HSC Physics Question Thread

[conic curve]

Where are you getting these questions from? Sounds like something out of a textbook, homework you're supposed to be doing or an Assignment. Don't mind if it's any but the last one, but just wondering!

A worksheet I was given. It's basically a worksheet for all the weak physics kids and require revision on the core basics

[jamonwindeyer]

Hey guys, need help with this question:

Describe what is meant by the term drift velocity, in terms of electrons moving in a solid conductor. Compare its magnitude to the thermal motion of electrons in a solid.

Cheers.

Hey there!! Drift velocity refers to the velocity attained by an electron due an applied electric field. A quick bit of research reveals that there is actually a formula for drift velocity!



Drift velocity is the product of the magnitude of the applied electric field and the electron mobility of the material (this is new for me too, I'm learning with you!)

Now, electrons normally vibrate randomly in solids due to thermal energy. Remember, heat causes atoms in solids to vibrate, that's what thermal energy is. This causes small random movements of election, but this doesn't cause any net drift velocity. Applying the electric field will cause larger movement, and they will be aligned in a single direction  i hope this helps!!

[wyzard]

Hey there!! Drift velocity refers to the velocity attained by an electron due an applied electric field. A quick bit of research reveals that there is actually a formula for drift velocity!



Drift velocity is the product of the magnitude of the applied electric field and the electron mobility of the material (this is new for me too, I'm learning with you!)

Now, electrons normally vibrate randomly in solids due to thermal energy. Remember, heat causes atoms in solids to vibrate, that's what thermal energy is. This causes small random movements of election, but this doesn't cause any net drift velocity. Applying the electric field will cause larger movement, and they will be aligned in a single direction  i hope this helps!!

Pretty much as explained

As for the magnitude, if you run through the calculations you'll find that drift velocity have magnitudes around , while the velocity of the electron from the thermal motion is about . So the thermal motion is about 1 billion times faster than the thermal motion.

The resulting motion is the electron jiggling around at really high speed, while slowly drifting in the opposite direction of the applied electric field as shown in the attached image.

[MysteryMarker]

Thanks guys. 

Another Question,  for the dot point 'Identify Einstein contribution to quantum theory and its relation to black body radiation. What is his relation to black body radiation? Didn't he just extend upon Planck's model of energy being quantised and use it to explain the photoelectric effect? With threshold frequency, work function, E = hf and all that stuff? Just confused as to what Einstein had to do with 'Black body radiation'.

Cheers.

[Swagadaktal]

An observation it could not explain is the photoelectric effect : In theory if it were a wave it would reflect off a surface even at lower frequencies. However, there is a stopping voltage which can overcome the emission of light which suggests that light isn't a wave but rather a particle.  - Not sure if I worded this correctly so any help would be appreciated

I think this was lost in the sea of posts - can someone validate whether this is correct? Dont have a solid understanding of this topic quite yet

[Happy Physics Land]

Thanks guys. 

Another Question,  for the dot point 'Identify Einstein contribution to quantum theory and its relation to black body radiation. What is his relation to black body radiation? Didn't he just extend upon Planck's model of energy being quantised and use it to explain the photoelectric effect? With threshold frequency, work function, E = hf and all that stuff? Just confused as to what Einstein had to do with 'Black body radiation'.

Cheers.

Hey Mysterymarker!

If you were at the physics lecture last week you might have heard me talking about this - both einstein and planck agreed on the mechanism to produce light but there differed in their explanation towards the nature of light. They agree that electromagnetic radiation or electromagnetic wave is produced as a result of change in quantised energy levels. But Planck is actually still partly a classical physicist and believed that energy is evenly distributed across wavefronts. On the other hand, Einstein ascertained the particle nature of light through the photoelectric effect and the 1:1 interaction between photons and electrons, hence affirming light as a stream of photons. So there is a fundamental shift from Planck to Einstein in terms of the explanation towards the nature of light. Obviously through photoelectric effect and the stopping voltage experiment, Einstein was able to confirm Planck's second postulate E=hf and this confirms Planck's explanation towards the blackbody radiation in terms of UV catastrophe and peak wavelength radiation. So essentially Planck theorised a relationship between frequency and quantised energy but Einstein used photoelectric effect and the related implications as evidences to E=hf.

