I've read a lot of comments on the fact that ions of different mass will emerge at different velocities from the accelerating potential. These are really good questions about the accelerating potential issue: absolutely the heavier particles will emerge with a lower velocity. If the speed selector was fixed at one value and the accelerating potential was fixed at one value, you would miss the vast majority of atoms/'molecules emerging from the ionization chamber. I ran into these comments while preparing to produce a video for a physics course, which inspired me to do more research on the issue, and the answer is very simple: the experimenter can turn the knobs to change these parameters in order to scan through ions with different charge to mass ratios! Mass spectrometers simply modify the parameters to rapidly scan through m/z values to produce a relative abundance vs. m/z plot. In the physics textbooks I've taught from, they always present it as "one velocity, many different landing positions", but clearly this can't be true unless the particles are very close in mass. In my research over the last couple days, I ran into a cool review article on the history of the mass spectrometer: www.researchgate.net/publication/277275153_Colloquium_100_years_of_mass_spectrometry_Perspectives_and_future_trends In the article, we see that Dempster's magnetic sector analyzer (1918) looks very much like the schematic we're used to seeing in introductory physics, except that *it has only a single detector* instead of a spread-out photographic plate. This made me realize that with experimental control over the accelerating potential and the magnetic field in the analyzer region, we can detect different particles at the same position by sweeping through the parameters of the device. It is interesting to note that Dempster's design did NOT have a speed selector, but he acknowledged the loss of resolution due to the distribution of ion velocities entering the analyzer and proposed adding the Wien filter (velocity selector) in order to improve the resolution of the device. I think the addition of the Wien filter is what distinguishes this canonical example as the "Bainbridge Mass Spectrometer". I ended up making my video with a spectrometer schematic that has a single fixed detector, assuming we have the experimental control to sweep through parameters and detect different ions by modifying the accelerating potential, speed selector and/or analyzer magnetic field. ua-cam.com/video/vFUMH5WVuc4/v-deo.html
omg thank you thank you thank you!!!! I was looking for this!!! I love your videos please make more!!! I only have 14 more days till ap physics 2, and 11 days till ap physics 1self studying them. these hellps super super much
I tried the exercise and everything is okay for the first radius, in the second part though I cannot understand how did you calculated the mass. Because if I try the result is 9.95x10^-26kg and if i put this in the formula the second radius results 5.2 instead of 6.. What did I do wrong?
I am a physics teacher. I would like to correct you on the hand rules…. To find the the direction of a force on a moving charge you ALWAYS use Fleming’s left hand rule. The right hand rule is only used to find the direction of an induced current in a magnetic field.
I use 2 right hand rules. I never liked the left hand rule because then you need to remember when to use right vs left. Instead I teach 2 right hand rules one for currents and one for for cross products. It’s worked for my students for over 20 years.
How are the isotopes moving with the same velocity after the velocity selector? Wouldn't the orange isotope move slower than the red one before entering the selector, so it will get deflected?
Also, I had a similar mass spectrum question on my textbook and in order to find velocity, they used conservation of energy equations... why could they not use the equation for circular motion and find velocity?
The energy that the ions gain during accelaration is same if we consider that all ions have same charge. So The heavier molecules will have smaller velocity and the ligther will have greater velocity. But velocity selector let pass only the ions with certain velocity. So how the spectrometer detects the other ions? It doesn't make sense.
That's assuming that they have an initial velocity of 0m/s. In general you have an ionized gas so they are entering the E-field region with different velocities. So you can have heavy and light molecules that are accelerated through the potential difference. I considered only the case with initial kinetic = 0 but this is just a specific case.
