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Altius Covid Exam 5 Question 46
Sukham_5277
#1 Posted : Saturday, July 18, 2020 5:24:13 PM
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Hello,
For this question I don't understand how the right hand rule was applied to find the force.
I understand that your thumb points in the direction of the current and your finger curl gives the direction of the magnetic field but I am not sure how magnetic force fits into this right hand rule.

Thank you!
INSTR_Katerina_102
#2 Posted : Saturday, July 18, 2020 6:24:17 PM
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Hi,

I've drawn how one might solve this question on the MCAT: https://ibb.co/3rx9zWR

The trick here is that you are using the right hand rule twice in two different ways:

The first is with the thumb in the direction of the current and the thumbs coiling in the direction of the magnetic field. You use this to get the direction of the magnetic field (which I have denoted as X for going into the page and . for going out of the page. You do not denote force at this point (or if you can I'm not sure how you do that).

The second is the conventional one for a point charge (because a current is almost a line of point charges moving in a kind of conga line). I then use my index finger in the direction of the charge, middle finger for the force, and thumb for the magnetic field I got in the first step.

I hope this helps!

Katt
Sukham_5277
#3 Posted : Saturday, July 18, 2020 7:50:09 PM
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Hi,
the right hand rule makes sense now, thanks! But I was wondering why the direction of the point charge is opposite from that of the current? Is is because the point charge is equivalent to an electron?
INSTR_Katerina_102
#4 Posted : Saturday, July 18, 2020 8:07:36 PM
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Hi Sukham,

"the right hand rule makes sense now, thanks! But I was wondering why the direction of the point charge is opposite from that of the current? Is is because the point charge is equivalent to an electron?"

The MCAT uses conventional current, where you imagine a positive charge moving in the same direction as I.

I think where you are getting mixed up is that the force of the magnetic field B1 generated by wire 1 must operate on the wire 2, which has current I in opposite direction to wire 1.

Likewise, the B2 generated by wire 2 will act on wire 1.

This is probably where you get the illusion of a switched point charge direction.

Please let me know if this is unclear,

Katt

Sukham_5277
#5 Posted : Saturday, July 18, 2020 10:08:38 PM
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Thank you! That cleared up the confusion for me :)
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