Best Regards
Happy Physics Land

[Spencerr]

I think this was lost in the sea of posts - can someone validate whether this is correct? Dont have a solid understanding of this topic quite yet

I haven't seen the original post but i'm assuming the question is why the wave model of light couldn't account for the photoelectric effect.

The classical wave model of light proposed that the energy of light was related to its intensity. So pretty much if you shine light onto a photemitting metal (no matter the frequency), after a while, the electrons in the metal would have enough energy and they would be freed. However experiments showed that this was not the case, as only light above a certain frequency (called the threshold frequency) would cause the electrons to be freed.
As a result 3 problems came up
1. The existence of a threshold frequency
2. The absence of a time delay or immediate release
3. How does intensity affect the emission of photoelectrons.

In order to solve these problems, Einstein extended Planck's idea of quantised energy and suggested that light was also quantised into small packets called photons. These photons would have energy equal to E=hf (linking energy with frequency) and would interact with the electrons on the surface of the metal on a 1:1 basis. If the photons had enough energy, it would transfer all of it to the electron and the electron would have the requried energy to free it self from the attractive force of the metal. However if the photon did not have enough energy, it would be absorbed by the metal and then reemitted. This particle nature of light explained the absence of a time delay and the existence of a threshold frequency. Intensity refers to how many photons of light are there in the beam of light and is independent  of the energy of the photons.
 

[Swagadaktal]

I haven't seen the original post but i'm assuming the question is why the wave model of light couldn't account for the photoelectric effect.

The classical wave model of light proposed that the energy of light was related to its intensity. So pretty much if you shine light onto a photemitting metal (no matter the frequency), after a while, the electrons in the metal would have enough energy and they would be freed. However experiments showed that this was not the case, as only light above a certain frequency (called the threshold frequency) would cause the electrons to be freed.
As a result 3 problems came up
1. The existence of a threshold frequency
2. The absence of a time delay or immediate release
3. How does intensity affect the emission of photoelectrons.

In order to solve these problems, Einstein extended Planck's idea of quantised energy and suggested that light was also quantised into small packets called photons. These photons would have energy equal to E=hf (linking energy with frequency) and would interact with the electrons on the surface of the metal on a 1:1 basis. If the photons had enough energy, it would transfer all of it to the electron and the electron would have the requried energy to free it self from the attractive force of the metal. However if the photon did not have enough energy, it would be absorbed by the metal and then reemitted. This particle nature of light explained the absence of a time delay and the existence of a threshold frequency. Intensity refers to how many photons of light are there in the beam of light and is independent  of the energy of the photons.

Thank you for the response!
Um the thing I was inquiring about was in the quote: I'll write it bellow - thanks for the explanation though! I was more specifically asking about whether this point was valid or I've mistaken it with something else:

An observation it could not explain is the photoelectric effect : In theory if it were a wave it would reflect off a surface even at lower frequencies. However, there is a stopping voltage which can overcome the emission of light which suggests that light isn't a wave but rather a particle.  - Not sure if I worded this correctly so any help would be appreciated

[Spencerr]

Thanks guys. 

Another Question,  for the dot point 'Identify Einstein contribution to quantum theory and its relation to black body radiation. What is his relation to black body radiation? Didn't he just extend upon Planck's model of energy being quantised and use it to explain the photoelectric effect? With threshold frequency, work function, E = hf and all that stuff? Just confused as to what Einstein had to do with 'Black body radiation'.

Cheers.

Happy Physics Land gave an excellent answer but here's my take on how to answer the question if it was in an exam.

