HSC Physics Question Thread

[conic curve]

Outline Einsteins explanation for the photoelectric effect. (6marks)

How would i go about this question to get the maximum? The marking criteria says 'an explanation of at least three observations', what does this mean?

Thanks guys.

I think it means each observation is worth two marks

[conic curve]

Definitely not - there were way less than 6 observations.

I no longer remember the practical off by heart though - will wait for Jamon or Jacky or Jake someone. (Tbh I just wanted to use the 3 J's)

Sorry I meant 2. I'll change my comment

[RuiAce]

Sorry I meant 2. I'll change my comment

Lol all good.

Hard to say. Whilst you can keep that frame of mind (and I reckon you could be right), 6 markers become ambiguous because the examiner probably subtly wants more.

[MysteryMarker]

Could you please explain what the observations are, and how many Einstein accounted for with his 'photoelectric effect'?

Also, what are some experimental evidence that the wave model of light could not explain for the photoelectric effect?

Thanks Guys.

[Happy Physics Land]

Could you please explain what the observations are, and how many Einstein accounted for with his 'photoelectric effect'?

Also, what are some experimental evidence that the wave model of light could not explain for the photoelectric effect?

Thanks Guys.

Hey MysteryMarker!

The question is a bit poorly worded here but I think I know approximately what you wanna ask, I will break my answer down into three sections, keeping in mind that Einstein wasn't the first to observe photoelectric effect, nor did he perform the experiment to ascertain the photoelectric effect (He just took the credit). And with "how many" I think you perhaps mean how many electrons did he see getting emitted? Well this emission of electrons is something thats theorised, even nowadays with the really advanced technologies we still can't see the movement of electrons, we can only propose likely reasons to explain the occurrence of certain events. So no it wouldnt be possible for him to see the amount of electrons being emitted. If you are talking about how many times he observed the photoelectric effect taking place, I wouldnt be sure and this is definitely something you dont need for physics.

First observation of photoelectric effect - By Heinrich Hertz:
- He conducted an experiment to ascertain the existence of electromagnetic radiation
- Because the spark at the receiver was perhaps a little hard to see, he decided to perform the experiment in a dark room
- He saw that the intensity of spark in a sun-lit bright room increased comparing to the intensity in a dark room

Photoelectric effect postulate - By Einstein:
- He theorised that when an incident light hits a metal surface, as long as the light possess frequency that is above the threshold frequency (i.e. if E=hf of incident light can overcome the work function), then a valence electron would be ejected with maximum kinetic energy (maximum because it takes minimum energy to eject a valence electron which is least tightly held to nucleus)
- He theorised that electrons and photons interact 1:1
- From this he theorised that light is a stream of photons and hence possess particle nature

Evidence for photoelectric effect - credited by Einstein and Planck:
- Blackbody radiation was a solid proof to photoelectric effect
- Einstein made his photoelectric effect postulates based upon the shape of the experimental black body radiation curve and Planck's explanation of the blackbody curve
- Experiment involving stopping voltage: This is the most important piece of evidence. So of course we can't seen current macroscopically, nor is there any microscopes to help Einstein to see the electrons. Therefore his assistant performed this experiment which aimed to show that yes there is a current (even though we couldnt see it) by shining a light onto the cathode and observe the reading on the ammeter. There was a current reading and therefore it was ascertained that a photocurrent was formed. To further detect the magnitude of the frequency of incident light ray, the anode was made negative (by supplying it with a negative potential) to repel the electron current. The moment ammeter reading --> 0, Einstein could apply the formula Ek = hf - work function to figure out frequency.
 

[Goodwil]

Hi, I'm having trouble understanding how to get the answer for this question (correct answer is A)

[RuiAce]

Hi, I'm having trouble understanding how to get the answer for this question (correct answer is A)

I'll let someone else do the more comprehensive answer since I'm just on my phone but in short:

1. Derive the formula v=sqrt(GM/r) or just quote it cause it's multiple choice.

2. GM is fixed. r varies. Note that v/sqrt(r)=sqrt(GM)

Therefore first v/sqrt(r) equals second V/sqrt(R)

[jamonwindeyer]

I'll let someone else do the more comprehensive answer since I'm just on my phone but in short:

1. Derive the formula v=sqrt(GM/r) or just quote it cause it's multiple choice.

2. GM is fixed. r varies. Note that v/sqrt(r)=sqrt(GM)

Therefore first v/sqrt(r) equals second V/sqrt(R)

Can do 

First we derive the formula for orbital velocity by equating gravitational and centripetal forces:



So let's let that expression be the orbital velocity of P, since it suits. Now, if instead we have a distance of 2R:



Thus yielding the answer of A 

[EEEEEEP]

I'll let someone else do the more comprehensive answer since I'm just on my phone but in short:

1. Derive the formula v=sqrt(GM/r) or just quote it cause it's multiple choice.

2. GM is fixed. r varies. Note that v/sqrt(r)=sqrt(GM)

Therefore first v/sqrt(r) equals second V/sqrt(R)

((SLIGHTLY DIFF METHOD THan the other two.. but never the less ))
v=sqrt(GM/r)

>>> * Let the two different radius be 1 and 2 (planet 1 and 2 respectively ) >>>>

Planet one .
First we square the formula.
V^2 = GM/1
GM  = V^2 (important for planet 2)   

Planet two
Orbital velocity 2  = sqrt (GM/2)

THUS
Orbital velocity 2 = sqrt (V^2 / 2)
= V / sqrt (2)
 = A

[MysteryMarker]

Hey MysteryMarker!

