TPR Physcis - Possible Mistake?

[5words]

Which of the following describes the motion of a positive ion with charge +e and an electron each moving perpendicular to the same uniform magnetic field with the same speed?

A. The ion and the electron will move in circular paths in the same direction but with different radii.
B. The ion and the electron will move in circular paths in opposite directions but with the same radii.
C. The ion and the electron will move in circular paths in opposite directions and with different radii.
D. The ion and the electron will move in straight lines in opposite directions.

So i chose B , because the positive charge described above is a positron which supposedly has the same mass as an electron. But TPR's answer is C , how is that possible when both particle have the same mass??

Since the Magnetic force is supplying the centripedal force here .. so qvB = mv^2/r so if m and v and |q| are the same why is the radii changing?

Question: is this a mistake from them? Thanks.

@aldol16

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[aldol16]

If the particles have the same speed and mass and are moving through the same uniform magnetic field, then the radii will be the same. Are you sure they're still talking about a positron? Because usually we don't refer to positrons as "ions."

 

[5words]

If the particles have the same speed and mass and are moving through the same uniform magnetic field, then the radii will be the same. Are you sure they're still talking about a positron? Because usually we don't refer to positrons as "ions."

You are right, just check again, protons have +1 charge as well, as a matter of fact once an atoms lose an electron it acquire a positive +e charge as well... So, my question to you aldol is , in order to consider a positive charge as a positron, the pssage or question stem has to mention it right?

 

[aldol16]

You are right, just check again, protons have +1 charge as well, as a matter of fact once an atoms lose an electron it acquire a positive +e charge as well... So, my question to you aldol is , in order to consider a positive charge as a positron, the pssage or question stem has to mention it right?

Well it makes a difference if you're talking about protons. A huge difference. Protons have a mass that is roughly 1800 times that of an electron. Which means it would have a larger radius of deflection. A positron is completely different - it's antimatter. It's only produced via radioactive decay.

 

[neurodoc]

Well it makes a difference if you're talking about protons. A huge difference. Protons have a mass that is roughly 1800 times that of an electron. Which means it would have a larger radius of deflection. A positron is completely different - it's antimatter. It's only produced via radioactive decay.

And in any case protons are not ions, they are subatomic particles. "Ion" is a term meaning a charged (+ or -) atom or molecule. It is possible to overthink test questions like this.

 

[aldol16]

And in any case protons are not ions, they are subatomic particles. "Ion" is a term meaning a charged (+ or -) atom or molecule. It is possible to overthink test questions like this.

Proton is an ion - it's one of the ions you get upon acid dissociation. In fact, we usually consider protons in their capacity as hydrogen cations and not as subatomic particles.

 

[neurodoc]

Proton is an ion - it's one of the ions you get upon acid dissociation. In fact, we usually consider protons in their capacity as hydrogen cations and not as subatomic particles.

I suppose you could call a bare proton an "ion," if you want, and I appreciate your argument, but the generally accepted meaning of ion is that it is a positively or negatively charged "atom" or "molecule." A proton is a hydrogen nucleus (at least for one hydrogen isotope), and I believe an internet search will confirm that most folk do not consider bare protons to be "ions." Also, in aqueous acidic solutions it is the hydronium ion complex that is being referred to, not "H+" I am not trying to be pedantic, just to help you understand how overthinking led you to the wrong answer.

 

[aldol16]

I suppose you could call a bare proton an "ion," if you want, and I appreciate your argument, but the generally accepted meaning of ion is that it is a positively or negatively charged "atom" or "molecule." A proton is a hydrogen nucleus (at least for one hydrogen isotope), and I believe an internet search will confirm that most folk do not consider bare protons to be "ions." Also, in aqueous acidic solutions it is the hydronium ion complex that is being referred to, not "H+" I am not trying to be pedantic, just to help you understand how overthinking led you to the wrong answer.