Planck had a problem applying classical physics to black body radiation observations (UV catastrophe)
Thus, in order to account for experimental observations, Planck suggested that EMR emitted or absorbed by black bodies were quantised i.e. they existed as discrete separate packets. This explained by the black body radiation curve peaked and then declined as it approached higher frequencies. He proposed that the energy of these quanta would be E=hf, at that time, Planck himself thought this was crazy and it was all theoretical. However Einstein used Planck's idea of quantised energy to explain the photoelectric effect and solved many problems with it such as the threshold frequency etc. Einstein's practical application of Planck's idea of quantised energy in explaining the photoelectric effect greatly validated the theory and laid the foundation for quantum theory.

Thank you for the response!
Um the thing I was inquiring about was in the quote: I'll write it bellow - thanks for the explanation though! I was more specifically asking about whether this point was valid or I've mistaken it with something else:

An observation it could not explain is the photoelectric effect : In theory if it were a wave it would reflect off a surface even at lower frequencies. However, there is a stopping voltage which can overcome the emission of light which suggests that light isn't a wave but rather a particle.  - Not sure if I worded this correctly so any help would be appreciated

Hey there, to my knowledge I don't know any links between stopping voltage and the emission of light. I think how stopping voltage works is that, its the voltage at which electrons with max KE cannot reach the electrode and as for the first part, I don't think there is a relevance between wave reflections and the photoelectric effect.

If someone knows more, then I want to hear a better explanation but I don't think it's completely valid.

Moderator action: Merged double post. You can edit and include multiple quotes in the same post.

[jamonwindeyer]

Thank you for the response!
Um the thing I was inquiring about was in the quote: I'll write it bellow - thanks for the explanation though! I was more specifically asking about whether this point was valid or I've mistaken it with something else:

An observation it could not explain is the photoelectric effect : In theory if it were a wave it would reflect off a surface even at lower frequencies. However, there is a stopping voltage which can overcome the emission of light which suggests that light isn't a wave but rather a particle.  - Not sure if I worded this correctly so any help would be appreciated

Sorry Swag! Must have gotten lost I'm not 100% sure what you are driving at in that statement, which leads me to think the wording may be a little off, reckon you could explain what you mean in another way? Not quite clicking with it right now that's all

[Happy Physics Land]

Happy Physics Land gave an excellent answer but here's my take on how to answer the question if it was in an exam.

Planck had a problem applying classical physics to black body radiation observations (UV catastrophe)
Thus, in order to account for experimental observations, Planck suggested that EMR emitted or absorbed by black bodies were quantised i.e. they existed as discrete separate packets. This explained by the black body radiation curve peaked and then declined as it approached higher frequencies. He proposed that the energy of these quanta would be E=hf, at that time, Planck himself thought this was crazy and it was all theoretical. However Einstein used Planck's idea of quantised energy to explain the photoelectric effect and solved many problems with it such as the threshold frequency etc. Einstein's practical application of Planck's idea of quantised energy in explaining the photoelectric effect greatly validated the theory and laid the foundation for quantum theory.

Hey there, to my knowledge I don't know any links between stopping voltage and the emission of light. I think how stopping voltage works is that, its the voltage at which electrons with max KE cannot reach the electrode and as for the first part, I don't think there is a relevance between wave reflections and the photoelectric effect.

If someone knows more, then I want to hear a better explanation but I don't think it's completely valid.

Moderator action: Merged double post. You can edit and include multiple quotes in the same post.

Yep yep just combine my answer and Diii's answer together you will get an almost perfect response! Nice job!

[jamonwindeyer]

Thanks

1. Why is it difficult to see newton's first law?
2. What is the definition of a normal force (I tried to research this but don't understand it). Are normal forces the reaction force of gravity?
3. What is the relative velocity formula and what does it tell us?
4. For centripetal force, why do we say that objects travelling in a circular motion are still accelerating?
5. Why are speed limits a good idea

Definitely can help you with Question 2! A normal force does sort of manifest as the reaction force to gravity, but a little more complicated. Basically, objects are accelerated towards the centre of the earth by a gravitational force. Eventually, this means objects will hit the floor. At this point, the gravitational force is still there, and yet the object is now sitting still. There must be another force cancelling the gravitational force! This is the normal force, and it comes from the ground pushing back against the object pushing against it.

