HSC Physics Question Thread

[bsdfjnlkasn]

Hi there,

Just reviewing my Ideas to Implementation summary and have a few questions

Why do we use a high voltage source in cathode ray tubes? And why can it only be sourced from an induction coil? Why can't a transformer be used?

Do we need to know why/how Hertz was inaccurate in his experiments which supposedly proved the wave properties of light?
If so, how does a cathode tube with some gas remaining (not completely evacuated) cause no observable deflection when an electric field is applied?

Considering that F = qvBsin(theta) and F = qE, does E = vbson(theta)?

Is theta always 90° when the magnetic field is going in or out of the page and the particle is moving left/right/up/down, or in any direction around the page?

Thank you so much!!
(Expect some edits with more questions )

[jamonwindeyer]

Hi there,

Just reviewing my Ideas to Implementation summary and have a few questions

Sure thing!

Why do we use a high voltage source in cathode ray tubes? And why can it only be sourced from an induction coil? Why can't a transformer be used?

A high voltage is required for the air in the tube (even the reduced amount we get by using an evacuated tube) to break down and allow a flow of electrons through it. This is called breakdown. Essentially, it is the point where even the resistance of the air in the tube isn't enough to stop a current flow between the cathode and anode.

Pretty sure you can get this voltage from anywhere you want to, but it needs to be DC. You can't use a transformer directly because that would be AC, but you could rectify the voltage coming out of a transformer if you wanted to

Do we need to know why/how Hertz was inaccurate in his experiments which supposedly proved the wave properties of light?

I wouldn't put too much effort into it. Just say lack of accurate measuring equipment, that's what I'd do!

If so, how does a cathode tube with some gas remaining (not completely evacuated) cause no observable deflection when an electric field is applied?

Not quite sure what you are asking here sorry, but no cathode ray tube is completely evacuated. There's always some air left inside. The amount of air will change the voltage you need across the plate, as well as the striation patterns, but within reason small differences in the amount of air in a tube won't affect much else

Considering that F = qvBsin(theta) and F = qE, does E = vbson(theta)?

Not quite, those are two different forces. \(F=Bqv\sin{\theta}\) is the force on a moving charge due to a magnetic field, while \(F=Eq\) is the force on a charge due to an electric field. Together, we call the sum of these forces the Lorentz Force, but they aren't the same thing

Is theta always 90° when the magnetic field is going in or out of the page and the particle is moving left/right/up/down, or in any direction around the page?

Yep

Thank you so much!!
(Expect some edits with more questions )

You're welcome! Maybe ask the rest in a new post so we don't miss it

[Charlie_Sparkes]

Hi All,
Just having a bit of trouble with the following question, and I cant seem to find the right resource...

A 1W beam of light transfers 1J per second from one point to another. With reference to the particle model of light, contrast a 1W beam of red light and a 1W beam of blue light.

Thanks in advance for the help 

[katnisschung]

how did Hertz determine that radio waves travel in straight lines?

[kiwiberry]

Hi All,
Just having a bit of trouble with the following question, and I cant seem to find the right resource...

A 1W beam of light transfers 1J per second from one point to another. With reference to the particle model of light, contrast a 1W beam of red light and a 1W beam of blue light.

Thanks in advance for the help 

Hey, welcome to the forums!!
Blue light has a higher frequency than red light. Since E=hf, the energy carried by a photon of blue light will be much higher than a photon of red light. Therefore, less photons are needed to produce a 1J/s or 1W beam of blue light than red light.

[bsdfjnlkasn]

You're welcome! Maybe ask the rest in a new post so we don't miss it

You're a legend Jamon! Super detailed and hopeful

Still have a few more questions, if it's ok

Does the air in the tubes then ionise and because of this, charges are able to pass through the cathode ray tube?

To what detail should we know the striation patterns for the different pressures in discharge tube experiment?

Thank you

P.S. I'll just keep adding questions until things start to get answered - that way questions won't go unanswered.

[jamonwindeyer]

Does the air in the tubes then ionise and because of this, charges are able to pass through the cathode ray tube?

Yep, that's correct - The large potential differences ionises the air and allows the electrons to flow. Note that this is a little different to the thermionic emission method of electron ejection from a cathode

To what detail should we know the striation patterns for the different pressures in discharge tube experiment?

Not in huge detail - Understand why the pattern changes in the first place (changing air pressure, different amounts of air particles to collide with), and have a rough idea of how it changes (lower air pressures, less pattern/pattern focused close to the anode). As far as the names of the spaces and such, I don't think that is super important knowledge

[bsdfjnlkasn]

Thanks again Jamon!

Some more questions

I'm just reading up on Hertz's experiments and how he observed the photoelectric effect but 'failed to investigate' it and was wondering if this part of the syllabus outcome just needs you to detail the experiments and state that he didn't provide an explanation?

I'm a bit confused with the results he observed (and am not sure if we need to know why he noted what he did, if we don't an explanation would still be cool ). So when he enclosed his received loop to allow better observations, he saw the spark length (and intensity?) decrease. What's the relationship between spark length and intensity in this case? I imagine the more UV light the receiver is exposed to, the greater the photocurrent and so the longer the possible spark length. But how does intensity relate here? Should we just take for granted that as more photoelectrons are released, the brighter the stream is?

Thanks again!

[jamonwindeyer]

I'm just reading up on Hertz's experiments and how he observed the photoelectric effect but 'failed to investigate' it and was wondering if this part of the syllabus outcome just needs you to detail the experiments and state that he didn't provide an explanation?

Yep, that's correct!