@@adosar7261 I think you are absolutely right. From the equation v=sqrt(2e|deltaV|/m) we see that the velocity of each ion is inversely proportional to the square root of its mass-to-charge ratio (m/z). So at any selected velocity only one m/z passes though the velocity selector to the analyzing magnet sector, which means that only one ion type can be observed. Thus, it's clear that the system discussed in the video cannot operate in the way a real mass spectrometer should. Even if the ions arriving the accelerating electric field have a distribution of initial velocities, it won't make a crucial difference, as the largest part of the final velocities still comes from the accelerating field. The differing initial velocities may of course make it possible for a small number of different mass-to-charge ratios (around the "correct" one) to pass through the velocity selector, but most of them are still filtered out and stay undetected, especially those that differ significantly from the "correct" m/z. Moreover, this type of "mass spectrometer" can never yield quantitative results, as this would require that at least on some selectable velocity a wide range of different mass-to-charge ratios should be present in exactly the same abundance proportions as in the sample that is arriving the ionization chamber. As a result, I think the system under discussion is not representative of any usable real-world mass spectrometer. Instead, it is better described as a pedagogical thought experiment often encountered in introductory-level physics materials, although unfortunately a flawed one as such.
I don understand how isotopes with different masses reach the same speed under the same accelerating conditions I try to do my takehome assignment but I am desperate now
Your right, Isotopes with different masses will NOT reach the same final speed if the force is the same (qE) and they started from the same initial velocity because they would have different acceleration (a=qE/m, different masses, same charge). However they don't necessarily have the same initial velocities. Think of them as having a distribution of initial velocities. Typically they are created by collisions (not covered in this video).
I've read a lot of comments on the fact that ions of different mass will emerge at different velocities from the accelerating potential.
These are really good questions about the accelerating potential issue: absolutely the heavier particles will emerge with a lower velocity. If the speed selector was fixed at one value and the accelerating potential was fixed at one value, you would miss the vast majority of atoms/'molecules emerging from the ionization chamber. I ran into these comments while preparing to produce a video for a physics course, which inspired me to do more research on the issue, and the answer is very simple: the experimenter can turn the knobs to change these parameters in order to scan through ions with different charge to mass ratios! Mass spectrometers simply modify the parameters to rapidly scan through m/z values to produce a relative abundance vs. m/z plot. In the physics textbooks I've taught from, they always present it as "one velocity, many different landing positions", but clearly this can't be true unless the particles are very close in mass.
In my research over the last couple days, I ran into a cool review article on the history of the mass spectrometer: www.researchgate.net/publication/277275153_Colloquium_100_years_of_mass_spectrometry_Perspectives_and_future_trends
In the article, we see that Dempster's magnetic sector analyzer (1918) looks very much like the schematic we're used to seeing in introductory physics, except that *it has only a single detector* instead of a spread-out photographic plate. This made me realize that with experimental control over the accelerating potential and the magnetic field in the analyzer region, we can detect different particles at the same position by sweeping through the parameters of the device. It is interesting to note that Dempster's design did NOT have a speed selector, but he acknowledged the loss of resolution due to the distribution of ion velocities entering the analyzer and proposed adding the Wien filter (velocity selector) in order to improve the resolution of the device. I think the addition of the Wien filter is what distinguishes this canonical example as the "Bainbridge Mass Spectrometer".
I ended up making my video with a spectrometer schematic that has a single fixed detector, assuming we have the experimental control to sweep through parameters and detect different ions by modifying the accelerating potential, speed selector and/or analyzer magnetic field.
ua-cam.com/video/vFUMH5WVuc4/v-deo.html
Incredible explanation. I appreciate your attention to details, it helps me understand the matter much better. Thank you so much
Best video on the topic so far. And I've seen many.
The separation is not the difference of the radii, but the diameters, so it is not 1, but 2cm
This saved me in physics 1, thank you so much!
Yea mate, thanks!
omg thank you thank you thank you!!!! I was looking for this!!! I love your videos please make more!!! I only have 14 more days till ap physics 2, and 11 days till ap physics 1self studying them. these hellps super super much
I tried the exercise and everything is okay for the first radius, in the second part though I cannot understand how did you calculated the mass. Because if I try the result is 9.95x10^-26kg and if i put this in the formula the second radius results 5.2 instead of 6.. What did I do wrong?
