Kaplan Question (Right Hand Rule)

[TommyTman]

You are facing a square loop of wire that is suspended in a uniform magnetic field. The magnetic field is pointing upward. If a current begins flowing through the loop of wire in a clockwise direction, which of the following will most likely happen?
A. The wire square will remain still.
B. The square will rotate, its right side coming toward you and its left side moving away from you.
C. The square will rotate, its top coming toward you and its bottom moving away from you.
D. The square will rotate, its bottom coming toward you and its top moving away from you.

This question came from the Kaplan Question of the Day. The answer is C. I keep getting D. For the top of the square, index finger points up, middle finger right and thumb into the page. When they are talking about current, maybe its flow of electrons and not conventional current?

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[sps27]

You are facing a square loop of wire that is suspended in a uniform magnetic field. The magnetic field is pointing upward. If a current begins flowing through the loop of wire in a clockwise direction, which of the following will most likely happen?
A. The wire square will remain still.
B. The square will rotate, its right side coming toward you and its left side moving away from you.
C. The square will rotate, its top coming toward you and its bottom moving away from you.
D. The square will rotate, its bottom coming toward you and its top moving away from you.

This question came from the Kaplan Question of the Day. The answer is C. I keep getting D. For the top of the square, index finger points up, middle finger right and thumb into the page. When they are talking about current, maybe its flow of electrons and not conventional current?

I've read the right hand rule in Kaplan and found that to be confusing. That is not Flemings right hand rule they talk in there. I remember reading about Flemings right and left hand rules in school and so since this is a question of motor, not generator, I used Flemings Left hand rule for this. Flemings left hand rule is for motor type problems and Flemings Right hand rule is for generator type problems. Fore finger is for external magnetic field, middle finger is for induced current or flow of current in a loop, thumb points towards direction of force or movement of conductor. Here is a link....if you use that rule, the ans would be C.

http://www.docstoc.com/docs/31982789/Fleming′s-Right-Hand-Rule-Fleming′s-Left-Hand-Rule

 

[icedragon]

http://www.phys.ufl.edu/courses/phy2049/f07/lectures/2049_ch30B.pdf

Scroll down to slide 29, the right hand rule should still apply, albeit I hold my fingers in the direction of the magnetic field and point my thumb in the direction of the current. If you orient your hand that way then your palm points in the direction of the force. C is correct. Perhaps you mixed up clockwise with counterclockwise?

 

[TommyTman]

Thanks alot guys. I think Flemmings left hand rule is the correct rule to use. But it all depends on how you perform the right hand rule. If you use the method of icedragon it definitely works. I was always taught the right hand rule using three fingers (like the attached picture). But if you google image right hand rule, the fingers are labeled differently depending on the website. That's why I'm confused about it, there seems to be no consistency.

 

[Blueprint MCAT Tutor]

But if you google image right hand rule, the fingers are labeled differently depending on the website. That's why I'm confused about it, there seems to be no consistency.

The right hand rule is about correctly getting the orthogonal relationship between the two vectors and there are, in fact, multiple "right" ways to label your hand. Here's the mnemonic I use with the students:

"Fingers Field, Palm Push, Thumb "Hitchhikes" along with the velocity"

When doing the right hand rule for the equation Fb = qvBsinT, hold your right hand in a flat L shape. DO NOT CURL YOUR HAND. DO NOT MOVE YOUR FINGERS AROUND. As you try to orient your hand to the problem, you can move your wrist and your elbow, BUT NOT YOUR HAND. Keep it in a rigid L shape.

Okay the reason for starting here is simple - over the years I've watched dozens and dozens of student who were trying to apply the right hand rule to a problem on the MCAT screen and they get limp finger syndrome - they start curling their fingers around to make it "fit" the problem and that gets them the wrong answer.

So, first: when doing Fb = iLB or Fb = qVB, keep your hand LOCKED in a rigid L shape.

Then apply the mnemonic "fingers field, palm push (force vector), thumb hitchhikes along with the current (or moving charge)"

Here, I even made a picture for you: http://imgur.com/pWhYWia

Good luck!

__________________

Bryan, Next Step MCAT Tutor
Check out my Office Hours thread and feel free to ask any questions here:Link

 

[Blueprint MCAT Tutor]

I MAED U ANOTHER PRETTY PICKSHUR!!

http://imgur.com/2cbj9jP

Hope this helps 🙂

__________________

Bryan, Next Step MCAT Tutor
Check out my Office Hours thread and feel free to ask any questions here:Link

 

[TommyTman]

Thanks alot that was super helpful! The pictures are great.