The force exerted by a magnetic field of intensity B on a wire of length L is:
[tex]F=ILB \sin \theta[/tex]
where I is the current in the wire and [tex]\theta[/tex] is the angle between the wire and the direction of B.
In our problem, the length of the wire is L=260 m, the current is I=115 A, the intensity of the Earth's magnetic field is [tex]B=5.0 \cdot 10^{-5}T[/tex] and the angle between the wire and B is [tex]80^{\circ}[/tex], and by substituting these numbers in the previous expression we can find the magnitude of the force:
[tex]F=(115 A)(260 m)(5.0 \cdot 10^{-5}T)(\sin 80^{\circ})=1.48 N[/tex]
