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March 29, 2024, 03:34:11 am

Author Topic: VCE Physics Question Thread!  (Read 603289 times)  Share 

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mcpunjavu

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Re: VCE Physics Question Thread!
« Reply #2400 on: September 07, 2021, 09:04:18 am »
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Just about to do our final sac in physics, and no quite sure how to answer this question:
worth 3 marks
Explain with reference to quantised energy levels why only photons of certain wavelengths can be emitted by the atom.

thanks in advance

Billuminati

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Re: VCE Physics Question Thread!
« Reply #2401 on: September 07, 2021, 09:37:47 am »
+5
Just about to do our final sac in physics, and no quite sure how to answer this question:
worth 3 marks
Explain with reference to quantised energy levels why only photons of certain wavelengths can be emitted by the atom.

thanks in advance

I can answer your question based on the knowledge I learned in my 2nd year uni physical + analytical chem unit, with the caveat that the required explanation in VCE physics may be different. Basically what they mean by quantised is that in a chemical species, when electrons are excited by energy and move up to an orbital of higher energy, the associated quantity of energy absorbed (we call this a transition) is highly specific. Hence, this specificity translates to a very specific wavelength emitted given you already know the speed (c) and frequency (f) of the wave. The energy of each energy transition is usually plotted in what's known as a Grotian diagram.

That's why atomic absorption spectroscopy is highly selective, because each chemical species has distinct absorption bands that rarely overlaps with others so you'll almost never get a false positive or overestimation of concentration because some other chemical species with a similar absorption band is present. Basically how AAS works is a lamp containing the element of interest is used to emit light of the same wavelength of absorption, this light source is shined onto a burner which converts the element of interest in your sample into gaseous, ground state atoms which can absorb this electromagnetic radiation. This absorbance is detected by computers, but not before being wavelength-restricted to the intended wavelength by a monochromator
« Last Edit: September 07, 2021, 09:43:14 am by Billuminati »
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mcpunjavu

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Re: VCE Physics Question Thread!
« Reply #2402 on: September 09, 2021, 07:38:17 pm »
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I can answer your question based on the knowledge I learned in my 2nd year uni physical + analytical chem unit, with the caveat that the required explanation in VCE physics may be different. Basically what they mean by quantised is that in a chemical species, when electrons are excited by energy and move up to an orbital of higher energy, the associated quantity of energy absorbed (we call this a transition) is highly specific. Hence, this specificity translates to a very specific wavelength emitted given you already know the speed (c) and frequency (f) of the wave. The energy of each energy transition is usually plotted in what's known as a Grotian diagram.

That's why atomic absorption spectroscopy is highly selective, because each chemical species has distinct absorption bands that rarely overlaps with others so you'll almost never get a false positive or overestimation of concentration because some other chemical species with a similar absorption band is present. Basically how AAS works is a lamp containing the element of interest is used to emit light of the same wavelength of absorption, this light source is shined onto a burner which converts the element of interest in your sample into gaseous, ground state atoms which can absorb this electromagnetic radiation. This absorbance is detected by computers, but not before being wavelength-restricted to the intended wavelength by a monochromator

thats super helpful, thanks so much!

Newton is Nice

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Re: VCE Physics Question Thread!
« Reply #2403 on: September 26, 2021, 03:14:29 pm »
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Hi guys - got a question regarding saturation current:
2017 NHT- asked why beyond a certain voltage (1V), there is no increase in photocurrent, and here is what they said:
"At V = +1.0 V all of the available photoelectrons are being collected. Since there are no more
photoelectrons to be collected, increasing the voltage will not result in an increase in photocurrent."
However, if I were to increase the intensity of the light, then there would be an increase in the number of electrons emitted, thereby implying that there are still photoelectrons available in the metal that have not been emitted. This would contradict them saying "there are no more photoelectrons to be collected".
It would be great if anyone could kindly let me know if my reasoning is wrong, or if I am simply misinterpreting what they are saying.
Cheers.

eman27_hc

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Re: VCE Physics Question Thread!
« Reply #2404 on: October 29, 2021, 04:13:16 pm »
+2
Hi guys - got a question regarding saturation current:
2017 NHT- asked why beyond a certain voltage (1V), there is no increase in photocurrent, and here is what they said:
"At V = +1.0 V all of the available photoelectrons are being collected. Since there are no more
photoelectrons to be collected, increasing the voltage will not result in an increase in photocurrent."
However, if I were to increase the intensity of the light, then there would be an increase in the number of electrons emitted, thereby implying that there are still photoelectrons available in the metal that have not been emitted. This would contradict them saying "there are no more photoelectrons to be collected".
It would be great if anyone could kindly let me know if my reasoning is wrong, or if I am simply misinterpreting what they are saying.
Cheers.

"if I were to increase the intensity of the light, then there would be an increase in the number of electrons emitted, thereby implying that there are still photoelectrons available in the metal that have not been emitted."

I think that's where your logical reasoning goes wrong. I don't believe that jump in logic is sound. Just because more electrons are being emitted from the light does not mean there are still more available electrons on the cathode that are able to be ionized.

james.358

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Re: VCE Physics Question Thread!
« Reply #2405 on: October 29, 2021, 06:21:23 pm »
+1
Great answer by eman27_hc!

