Understanding Magnetic Field Hand Rules

[Mceestix]

Hey everyone. Just a quick high school physics question about determining magnetic fields using Hand rules.

The only thing I have trouble trying to understand is how to determine the way a compass arrow will point if you place it on top, or under an electric circuit. I know that they are opposites of each other , but i get confused trying to determine them. Any help would be appreciated.. thanks.

 

[jtbell]

You mean a (more or less) straight wire, right? Curl the fingers of your right hand around the wire so that your thumb points in the direction of the current. The magnetic field runs in circles around the wire, in the direction that your fingers are curling. This is also the direction that a compass needle will point if you place it near the wire.

 

[Mceestix]

You mean a (more or less) straight wire, right? Curl the fingers of your right hand around the wire so that your thumb points in the direction of the current. The magnetic field runs in circles around the wire, in the direction that your fingers are curling. This is also the direction that a compass needle will point if you place it near the wire.

Oh okay, that does make sense now... but what about the changes that happen to the compass if you place it on top of the wire, versus below the wire?

 

[Doc Al]

The compass needle will align itself with the field--as you move the compass up, down, and around the wire, the needle will adjust itself to always point in the direction of the field. So if you move the compass from above the wire to below the wire, the needle will swing 180 degrees to point in the other direction, since the magnetic field reverses direction.