Great explanation!
Electrostatic propulsion for spacecraft adopts the same principle to achieve high exhaust velocities and specific impulse.
thank you so much!!! this was incredibly helpful and so easy to understand!!! God bless!!!
If the ratio of masses is 60:58 then the ratio of the radii is 60:58, this gives a 2nd radius of 5.17 cm (where did 6cm come from?)
I am a physics teacher. I would like to correct you on the hand rules…. To find the the direction of a force on a moving charge you ALWAYS use Fleming’s left hand rule. The right hand rule is only used to find the direction of an induced current in a magnetic field.
I use 2 right hand rules. I never liked the left hand rule because then you need to remember when to use right vs left. Instead I teach 2 right hand rules one for currents and one for for cross products. It’s worked for my students for over 20 years.
How are the isotopes moving with the same velocity after the velocity selector? Wouldn't the orange isotope move slower than the red one before entering the selector, so it will get deflected?
thank you so much, such a great energy and video.
Also, I had a similar mass spectrum question on my textbook and in order to find velocity, they used conservation of energy equations... why could they not use the equation for circular motion and find velocity?
According to my calculations I get 100,000 m/s for the velocity.
Same bruh
@@drewcoghlan8910 Same with me.
Really good Mass Spec physics lecture! May the F=ma be with you Ninja! :)
The energy that the ions gain during accelaration is same if we consider that all ions have same charge. So The heavier molecules will have smaller velocity and the ligther will have greater velocity. But velocity selector let pass only the ions with certain velocity. So how the spectrometer detects the other ions? It doesn't make sense.
That's assuming that they have an initial velocity of 0m/s. In general you have an ionized gas so they are entering the E-field region with different velocities. So you can have heavy and light molecules that are accelerated through the potential difference. I considered only the case with initial kinetic = 0 but this is just a specific case.
@@PhysicsNinja So we need the velocity selector because the molecules have initial velocity?
@@adosar7261 There is generally a distribution of velocities and we want to take a small sample of the velocities.
@@PhysicsNinja I still don't understand it. How we know that we didn't miss any ion that didn't have the selected velocity? Thanks in advance.
@@adosar7261 I think you are absolutely right. From the equation v=sqrt(2e|deltaV|/m) we see that the velocity of each ion is inversely proportional to the square root of its mass-to-charge ratio (m/z). So at any selected velocity only one m/z passes though the velocity selector to the analyzing magnet sector, which means that only one ion type can be observed. Thus, it's clear that the system discussed in the video cannot operate in the way a real mass spectrometer should.
Even if the ions arriving the accelerating electric field have a distribution of initial velocities, it won't make a crucial difference, as the largest part of the final velocities still comes from the accelerating field. The differing initial velocities may of course make it possible for a small number of different mass-to-charge ratios (around the "correct" one) to pass through the velocity selector, but most of them are still filtered out and stay undetected, especially those that differ significantly from the "correct" m/z.
Moreover, this type of "mass spectrometer" can never yield quantitative results, as this would require that at least on some selectable velocity a wide range of different mass-to-charge ratios should be present in exactly the same abundance proportions as in the sample that is arriving the ionization chamber.
As a result, I think the system under discussion is not representative of any usable real-world mass spectrometer. Instead, it is better described as a pedagogical thought experiment often encountered in introductory-level physics materials, although unfortunately a flawed one as such.
Also I got... 9.989 times tenth to the -22. How did you get 10 000 m/s
I don understand how isotopes with different masses reach the same speed under the same accelerating conditions I try to do my takehome assignment but I am desperate now
Your right, Isotopes with different masses will NOT reach the same final speed if the force is the same (qE) and they started from the same initial velocity because they would have different acceleration (a=qE/m, different masses, same charge). However they don't necessarily have the same initial velocities. Think of them as having a distribution of initial velocities. Typically they are created by collisions (not covered in this video).
Thank you
You sound like Saul Goodman
Gamed 🤍