I'm a bit confused with the results he observed (and am not sure if we need to know why he noted what he did, if we don't an explanation would still be cool ). So when he enclosed his received loop to allow better observations, he saw the spark length (and intensity?) decrease. What's the relationship between spark length and intensity in this case? I imagine the more UV light the receiver is exposed to, the greater the photocurrent and so the longer the possible spark length. But how does intensity relate here? Should we just take for granted that as more photoelectrons are released, the brighter the stream is?

Thanks again!

I don't have an exact mathematical relationship, but quantitatively you are correct! Enclosing the coil gets rid of some of your photons which reduces the photocurrent. The intensity of a spark is based on the size of the current (I believe), so it makes sense that intensity and length would decrease

(And this isn't crucial to remember for HSC Physics)

[bsdfjnlkasn]

Yep, that's correct!

I don't have an exact mathematical relationship, but quantitatively you are correct! Enclosing the coil gets rid of some of your photons which reduces the photocurrent. The intensity of a spark is based on the size of the current (I believe), so it makes sense that intensity and length would decrease

(And this isn't crucial to remember for HSC Physics)

Awesome!!

Now for some questions about black body radiation, I'm just not sure what the relationship between temperature, intensity and wavelength are. I can make sense of it by picturing the curve (is it called a Rayleigh Jean curve, or is this just the law that predicted it's shape?) but don't know if I have to explain why increasing the temperature changes the peak intensity. And on that, why does the wavelength at which the maximum occurs decrease when temperatures increases?

Where is it relevant to mention Maxwell (because he is not mentioned in the syllabus) and his work with EMR?

Thanks again

[jamonwindeyer]

Now for some questions about black body radiation, I'm just not sure what the relationship between temperature, intensity and wavelength are. I can make sense of it by picturing the curve (is it called a Rayleigh Jean curve, or is this just the law that predicted it's shape?) but don't know if I have to explain why increasing the temperature changes the peak intensity. And on that, why does the wavelength at which the maximum occurs decrease when temperatures increases?

So that curve is just called a black body radiation curve, and the theoretical shape of it (the one that sparked the ultraviolet catastrophe) was governed by Rayleigh Jean's Law.

Here's a guide I wrote on Quantum Physics! It has a little on the black body curves and such - Basically, a black body of a higher temperature will have a smaller characteristic wavelength because the hotter a black body, the more likely it will be to have a large change in the energy of one of its electrons (more vibrations). Since the energy of a photon is dependent on some energy change within the atomic structure of the black body, this also means that the energy of emitted photons will increase. Energy is dependent on frequency, so characteristic frequency goes up (and characteristic wavelength goes down

This is a tough thing to explain, does this make sense? Happy to clarify

Where is it relevant to mention Maxwell (because he is not mentioned in the syllabus) and his work with EMR?
Thanks again

Never - You don't need Maxwell in HSC Physics

[winstondarmawan]

Hello! Just a querie about photocells:
My teacher said that the photocell only works when light strikes the p-type and not the n-type, because if electrons travel from the n-type through the circuit and fill a hole in the p-type, then the depletion layer becomes larger and larger as the n and p-type layers become more positive and negative respectively, stopping any more electrons from moving anywhere. This makes sense. However, other sources say that the electrons are released from the n-type. Just want to clarify this. TIA

[jamonwindeyer]

Hello! Just a querie about photocells:
My teacher said that the photocell only works when light strikes the p-type and not the n-type, because if electrons travel from the n-type through the circuit and fill a hole in the p-type, then the depletion layer becomes larger and larger as the n and p-type layers become more positive and negative respectively, stopping any more electrons from moving anywhere. This makes sense. However, other sources say that the electrons are released from the n-type. Just want to clarify this. TIA

Hey Winston! It can happen anywhere where the electric field has a substantial effect (I think most HSC sources just say it has to be in the depletion zone? In truth it doesn't matter too much). P-type or N-type, any released electrons will be forced through the external circuit by the electric field in the depletion layer

This said, it is the P-type which has more electrons in the depletion layer (the negatively charged silicon ions formed by recombination) - I'd wager that is why your teacher said P-type!

I've honestly never considered the idea that electrons flowing through the circuit would recombine with holes further back in the P-type. I don't think that quite works because then you'd end up with an asymmetric distribution of charge - Just doesn't make sense in my head...

All this said, you don't need to stress about these sort of little details in the HSC. Light strikes solar cell, released electrons flow through circuit due to electric field in the depletion layer. Doneskies

[limtou]

Hello! Have a few questions
1. What properties of cathode ray are determined, when it fluoresces?
2. Can you explain the changing acceleration of a rocket during launch in terms of the Law of Conservation of Momentum? (straight from the dot point haha) What are we expected to know for this?

Thanks

[JeffChiang]

Hello! Have a few questions
1. What properties of cathode ray are determined, when it fluoresces?
2. Can you explain the changing acceleration of a rocket during launch in terms of the Law of Conservation of Momentum? (straight from the dot point haha) What are we expected to know for this?

Thanks

Hello, limtou
1) Cathode ray fluorsence is detected using a fluorescent display screen. This demonstrates that it induces fluorescence on phosphors which is a property of electromagnetic waves.
2) You basically need to be able to describe the different stages of a rocket during launch in terms of force. I don't think you have to go into a lot of detail for it.
e.g. A rocket sits on the launchpad experiencing 1g.
      The rocket then launches upwards ejecting large amounts of exhaust gas downwards which opposes the direction of the rocket's motion. Due to the conservation of momentum, the rocket will go upwards as there is an equal and opposite reaction from the gas which propels the rocket.