The question is a bit poorly worded here but I think I know approximately what you wanna ask, I will break my answer down into three sections, keeping in mind that Einstein wasn't the first to observe photoelectric effect, nor did he perform the experiment to ascertain the photoelectric effect (He just took the credit). And with "how many" I think you perhaps mean how many electrons did he see getting emitted? Well this emission of electrons is something thats theorised, even nowadays with the really advanced technologies we still can't see the movement of electrons, we can only propose likely reasons to explain the occurrence of certain events. So no it wouldnt be possible for him to see the amount of electrons being emitted. If you are talking about how many times he observed the photoelectric effect taking place, I wouldnt be sure and this is definitely something you dont need for physics.

First observation of photoelectric effect - By Heinrich Hertz:
- He conducted an experiment to ascertain the existence of electromagnetic radiation
- Because the spark at the receiver was perhaps a little hard to see, he decided to perform the experiment in a dark room
- He saw that the intensity of spark in a sun-lit bright room increased comparing to the intensity in a dark room

Photoelectric effect postulate - By Einstein:
- He theorised that when an incident light hits a metal surface, as long as the light possess frequency that is above the threshold frequency (i.e. if E=hf of incident light can overcome the work function), then a valence electron would be ejected with maximum kinetic energy (maximum because it takes minimum energy to eject a valence electron which is least tightly held to nucleus)
- He theorised that electrons and photons interact 1:1
- From this he theorised that light is a stream of photons and hence possess particle nature

Evidence for photoelectric effect - credited by Einstein and Planck:
- Blackbody radiation was a solid proof to photoelectric effect
- Einstein made his photoelectric effect postulates based upon the shape of the experimental black body radiation curve and Planck's explanation of the blackbody curve
- Experiment involving stopping voltage: This is the most important piece of evidence. So of course we can't seen current macroscopically, nor is there any microscopes to help Einstein to see the electrons. Therefore his assistant performed this experiment which aimed to show that yes there is a current (even though we couldnt see it) by shining a light onto the cathode and observe the reading on the ammeter. There was a current reading and therefore it was ascertained that a photocurrent was formed. To further detect the magnitude of the frequency of incident light ray, the anode was made negative (by supplying it with a negative potential) to repel the electron current. The moment ammeter reading --> 0, Einstein could apply the formula Ek = hf - work function to figure out frequency.

Thanks man I understand it a lot better now, just got a few questions though,

1. Would the electrons still liberated from the metals surface if the incident light has a frequency equal to the threshold frequency, and would there be a photocurrent?
2. Trial question - Discuss the ability of the wave model of light to explain the photoelectric effect? In your answer refer to observations that it could/could not explain.

Cheers guys.

[jamonwindeyer]

Thanks man I understand it a lot better now, just got a few questions though,

1. Would the electrons still liberated from the metals surface if the incident light has a frequency equal to the threshold frequency, and would there be a photocurrent?
2. Trial question - Discuss the ability of the wave model of light to explain the photoelectric effect? In your answer refer to observations that it could/could not explain.

Cheers guys.

For Question 1, yes, but they would have no kinetic energy!! So the photocurrent would be zero at this precise point; any higher and it would increase  note that in reality having an exact match between the two is pretty rare 

Question 2 is actually something I've not seen before, so happy for my answer to be expanded upon/corrected. The wave model of light could, I believe, explain why electrons were emitted from metals exposed to electromagnetic energy. Energy transfer caused excitation, this made sense. However, it could not explain why this effect was not dependent on the amount of incoming light (that is, if it wasn't happening, why more light wouldn't make it happen). It also couldn't explain why the effect only started happening at approximately the ultraviolet range. According to classical theory, an intense radio wave should have induced a similar effect to a weak ultraviolet wave  hope this helps!

[conic curve]

What does it mean by one dimension, two dimension and three dimension in terms of waves?

[jamonwindeyer]

What does it mean by one dimension, two dimension and three dimension in terms of waves?

Hmm, not 100% sure what you mean by the question, but it would perhaps mean how many dimensions the wave oscillates/propagates in?

EG - Consider sending a pulse along a slinky. That propagates along the slinky in a single dimension.

Next, consider tying one end of a string to a wall, then moving the other end of the string up and down so it looks like a sine wave (called a transverse wave). That oscillates up and down, but propagates to the left/right, so two dimensions.

Finally, light waves consist of magnetic and electric fields which oscillate in two dimensions, but the wave propagates in a third.

Does this sort of make sense? The question seems a little strange to me that's all 

[conic curve]

Hmm, not 100% sure what you mean by the question, but it would perhaps mean how many dimensions the wave oscillates/propagates in?

EG - Consider sending a pulse along a slinky. That propagates along the slinky in a single dimension.

Next, consider tying one end of a string to a wall, then moving the other end of the string up and down so it looks like a sine wave (called a transverse wave). That oscillates up and down, but propagates to the left/right, so two dimensions.

Finally, light waves consist of magnetic and electric fields which oscillate in two dimensions, but the wave propagates in a third.

Does this sort of make sense? The question seems a little strange to me that's all 

Yeah I don't think I was explicit enough although I remember being asked a question like that in a booklet given to me at school

[Swagadaktal]

2. Trial question - Discuss the ability of the wave model of light to explain the photoelectric effect? In your answer refer to observations that it could/could not explain.

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