Question about right hand rule for physics

[Thundagere]

Question about "right hand rule" for physics

My school doesn't offer physics for freshmen right now, so I'm self studying it. Regarding the right hand rule for vectors, I'm slightly confused. I did excellent using it in mechanics, it was relatively simple, but I was at a loss as to how to use it for electricity and magnetism. For instance, consider this problem:

#9 THIS PAGE


I can't understand how to find the direction of electric and magnetic fields. They give an explanation, but again, I can't understand it. Can anyone simplify this for me?

 

[Dragynfyre]

During high school I learned an easier way to use the hand rules. First let your fingers represent the magnetic field and have your fingers point in the direction of the magnetic field (north to south). Then let your thumb point in the direction the charged particle is travelling. Your palm will then represent the direction of the force applied to the charged particle by the magnetic field. Use your right hand for positive particles and your left hand for negatively charged particles. As long as you know the direction of 2 of the variables you will always be able to turn your hand to find the direction of the 3rd.

 

[Thundagere]

During high school I learned an easier way to use the hand rules. First let your fingers represent the magnetic field and have your fingers point in the direction of the magnetic field (north to south). Then let your thumb point in the direction the charged particle is travelling. Your palm will then represent the direction of the force applied to the charged particle by the magnetic field. Use your right hand for positive particles and your left hand for negatively charged particles. As long as you know the direction of 2 of the variables you will always be able to turn your hand to find the direction of the 3rd.

Now that works! :)
But what about wires? A lot of the practice problems I'm doing involve wires carrying current, and sometimes I'm just given a magnetic field, only being told the magnitude of the force. What if I don't know the forces direction or the field's direction, how would I solve that?

 

[Dragynfyre]

Do you know the direction of current? Also can you give an example?

 

[Thundagere]

I guess the main one is on the Sparknotes page I gave, number 9. It's where I get most of my practice problems, but that one was annoying.

 

[Dragynfyre]

Ahh for that question you need to find the magnetic field created by each wire. A wire with a current creates a magnetic field that wraps in circles around the wire. To find the direction of the magnetic field stick out your thumb and make a fist. Your thumb points in the direction of current flow and your fingers represent the circular magnetic field. Again use your right hand for positive charges and your left for negative charges.

Picture
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/magcur.gif

 

[Thundagere]

Ah...but how would you find the direction of the force? THe magnetic field is circular, what about the forces exerted on one another?

 

[Dragynfyre]

Take a look at this picture. The direction of the field at the location of the second wire has a single, definite direction.

 

[Thundagere]

I saw that, but how do you directly find the direction of that force?

 

[Dragynfyre]

Use the hand rule I explained in my first post. Fingers in the direction of the magnetic field and choose the direction of the charge (current in this case) as the opposite direction to the current in the wire creating the magnetic field (so downwards in this case) and point downwards with your thumb. Your palm will represent the force. Make sure to use the proper hand which in this case looks to be your right hand since the hand in the diagram is of a right hand.

 

[Thundagere]

Use the hand rule I explained in my first post. Fingers in the direction of the magnetic field and choose the direction of the charge (current in this case) as the opposite direction to the current in the wire creating the magnetic field and point to the direction of the current with your thumb. Your palm will represent the force. Make sure to use the proper hand which in this case looks to be your right hand since the hand in the diagram is of a right hand.

DANG! I just tried it...thanks! Now it makes sense!

 

[nasu]

Assuming some directions for currents and applying the hand rules works, no doubt. However I think that for the given problem you were supposed to use the general knowledge that two parallel wires that carry currents in opposite direction repel each other (and attract each other when the currents are in the same direction). Of course, this rule comes from applying the hand rules but once you do it once, you can just make use of it.