HSC Physics Question Thread

[ATWalk]

Hi,

I'm having some trouble with this question right here. I'm not really sure what to say about the validity or accuracy. For example, I found  Vp/Vs to be greater than np/ns. That would mean that energy is lost, so would that affect the experiment's validity? Would the addition of a switch to limit heat be a good thing to mention? And also, is it enough just to say that the experiment should be repeated in order to improve its reliability? Basically, what exactly should I say for this question?

Thanks a lot in advance.

[jamonwindeyer]

Hi,

I'm having some trouble with this question right here. I'm not really sure what to say about the validity or accuracy. For example, I found  Vp/Vs to be greater than np/ns. That would mean that energy is lost, so would that affect the experiment's validity? Would the addition of a switch to limit heat be a good thing to mention? And also, is it enough just to say that the experiment should be repeated in order to improve its reliability? Basically, what exactly should I say for this question?

Thanks a lot in advance.

Hey ATWalk!

I always hated the validity/reliability/accuracy questions, got them confused heaps!

Validity concerns how well the variables in an experiment are controlled. That is, if we want to investigate the voltage relationship in a transformer, the validity of that investigation would hinge on how well we can achieve flux linkage between the coils. In this situation, you have correctly identified through some calculations, that we have definitely lost energy to the environment. We haven't effectively controlled all variables. Therefore, your assessment would be that this experiment is not completely valid. It does not ask you to suggest an improvement for validity, so this is all you need.

To improve reliability, you are looking at larger sample sizes/more repetition. However, it wants a change to the apparatus, so perhaps "repetition" isn't the best answer. Perhaps replace the coils with solenoid-like apparatus where the amount of turns can be adjusted, then you could get results for a variety of different turn-to-turn relationships, thus improving reliability 

[mijomo]

Hi everyone, just wondering if someone could explain how to do this question thanks.
Q) Just after launch the engines of a spaceship (mass 6.8x10^5 kg) produce a thrust of 10^7N. What is the force that a 100kg astronaut exerts on the launch lounge at this time?
(a) 100N
(b) 490N
(c) 980N
(d) 1470N

[jakesilove]

Hi everyone, just wondering if someone could explain how to do this question thanks.
Q) Just after launch the engines of a spaceship (mass 6.8x10^5 kg) produce a thrust of 10^7N. What is the force that a 100kg astronaut exerts on the launch lounge at this time?
(a) 100N
(b) 490N
(c) 980N
(d) 1470N

Hey Mijomo!

This question requires multiple applications of the formula



First, you have to identify that the Thrust force is the same as the force on the rocket, ie.



Using this equation, we can solve to find the total acceleration of the rocket, which is



Now, we can figure out the downward force that the Astronaut exerts based on this acceleration! Using F=ma again,



And so, the answer is D.

Hope that this helped!

Jake

[RuiAce]

Adding on to Jake's response:

Observe that the acceleration of the rocket = the acceleration on the astronaut. This occurs because the astronaut is essentially inside the rocket, so they are the same system. You could be fancy, and say that they are in fact the same non-inertial frame of reference as well.

This is why the acceleration calculated for the rocket can be applied immediately to that of the astronaut.

[gstaah]

Hey, this is from the 2005 HSC physics paper. From what I've learnt I would think it's B, but its C. Isn't there still minuscule eddy currents generated in R?

[RuiAce]

Hey, this is from the 2005 HSC physics paper. From what I've learnt I would think it's B, but its C. Isn't there still minuscule eddy currents generated in R?

From memory this question has been disputed. The conclusion among many people I've consulted with have agreed that it should be B

[gstaah]

From memory this question has been disputed. The conclusion among many people I've consulted with have agreed that it should be B

Oh no wonder. Thanks!

[Meckenza]

Hello, I'm having trouble understanding the answer of circular motion to this question. Any clarification would be appreciated!

