Kaplan MCAT 2015 Diagnostic test Chem/Physics Qns 22 help

[pusillanimous]

Hello everyone! I've posted my question on the Kaplan MCAT forum sometime ago but realized this is a much better place to put up my question instead. =)

Just took the free online MCAT 2015 diagnostic test and I can't seem to get the right answer for question 22 in the Chem/Physics section of the test (please see uploaded file); I will always get the answer 'into the page' rather than 'out of the page'.

Here's how I derived my incorrect answer.
1.Since they are talking about electrons, I used the left-hand rule (read that left-hand rule is for electrons while the right-hand rule is for protons) please clarify me if my concept is wrong on this. My thumb=force, index finger=magnetic field, middle finger=current direction
2. the electrons are moving in a circular path, so the force must be pointing towards the centre of the circular path. So, taking a hypothetical electron situated on the right side of the path, the force (aka left thumb) should be pointing left (since it it pointing towards the center).
3. Looking at how the electrons are moving in a counter clockwise direction, the current must also be moving in the same counter-clockwise direction. So again, taking a electron on the right side of the path, the current (middle finger) should be pointing up.
4. hence, what's left is the index finger (magnetic field), which is pointing into the page.

Really sad to know i got this question incorrect cos i know it is a relatively straight forward question and I'm supposed to score on this. Please help me to clarify any wrong concepts or anything that can help me to understand this better. Thanks everyone and much appreciated! =)

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[NextStepTutor_1]

This is how I approach it. See if this makes sense to you. I use a different spin on the RHR.

So I use RHR (not LHR) first cause I'm more comfortable with it as my go-to, and then just flip the results if the question is asking about a negative charge, like it is here.
Also, my thumb is the velocity of the charge, my fingers are the magnetic field, and my palm is the magnetic force.
At the right side of the machine, look at any one of the paths where the electron is just about to enter the right portion of the cyclotron. The electron is traveling to the right. But we see once it enters that it's getting pushed vertically. This means that my thumb (velocity) is pointing to the right and my palm (force) is pointing up. When I do this, my fingers are pointing into the page. The last thing to do is flip them since I'm using my right hand and the question is talking about an electron. So instead, the magnetic field is pointing out of the page.

I hope this helps some!!

 

[m25]

This is how I approach it. See if this makes sense to you. I use a different spin on the RHR.

So I use RHR (not LHR) first cause I'm more comfortable with it as my go-to, and then just flip the results if the question is asking about a negative charge, like it is here.
Also, my thumb is the velocity of the charge, my fingers are the magnetic field, and my palm is the magnetic force.
At the right side of the machine, look at any one of the paths where the electron is just about to enter the right portion of the cyclotron. The electron is traveling to the right. But we see once it enters that it's getting pushed vertically. This means that my thumb (velocity) is pointing to the right and my palm (force) is pointing up. When I do this, my fingers are pointing into the page. The last thing to do is flip them since I'm using my right hand and the question is talking about an electron. So instead, the magnetic field is pointing out of the page.

I hope this helps some!!

But when it's on the right side, isn't the velocity also point upwards?
Also what do the two semicircle on the right and left represent and how do you know this?

 

[NextStepTutor_1]

The velocity is deflected upwards. Look at the space between the 2 left and right semicircles - on the lower half of the trajectories (those lines at the lower half), the electron is moving from left to right, and then once it moves into the right semicircle, it begins an ascent presumably due to the magnetic force it feels from the magnetic field that the right semicircle or portion of the cyclotron is creating.

Intuition tells me that the 2 semicircles are the parts of the cyclotron. This isn't something I knew beforehand or you should now know for the future. But you should be able to presume this because it's the only reasonable way you can get to the right answer considering what the MCAT expects you to know. You sometimes have to make the information fit together on questions like these (on foreign processes).

 

[m25]

The velocity is deflected upwards. Look at the space between the 2 left and right semicircles - on the lower half of the trajectories (those lines at the lower half), the electron is moving from left to right, and then once it moves into the right semicircle, it begins an ascent presumably due to the magnetic force it feels from the magnetic field that the right semicircle or portion of the cyclotron is creating.

Intuition tells me that the 2 semicircles are the parts of the cyclotron. This isn't something I knew beforehand or you should now know for the future. But you should be able to presume this because it's the only reasonable way you can get to the right answer considering what the MCAT expects you to know. You sometimes have to make the information fit together on questions like these (on foreign processes).

Ooh, thank you, so in this case, the two semicircles are the area that actually have electric and magnetic field, and there is no electric/magnetic field present in the middle?
And the direction of velocity is the original direction of the electron before encountering electric/magnetic field?

 

[NextStepTutor_1]

There may be magnetic fields in the middle, but that's a complicated thought process, or at least makes it more complicated. With the info you're given, it's easier to just thing that areas of the semicircles are where the magnetic fields exist.
And yes to your 2nd question.

 

[m25]

There may be magnetic fields in the middle, but that's a complicated thought process, or at least makes it more complicated. With the info you're given, it's easier to just thing that areas of the semicircles are where the magnetic fields exist.
And yes to your 2nd question.

Okay, so the area of semicircle is where magnetic AND ELECTRIC field exist, right? And is electric field is pointing DOWN on the right side and pointing UP on the left side since direction of force and electric field is the same?

 

[pusillanimous]

Ah all of these makes sense! Thanks all for such wonderful discussions and inputs! =)