28. Magnetic Fields and Forces

A rectangular loop of wire carries a 2.0-A dc current and lies in a plane which also contains a very long straight wire carrying a 10.0-A current as shown in Fig. 28–58. Determine (a) the net force and (b) the net torque on the loop due to the straight wire.

<IMAGE>

(II) An iron atom has a magnetic dipole moment of about 1.8 x 10⁻²³ A • m² . (a) Determine the dipole moment of an iron bar 15 cm long, 1.2 cm wide, and 1.0 cm thick, if it is 100 percent saturated. (b) What torque would be exerted on this bar when placed in a 0.80-T field acting at right angles to the bar?

A motor run by a 9.0-V battery has a 20-turn square coil with sides of length 5.0 cm and total resistance 24 Ω . When spinning, the magnetic field felt by the wire in the coil is 0.020 T. What is the maximum torque on the motor?

(I) If the restoring spring of a galvanometer weakens by 15% over the years, what current will give full-scale deflection if it originally required 25μA?

(I) A galvanometer needle deflects full scale for a 63.0-μA current. What current will give full-scale deflection if the magnetic field weakens to 0.700 of its original value?

(I) A 13.0-cm-diameter circular loop of wire is placed with the plane of the loop parallel to the uniform magnetic field between the pole pieces of a large magnet. When 3.80 A flows in the coil, the torque on it is 0.185 m • N What is the magnetic field strength?

(I) If the current to an electric motor drops by 18%, by what factor does the output torque change?

(I) How much work is required to rotate the current loop (Fig. 27–23) in a uniform magnetic field B (→ above B) from (a) θ = 0° (μ (→ above μ) ∣∣ B (→ above B)) to θ = 180°

, (b) θ = 90° to θ = ― 90°