Q: The mass spectrometer is used to determine the mass of particles. It operates by projecting particles, with known charge and velocity, through an evacuated chamber into a region of uniform magnetic field acting perpendicular to the velocity of the particles. Explain how the charged particles will behave as they enter the magnetic field and describe how this behaviour allows the mass to be determined.

Thanks!

[jakesilove]

Hello, I'm having trouble understanding the answer of circular motion to this question. Any clarification would be appreciated!

Q: The mass spectrometer is used to determine the mass of particles. It operates by projecting particles, with known charge and velocity, through an evacuated chamber into a region of uniform magnetic field acting perpendicular to the velocity of the particles. Explain how the charged particles will behave as they enter the magnetic field and describe how this behaviour allows the mass to be determined.

Thanks!

Hey!

Let's start by thinking about what is actually going on in your scenario. We have a charged particle, fired into an area with a constant magnetic field. This means that, by the right hand rule, there will be a force acting on the particle. An example of this is displayed in the image.



However, remember that the force acting on the particle is CONSTANT (as it is proportional to the magnetic field, and the charge of the particle, neither of which are changing). Recalling the formula



We can see that if the force is constant, and the mass is constant, there will also be a constant acceleration! This means that the particle is 'rotating' through the field at the same, constant rate. The curve, therefore, will be uniform, and therefore traces out a circular path! See the image below for clearer details.



You can use the right hand rule along the entire path of the particle (just move your hand as though it IS the particle) to show that the force will always be towards the centre of the circle (ie. it is always accelerating towards the centre). If the circle is entirely contained within the magnetic field, it will trace the following path.



Okay great, so we know that the particle follows a curved path, accelerating towards the centre only. This sounds pretty familiar! Space shuttles, and orbits, work exactly the same way! So we can apply formulas used in the Space section here!

The next derivation is similar to the method used by JJ Thompson to figure out the mass:charge ratio of electrons. It's important to know the derivation for the HSC, although very few questions ask about it.

We know that the force acting on a moving charge in a uniform magnetic field is



However, we also know the force exerted on an object in orbit. The formula for centripetal force is



Since these two forces must be the same, we can equate them!




Since we know the charge, velocity and Magnetic field strength, and we can measure the radius of the spinning charge, we can figure out the mass!

Very rarely, HSC questions will make you derive things like this. You just need to think about what is actually HAPPENING, and try work things out from there. I hope that this explanation helped! Let me know if I can clarify anything.

Jake

[Meckenza]

Ahh, ok! That makes sense. Thanks Jake!

[jakesilove]

Ahh, ok! That makes sense. Thanks Jake!

No problem! That was quite a difficult (and, as far as my experience goes, unique) question. Keep looking through difficult material, and I look forward to seeing you on the forums!

Jake

[RuiAce]

Actually can I make a comment... questions that ask you to equate F_B=F_C for magnetic field deflection of an electron are actually pretty common. It's expected that students should be able to handle this.

Some students even remember the equations in JJ Thomson's experiment and know the full derivation to q/m=E/(rB²)

To be fair, it is hard, but it something that you should know how to do after first time exposure to it at least.

[jakesilove]

Actually can I make a comment... questions that ask you to equate F_B=F_C for magnetic field deflection of an electron are actually pretty common. It's expected that students should be able to handle this.

Some students even remember the equations in JJ Thomson's experiment and know the full derivation to q/m=E/(rB²)

To be fair, it is hard, but it something that you should know how to do after first time exposure to it at least.

Just following on from this (and I totally agree with RuiAce; you should try to learn the JJ. Thompson equations derivation, if possible), I thought I would post the derivation for anyone interested. If you want a comprehensive formula sheet, including a whole bunch of important information, check out the link here!



Jake

[Neutron]

Hey guys!

So I started learning about Cathode Ray Tubes and I was wondering how the control electrode actually controls the number of electrons coming off the cathode? And also, how does the accelerating anode work? Like I get how the electrons are attracted to the anode but how does the electric field produced by the accelerating anode actually speed up the electrons? Many thanks!

Neutron