vectors and magnetic force

[ACLerok]

A straight, vertical wire carries a current of 1.13A downward in a region between the poles of a large superconducting electromagnet, where the magnetic field has a magnitude of 0.553T and is horizontal.

What is the magnitude of the magnetic force on a section of the wire with a length of 1.00cmthat is in this uniform magnetic field, if the magnetic field direction is 29.0degrees south of west?

i read the textbook and tried using F= current*length*magnetic field*sin(theta) and i got .00303 N.. is this correct or am i going about this all wrong?

[Doc Al]

i read the textbook and tried using F= current*length*magnetic field*sin(theta) and i got .00303 N.. is this correct or am i going about this all wrong?
In that formula, theta is the angle between the current in the wire and the magnetic field. In this problem the wire is vertical (call it along the z-axis) and the magnetic field is horizontal (somewhere in the x-y plane). So what is the angle between them?

[ACLerok]

i just tried taking the cross product of I and B but that didnt work out right? how wold i find the angle between the magnetic field and the current?

[Doc Al]

The cross product would work fine if you used the correct angle between the vectors. Hint: the wire is vertical and the field is horizontal. :smile:

[ACLerok]

would that angle just be the one given then?

[Doc Al]

would that angle just be the one given then?
If by that you mean "29.0 degrees", then no. That's the angle the field makes with west, not the angle it makes with the wire.

The way I'm picturing this coordinate system is: the wire is along the z-axis, north is along the y-axis, and east is along the x-axis. Specifying the angle that the field makes with west (the -x axis) doesn't change the fact that the field is horizontal and thus perpendicular to the wire.

[ACLerok]

help please!

[ACLerok]

Anyone help this homework problem

[Doc Al]

Did you read my response? Do you know what perpendicular means?

[ACLerok]

so that angle is 90 degrees? if so, what does the 29 have to do with anything?

[Doc Al]

so that angle is 90 degrees?
Yes. if so, what does the 29 have to do with anything?
You'd need that to find the direction of the magnetic force.

[ACLerok]

Yes.
You'd need that to find the direction of the magnetic force.

i guess now i'm trying to find the angle the magnetic force will make relative to some axis, say north. time to use the right hand rule correct?

[Doc Al]

Yes, use the right hand rule.

[ACLerok]

Yes, use the right hand rule.

in determining the direction and the angle the magnetic force on teh wire makes with the y-axis (north), i should be applying the right hand rule to the vectors of the current and magnetic field correct?

[Doc Al]

That is correct.