is 0 eventually and Ep is always negative, but i really need a solid answer explaining the features any help appreciated.

[Zainbow]

hmmmm the answer is D in this book, but thankyou bsdfjnlkasn i get where you coming from. ok so i had this in my half yearly, and the answer was B, but in this textbook the answer is D, so which is it and why? -- there is a lot of dispute in my class and im arguing for D, because it escapes so it's Ep is 0 eventually and Ep is always negative, but i really need a solid answer explaining the features any help appreciated.

I'll try to explain this.

Ok so, we know that potential energy increases the further you move away from the planet, and, as you said, it increases to zero. This would make options A and B incorrect, because both their Ep graphs are decreasing. This leaves options C and D, and we have to decide whether the relationship is linear or parabolic. Given the kinetic energy formula, Ek=1/2 mv^2 where v is squared, and knowing that velocity increases due to the rocket accelerating, the graph must be increasing parabolically. Hence, the answer must be D.

I don't know if this makes sense (or if it's right), but I hope it helps.

[jamonwindeyer]

hmmmm the answer is D in this book, but thankyou bsdfjnlkasn i get where you coming from. ok so i had this in my half yearly, and the answer was B, but in this textbook the answer is D, so which is it and why? -- there is a lot of dispute in my class and im arguing for D, because it escapes so it's Ep is 0 eventually and Ep is always negative, but i really need a solid answer explaining the features any help appreciated.

I'll try to explain this.

Ok so, we know that potential energy increases the further you move away from the planet, and, as you said, it increases to zero. This would make options A and B incorrect, because both their Ep graphs are decreasing. This leaves options C and D, and we have to decide whether the relationship is linear or parabolic. Given the kinetic energy formula, Ek=1/2 mv^2 where v is squared, and knowing that velocity increases due to the rocket accelerating, the graph must be increasing parabolically. Hence, the answer must be D.

I don't know if this makes sense (or if it's right), but I hope it helps.

Throwing my answer behind Zainbow here, the answer is definitely D further, if a rocket has a constant thrust, the acceleration will increase over time as the mass of the rocket decreases (due to lost fuel) - This means velocity will be increasing at an increasing rate, which does more to explain the accelerated increase shown in the final graphs

Beyond that, \(E_p\) must be negative and is increasing as time passes - That's the big thing to spot

I think the answer might be A) as it's the only one which obeys the law of conservation of energy. Considering that you can't create nor destroy energy, you know that that as one form increases another must decrease to ensure that the total energy in a system stays the same. So let's apply this to the problem. When the rocket is sitting on the launch pad, it's KE = 0 as it's not moving. We're told that the rocket rises with constant thrust so we we're looking for linear relationships. The faster the rocket goes, the more kinetic energy it has meaning that the potential energy (gravitational in this case) will have to decrease as the total energy in the system has to remain constant. You can show this with the first graph by superimposing the two lines. You will end up with a horizontal line at the height of KE @ t=0 i.e. a constant amount of energy. This obeys the law of conservation of energy unlike any of the others which means a) is our answer.
Hopefully this helped!

I love your way of thinking here, but only one problem - We inject energy into the system from the fuel. So the Conservation of Energy need not apply, at least not without considering the potential chemical energy in the fuel of the rocket

[jamonwindeyer]

We recently covered the discharge tubes prac and had to observe how the different gas pressures influenced the ray produced. One of the features we looked at was striations which appear as pressure is reduced - this made me wonder if we had to know specific explanations for why these occurred. Looking online, I found that they were due to changing electric field strengths (which I now understand to be the result of varying charge concentrations around the discharge tube), but if this isn't relevant to the syllabus then don't worry about my side comment from the previous post haha.

Definitely totally unnecessary to understand at the HSC level I know Sydney University has a document going into specifics, and perhaps that is what you used, but it is definitely unnecessary to consider it to this level of depth in the HSC (interesting read though, if you haven't already)

For the HSC, you need only know that the different striation patterns are caused by differing air pressures, as it changes the amount of particles in the tube for the electrons to collide with. Speaking roughly, lower air pressures produce less prominent striations for exactly this reason - Less obstructions, and so, less collisions. The colour and nature of the striations can also depend on the gas inside the tube. Understanding these points, and perhaps (even this isn't really necessary but good to know if you can) being able to identify different parts of the patterns (dark spaces and the like), is all you need

But as you mentioned deflection plates/coils, will we need to know how changing the voltage on the X and Y plates will influence the degree of deflection? I haven't heard of the use of coils in CRTs yet, so if you're free to, could you please offer some resources that do a good job of explaining how coils are involved (or if you have time, possible write something short up - whatever suits, either is awesome ).

Yes you are, but qualitatively, not quantitatively. Basically, just the knowledge that plates and/or coils are used to deflect an electron beam in a Cathode Ray Television/Oscilloscope is what is required. Exactly how the beam is deflected depends on application:

For a CRO, we need the horizontal axis to represent time, and the vertical to represent some input signal. So, the horizontal deflection plates are set up with a time varying sawtooth wave, which sweeps the beam from the left side of the screen to the right, then immediately jumping back to the starting point to repeat. The vertical deflection plates are connected to an input voltage, so the amount of vertical deflection is completely dependent on your input. The net effect?



For a television, we need to sweep the electron beam across the screen. So both vertical/horizontal plates are time varying, with the periods adjusted such that the beam will touch every point on the screen, by zig zagging left and right as it moves up and down.

Hopefully this helps! Let me know if you need anything clarified

[katnisschung]

doing some revision questions and i came to wonder what the theory is for how the increase
in coils in a solenoid increases the magnetic field strength with current kept constant?
thanks