I just want to quickly add that the problem with Newton is Nice's answer isn't that it is necessarily wrong, it just doesn't answer what the question asks for. The question says why an increase in voltage doesn't lead to an increase in photocurrent. This is because all the available photoelectrons are ejected, so an increase in voltage (which basically makes the receiving plate more "attractive") wouldn't do anything.

However, an increase in intensity would increase the photocurrent. Remember that this is a circuit, so if you increase the intensity, the rate of incident photons will increase, so more electrons will be ejected and hence a higher current will be achieved.

Quote
are still more available electrons on the cathode that are able to be ionized.

This is a very minor detail, but note that the electrons are not being ionised. Ionisation would imply that the metal becomes positively charged after losing the electron. However, as the photelectric effect takes place in a circuit, the lost electrons are replenished and hence no ions are formed.

Hope this helps,
James
« Last Edit: October 29, 2021, 06:22:58 pm by james.lhr »
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miyukiaura

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Re: VCE Physics Question Thread!
« Reply #2406 on: November 05, 2021, 06:46:55 pm »
+1
Hey guys, I just wanted to know what you think was the hardest exam, and the most poorly done topics in physics?
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eman27_hc

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Re: VCE Physics Question Thread!
« Reply #2407 on: November 05, 2021, 08:13:33 pm »
+4
Hey guys, I just wanted to know what you think was the hardest exam, and the most poorly done topics in physics?

Hi, from the past VCAA exams that I've done, I don't think there's any particular exam that stands out to be clearly 'harder' than the others. In terms of the big practical investigation problem that they include every year, the 2019 VCAA paper is probably the hardest/most different from other years - just because it comprises of two strings which are concurrently used, and so has the potential to throw some people off.

In terms of which topics are done most poorly, I'm not sure with 100% certainty which topic it is. Though from what I can recall, I believe the chief assessor for physics said it was special relativity. In my opinion, the most poorly done problems are generally the explanation type questions. This is from looking at the chief examiner's report.

Hope it helps

eman27_hc

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Re: VCE Physics Question Thread!
« Reply #2408 on: November 05, 2021, 09:01:44 pm »
+1
Why is answer A here wrong (q. 12)? isnt the velocity function a linear equation for objects falling under the influence of gravity, and hence isnt the speed of da object increasing at a constant rate? i wouldve thought both A and C are correct but apparently its just C

Hi, for physics, you have to break down the velocity into its vertical and horizontal components. Expanding upon your statement "isnt the velocity function a linear equation for objects falling under the influence of gravity", this assertion only applies to the vertical component of the velocity. This will be important to note.

Secondly, the speed of the object is a combination of the vertical and horizontal component of the velocity. Hence, you have to apply Pythagoras theorem to find the speed of the object. I've attached the mathematics behind how this works.

If you look at the final speed equation that I arrive at, it is clear that the speed does not increase at a constant rate - it is actually quadratic expression that is all under a square root!


Hopefully this helps!

miyukiaura

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Re: VCE Physics Question Thread!
« Reply #2409 on: November 06, 2021, 12:30:59 pm »
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Just wondering whether using split ring commutators instead of slip rings in a generator makes the graph of flux vary from 0 to maximum (instead of a sin or cos graph)? I know the split ring commutator reverses the direction of current every half turn hence making emf always in the same direction, but does it affect the actual flux going through the loop?

Thanks so much!
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james.358

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Re: VCE Physics Question Thread!
« Reply #2410 on: November 06, 2021, 12:38:19 pm »
+3
Nah. The only difference using a split ring would be to rectify the emf graph. So rather than sin(x) for example, the graph would look like |sin(x)|. Other than that, every other feature should be identical (e.g. maximum emf, frequency, flux).
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TnGn74

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Re: VCE Physics Question Thread!
« Reply #2411 on: June 01, 2022, 06:21:04 pm »
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Do we need to know about relativity of simultaneity explicitly for VCE Physics?

nillyadis

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Re: VCE Physics Question Thread!
« Reply #2412 on: June 20, 2022, 08:40:27 pm »
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Is the frequency in a compression different to one in a rarefaction?

nillyadis

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Re: VCE Physics Question Thread!
« Reply #2413 on: June 20, 2022, 09:20:05 pm »
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How does polarisation support the theory that light is a transverse wave and not longitudinal?
Is is because:
it uses a vertical filter
the vertical filter stops every transverse wave that doesn't oscillate vertically and only allows a vertically oscillating wave to pass through
a longitudinal wave oscillates parallel to the direction the wave propagates
it is not longitudinal because we do not observe all waves passing through the filter,   

Bri MT

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Re: VCE Physics Question Thread!
« Reply #2414 on: June 22, 2022, 08:33:44 pm »
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How does polarisation support the theory that light is a transverse wave and not longitudinal?
Is is because:
it uses a vertical filter
the vertical filter stops every transverse wave that doesn't oscillate vertically and only allows a vertically oscillating wave to pass through
a longitudinal wave oscillates parallel to the direction the wave propagates
it is not longitudinal because we do not observe all waves passing through the filter,   

Polarisation selects a particular direction (not necessarily vertical) of oscillation. Evidence for this is indicated by light being able to pass through one polarisation filter, but not a second one that selects for the perpendicular direction of oscillation. In longitudinal waves, oscillation is parallel to the direction of wave movement whereas in transverse waves particles oscillate perpendicular to the direction the wave is moving in. Thus, only transverse waves undergo polarisation, and light being able to be polarised suggests that it is a transverse wave.