Chapter 20, Problem 89PP

BIO Studying magnetic bacteria. Some types of bacteria contain chains of ferromagnetic particles parallel to their long axis. The chains act like small bar magnets that align these magnetotactic bacteria with the earth’s magnetic field. In one experiment to study the response of such bacteria to magnetic fields, a solenoid is constructed with copper wire, 1.0 mm in diameter, evenly wound in a single layer to form a helical coil of length 40 cm and diameter 12 cm. The wire has a very thin layer of insulation, and the coil is wound so that adjacent turns are just touching. The solenoid, which generates a magnetic field, is in an enclosure that shields it from other magnetic fields. A sample of magnetotactic bacteria is placed inside the solenoid. The torque on an individual bacterium in the solenoid’s magnetic field is proportional to the magnitude of the magnetic field and to the sine of the angle between the long axis of the bacterium and the magnetic-field direction.

89. What current is needed in the wire so that the magnetic field experienced by the bacteria has a magnitude of 150 μT?

1. A.    0.095 A
2. B.     0.12 A
3. C.     0.30 A
4. D.    14 A