Right hand rule/ Left hand rule (Solved)

[Scrapbook]

Hi,

I'm confused when to use the right hand rule and when to use the left hand rule. I thought when the directions of current and magnetic field are given, we always use the left hand rule to find the force. But the following question of the day from Kaplan state otherwise.

Question:
Epidemiological studies often try to link environmental factors to patterns of disease. There is ongoing debate as to whether exposure to magnetic fields can increase risks for cancer; recent studies, however, have failed to find such an association. 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?

1. The wire square will remain still.
2. The square will rotate, its right side coming toward you and its left side moving away from you.
3. The square will rotate, its top coming toward you and its bottom moving away from you.
4. The square will rotate, its bottom coming toward you and its top moving away from you.

Answer: 3

Explanation:
Draw a diagram of the square wire loop with the current and magnetic field. Use the right-hand rule to determine the direction of magnetic forces on the horizontal sides of the square. (Vertical sides of the square experience no magnetic forces as they’re parallel/antiparallel to the magnetic field). For the top side of the square, the magnetic force is pointing out of the page; for the bottom side of the square, it’s pointing into the page. Therefore, the square will rotate, its top coming toward you and its bottom moving away from you — answer choice (C). The loop would not remain still (A) as torque causes it to rotate and there are magnetic forces on the loop even though the net force is zero.

Why are we using the right hand rule here?

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[FutureD0C17]

Why would you not use the right hand rule might be a better question. The only time I remember using the LHR, which isn't even necessary and I only vaguely remember using it, was dealing with negative charges.

 

[aldol16]

Always use the right hand rule. Just forget about the left hand rule. In fact, forget the left hand rule exists. Now, the relevant equation here is vector I times L crossed by B. Apply the right-hand rule for the I and B vectors. You'll get the correct result.

You're just confused because you're thinking about current in the logical way, not in the conventional way physicists think about it. Current is actually the flow of electrons, but in any electric circuit, you define current as the flow of positive charge, not the other way around.

Now, again, just forget about the left-hand rule and just reverse the direction whenever you're dealing with a negative charge. Don't make things harder than they are.

 

[Scrapbook]

Oh shoot I got the rules mixed up. Thank you both for making it clear.
Bottom line: Right hand rule for all positive charges; reverse the direction for negative charges.

 

[sfgiants23]

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