A positively charged hydrogen is a proton. Hydrogen is an atom. Therefore, a proton is an ion. If you don't believe me, you can take a look at the encyclopedia definition: https://www.britannica.com/science/hydrogen-ion

When refer to acid-base chemistry, we refer to "acidic protons." For instance, the acidic protons of acetylacetone are at the C-3 position. A base like LDA will come in and pluck one of those off - they will not pluck off a hydronium. They pluck off a proton, which is cationic hydrogen.

Also, please note who you're responding to - I'm not OP and I'm not the one who got the question wrong. I have a PhD in chemistry and so I'm trying to explain the chemistry of the problem to OP, as I regularly do on the MCAT threads.

 

[neurodoc]

A positively charged hydrogen is a proton. Hydrogen is an atom. Therefore, a proton is an ion. If you don't believe me, you can take a look at the encyclopedia definition: https://www.britannica.com/science/hydrogen-ion

When refer to acid-base chemistry, we refer to "acidic protons." For instance, the acidic protons of acetylacetone are at the C-3 position. A base like LDA will come in and pluck one of those off - they will not pluck off a hydronium. They pluck off a proton, which is cationic hydrogen.

Also, please note who you're responding to - I'm not OP and I'm not the one who got the question wrong. I have a PhD in chemistry and so I'm trying to explain the chemistry of the problem to OP, as I regularly do on the MCAT threads.

OK. I'm sorry if my comment offended you. Thanks for letting me know that it's perfectly OK to refer to bare protons as ions. I was taught differently, but that was a long time ago at Berkeley. And I really am sorry I mistook you for the OP. I still think he was overthinking the question, and probably the folks who wrote it were mistaken and thought it was common practice to not consider a bare subatomic particle, like a proton, to be an ion.

 

[5words]

I suppose you could call a bare proton an "ion," if you want, and I appreciate your argument, but the generally accepted meaning of ion is that it is a positively or negatively charged "atom" or "molecule." A proton is a hydrogen nucleus (at least for one hydrogen isotope), and I believe an internet search will confirm that most folk do not consider bare protons to be "ions." Also, in aqueous acidic solutions it is the hydronium ion complex that is being referred to, not "H+" I am not trying to be pedantic, just to help you understand how overthinking led you to the wrong answer.

You are actually agreeing with me. Hence why I made the question, but we are wrong because a proton can be consider ion and if we were right the answer would have been B. If the positive and negative charge were "just" ion (and not a proton), as in the case, they would have had the same radii. The radii is changing because what Aldo posted above.

Proton and electrons have different masses. And most ions are electrons ( or so I thought, hence why when I saw +e and -e, I automatically thought positron and electron inoted of proton aND electron.

I guess what I got from this question is that the Q stem or paragraph literally has to mentioned positron, otherwise a +1e or positive ions is assumed to be a proton.

 

[neurodoc]

You are actually agreeing with me. Hence why I made the question, but we are wrong because a proton can be consider ion and if we were right the answer would have been B. If the positive and negative charge were "just" ion (and not a proton), as in the case, they would have had the same radii. The radii is changing because what Aldo posted above.

Proton and electrons have different masses. And most ions are electrons ( or so I thought, hence why when I saw +e and -e, I automatically thought positron and electron inoted of proton aND electron.

I guess what I got from this question is that the Q stem or paragraph literally has to mentioned positron, otherwise a +1e or positive ions is assumed to be a proton.

Yes. I got distracted by the discussion about ions and whether or not a proton is considered to be an ion. In fact the question just mentioned a positive ion and didn't specify a proton at all. Clearly any positive ion is going to be a lot more massive than an electron and as Aldol said you can't consider a positron to be a positive ion. I appreciate Aldol's correcting my misunderstanding that protons shouldn't be considered ions...I remember learning that H1 nuclei stripped of their electron (protons) when in solution bond immediately with water molecules to form more stable ions (H3O and more complex assemblages). Anyhow, your post provides a good example about how overthinking a question can make it more difficult to get the answer the test writer considers correct, and my responses to these posts show how folks get off track by not paying attention to what is written!.