TBR CBT#3, q25

[TinaBina22]

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So for this question, I selected C - but I don't really understand why it's incorrect because I don't get TBR's logic. And even following their logic, how would we come to that conclusion? Because to me it looked like a direct "look at the equation," question... Help?

25. If the mass of an ion in the mass spectrometer described in the passage were doubled, then the radius of its deflection path would:




B. increase by a factor of (2)½. B is the best answer. The radius of the path followed by an ionized, radially accelerated, moving particle in a mass spectrometer's magnetic field is given by Equation 1:

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The velocity of the particle depends on its mass (see Equation 2). Substituting Equation 2 into Equation 1 gives Equation 3:

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The radius is proportional to the square root of the particle's mass (r ∝ (m)½). If the mass were doubled, the radius would increase by a factor of , which is answer choice B. The reason choice C is not correct is that not all particles enter the mass spectrometer's magnetic field with the same initial velocity. The lightest particles generally enter the field with the greatest velocity. If the velocities of all particles as they entered the field were equal, then the radius of the path of any particle would double uniformly along with its mass. The best answer is B.

 

[SaCkO]

Yes I choose the wrong answer in that question too.. This question is tricky because when you are changing the mass you are changing the velocity too.. So you need to rewrite the formula and replace v for the new v...
Their answer is right , we should be aware of these kinds of questions because they might look easy but it tricky.

 

[411309]

So for this question, I selected C - but I don't really understand why it's incorrect because I don't get TBR's logic. And even following their logic, how would we come to that conclusion? Because to me it looked like a direct "look at the equation," question... Help?

25. If the mass of an ion in the mass spectrometer described in the passage were doubled, then the radius of its deflection path would:




B. increase by a factor of (2)½. B is the best answer. The radius of the path followed by an ionized, radially accelerated, moving particle in a mass spectrometer's magnetic field is given by Equation 1:

​

The velocity of the particle depends on its mass (see Equation 2). Substituting Equation 2 into Equation 1 gives Equation 3:

​

​

The radius is proportional to the square root of the particle's mass (r ∝ (m)½). If the mass were doubled, the radius would increase by a factor of , which is answer choice B. The reason choice C is not correct is that not all particles enter the mass spectrometer's magnetic field with the same initial velocity. The lightest particles generally enter the field with the greatest velocity. If the velocities of all particles as they entered the field were equal, then the radius of the path of any particle would double uniformly along with its mass. The best answer is B.

was the second formula given in the passage? because i wouldnt hae know it was 2.5 unless i saw the second formula; off the top of my head i wouldnt know it.

 

[SaCkO]

Yes it was .. both formulas were given.. its just a tricky question..

 

[TinaBina22]

Oh I getttt it, you're technically increasing both - that's why. I don't remember AAMC being this tricky...

 

[SaCkO]

Oh I getttt it, you're technically increasing both - that's why. I don't remember AAMC being this tricky...

Actually you are decreasing velocity , so the outcome of decreasing V by 2^1/2 and increasing mass by by 2 .
so 2*2^-1/2 = 2^1/2 or 1.4 .. I know it sounds tricky and weird

 

[Freeinfinity]

Actually you are decreasing velocity , so the outcome of decreasing V by 2^1/2 and increasing mass by by 2 .
so 2*2^-1/2 = 2^1/2 or 1.4 .. I know it sounds tricky and weird

This. Basically you double the mass, but the velocity is also inversely proportional to the square root of mass. So if you double the mass, you also decrease the velocity by root 2 at the same time. Hence, the overall change in the radius is 2/(root2), which is just root 2.