"a 2.0-m straight wire carrying a current of 0.60 a is oriented parallel to a uniform magnetic field of 0.50 t. what is the magnitude of the magnetic force on it? "

The magnitude of the magnetic force acting on the wire is zero, because the magnetic field is parallel to the wire.

In fact, the magnetic force exerted by the magnetic field on the wire is

where I is the current in the wire, L the length of the wire, B the magnetic field intensity and  the angle between the direction of B and the wire. In our problem, B and the wire are parallel, so the angle is  and so , therefore the magnetic force is zero: F=0.

therefore, the efficiency is: