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VCE Physics Question Thread!
mcpunjavu:
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:
--- Quote from: mcpunjavu on September 07, 2021, 09:04:18 am ---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
--- End quote ---
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
mcpunjavu:
--- Quote from: Billuminati on September 07, 2021, 09:37:47 am ---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
--- End quote ---
thats super helpful, thanks so much!
Newton is Nice:
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:
--- Quote from: Newton is Nice on September 26, 2021, 03:14:29 pm ---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.
--- End quote ---
"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.
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