For the others, give your textbook a bit of a flick! Us feeding you answers won't benefit you, you should come to us with things you've tried to understand but can't, or are confused about, or want to confirm, etc. We can answer all your questions, but that won't help you.

Incidentally, Question 1 has primarily to do with friction forces. Think of it this way; if you roll a ball along the ground, will the ball keep rolling forever? Given Newton's 1st Law, would you expect it to? Why doesn't it then?

I'll give you this link to help you with Question 3.

Question 4 is a matter of convention. We say that acceleration is any change in velocity. Velocity is a vector, meaning it has a direction and a magnitude. While circular motion may not change the magnitude of an objects velocity, it definitely changes the direction as it moves in a circle, and thus, it is 'accelerating.'

I'll leave Question 5 for you. Have a bit of a think, I reckon you'll come up with some ideas, remember to tie in things like the impulse formula:

[jakesilove]

Happy Physics Land gave an excellent answer but here's my take on how to answer the question if it was in an exam.


Hey there, to my knowledge I don't know any links between stopping voltage and the emission of light. I think how stopping voltage works is that, its the voltage at which electrons with max KE cannot reach the electrode and as for the first part, I don't think there is a relevance between wave reflections and the photoelectric effect.

If someone knows more, then I want to hear a better explanation but I don't think it's completely valid.

Moderator action: Merged double post. You can edit and include multiple quotes in the same post.

Just quickly, there IS a relationship between the quantisation of light and stopping voltage, although I don't remember how much you needed (if any) to know in the HSC. Basically, there were three key points that suggested the photoelectric effect was caused by a "particle" like light, rather than a wave.

1. Stopping voltage
If light were a wave, there wouldn't be a minimum frequency (ie. work function) for which electrons would be ejected. We know that the energy of a wave is proportional to the square if its amplitude (don't worry about this, not important). But what that means it that, if light were a wave, if would CONTINUOUSLY add energy to the electron (as more amplitudes pass through the electron) until, eventually, it should have enough energy to jump off the surface of the metal. This doesn't happen: you can shine light at a low enough frequency for as long as you want onto a surface, and if it below the work function nothing will happen. Therefore, stopping voltage is related to the photoelectric effect, and it's explanation.

2. Time delay
This is a similar point as above; the only time delay between the light being switched on and the electrons jumping off was the distance between the light and the surface, divided by the speed of light (ie. the light had to GET to the surface, but after that the electrons jumped off instantly). Again, this is indicative of the particle nature of light (think all or nothing principle, rather than a gradual build up).

Frequency, not intensity, dependent
This one is straight from the course, so I'll assume you understand this!

If you want a super comprehensive, impressive answer, you could always just cite the three things talked about above. Nice to understand them though

Jake

[MysteryMarker]

Hey guys

I dont understand why the answer for Q14 is C, or why the answer for Q19 is D. Any help is appreciated.

Thanks.

[zsteve]

Hey guys

I dont understand why the answer for Q14 is C, or why the answer for Q19 is D. Any help is appreciated.

Thanks.

Hey MysteryMarker,
For Q14, the train is turning right, (option C as specified). Try to visualise the situation (this might be hard for some people!)

In the reference frame of the ground, as the train begins to turn right, the ball travels in a straight line. Newton III predicts that the ball will tend to retain its straight path in the inertial frame of the ground. This is achieved more or less, assuming friction between ball and table isn't massive.

In the non-inertial frame of the turning train carriage, the ball appears to bend as shown.

I'm having severe difficulties with explaining this one . Best if you can visualise it by putting yourself 'in' the carriage and doing it intuitively.

Q19.
This is rather weird, because from theory we know that their accelerations are the same. However, this comes to mind...
\( a = \dfrac{F_g}{m}\), so we see that acceleration is inversely proportional to mass. (considering only gravity here)

But \(F_g \propto m\) as well, so if we combine this with a, we find that m cancels and acceleration is independent of mass.

Not a well-phrased question in my opinion.