What is the Direction of Magnetic Field Lines?

[ppppparker]

This is completely confusing me, please help. The word that is giving me problems is "direction".
The text talks about the "direction of the magnetic field lines" . As I understand it, the fied lines curve around a straight wire. So you can only have two directions, clockwise or anticlockwise. But are they saying that clockwise or anticlockwise is a a direction??
Im also confused when field lines are described as "into the page" or "out of the page". If the mag field lines wrap around the wire, then wouldn't that mean the field lines are going into the page on one end of wire and out of page on the other end (if you looking top down onto a wire)? So it should be both into and out of paper right?

Theres also this practice question in the text (that I will separete from this post and repost in homework questions, if necessary?) and it specifically asks this:

"A moving negative charge placed in an external magnetic field circulates counterclockwise in the plane of the paper. In which direction is the magnetic field pointing?"

-Is that a typo? Shouldnt it be asking about the direction of the magnetic force? (because the field lines go either clockw or anticlockw)? Or is this just more of me not understanding fields?

Thanks for any help. I really want to understand RHR...Its making me crazy.

 

[muscaria]

Imagine the centre of mass of an object is located at some point in space. Then you say it is rotating about the centre of mass point say at some angular frequency, but you need to specify an axis about which it is rotating. It is because you have to choose an orientation in space for this axis (a line in space) that rotational problems require direction (in 3d, not in 2d though if you think about it). Once you have chosen your line axis for rotation, you still need to specify whether it is rotating around the line clockwise or anti-clockwise. This reflects the fact that if you were moving along the axis line, you could move upwards or downwards. I'm not saying that when things rotate they are moving up or down the axis of rotation, but just that once you have specified an axis for rotation, spinning one way represents upwards, and spinning the other way represents downwards. The whole point though about the need for "direction" and vectors in rotational problems boils down to the fact that you need to pick out an an axis for the rotation, which points along a direction in space. Does this help?

 

[muscaria]

Is that a typo? Shouldnt it be asking about the direction of the magnetic force? (because the field lines go either clockw or anticlockw)? Or is this just more of me not understanding fields?

No typo. The direction of the magnetic field (whether it's pointing into the page or out of the page) will determine whether the charge goes clockwise or anti-clockwise. The direction of the magnetic force for either case will always point towards the centre of the circle.

 

[Chandra Prayaga]

"The text talks about the "direction of the magnetic field lines" . As I understand it, the fied lines curve around a straight wire. So you can only have two directions, clockwise or anticlockwise. But are they saying that clockwise or anticlockwise is a a direction??"

There is ambiguity in your understanding ad in your statements. I quoted your first statement above. It is true that the field lines curve around a straight wire, but that does not specify which way they curve. Your textbook states it very clearly, and you should read that before you proceed.

"If the mag field lines wrap around the wire, then wouldn't that mean the field lines are going into the page on one end of wire and out of page on the other end (if you looking top down onto a wire)? So it should be both into and out of paper right?"

The above statement is not clear at all. If, as you yourself say, the field lines are curving "around" the wire, where do the ends of the wire come into the picture? You should draw a diagram, so that the statements are clear to you as well as to others.

I really suggest you draw diagrams. Questions about which way a charge moves in a magnetic field should follow only after you clearly know how to figure out the direction of the magnetic field due to a current.

 

[sophiecentaur]

The magnetic field lines form a closed loop around any shape of conducting loop so they are pointing all over the place. The direction that you use is the direction of the lines at the point of interest. CLockwise or Anticlockwise are just general descriptions. These ideal circular patterns will only be found for a perfectly straight wire, very long and far away from anything else.