28. Magnetic Fields and Forces

A 5-m current-carrying wire (red line) is ran through a 4 T magnetic field (blue lines), as shown. The angle shown is 30°. What must the magnitude and direction of the current in the wire be when it feels a 3 N force directed into the page?

A small but rigid U-shaped wire carrying a 5.0-A current (Fig. 28–67) is placed inside a solenoid. The solenoid is 15.0 cm long and has 600 loops of wire, and the current in each loop is 7.0 A. What is the net force on the U-shaped wire?

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(I) (a) What is the force per meter of length on a straight wire carrying a 7.40-A current when perpendicular to a 0.90-T uniform magnetic field? (b) What if the angle between the wire and field is 35.0°?

A bolt of lightning strikes a metal flag pole, one end of which is anchored in the ground. Estimate the force the Earth’s magnetic field can exert on the flag pole while the lightning-induced current flows. See Example 25–10.

Near the equator, the Earth’s magnetic field points almost horizontally to the north and has magnitude B = 0.50 x 10⁻⁴ T. What should be the magnitude and direction for the velocity of an electron if its weight is to be exactly balanced by the magnetic force?

(II) A stiff wire 50.0 cm long is bent at a right angle in the middle. One section lies along the z axis and the other is along the line y = 2x in the xy plane. A current of 20.0 A flows in the wire—down the z axis and out the wire in the xy plane. The wire passes through a uniform magnetic field given by B (→ above B) = (0.285î ) T. Determine the magnitude and direction of the total force on the wire.

(II) The force on a wire carrying 6.45 A is a maximum of 1.64 N when placed between the pole faces of a magnet. If the pole faces are 55.5 cm in diameter, what is the approximate strength of the magnetic field?