Chapter 20, Problem 91PP

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.

91. The solenoid is removed from the enclosure and then used in a location where the earth's magnetic field is 50 μT and points horizontally. A sample of bacteria is placed in the center of the solenoid, and the same current is applied that produced a magnetic field of 150 μT in the lab. Describe the field experienced by the bacteria. The field

1. A.    is still 150 μT.
2. B.     is now 200 μT.
3. C.     is between 100 and 200 μT, depending on how the solenoid is oriented.
4. D.    is between 50 and 150 μT, depending on how the solenoid is oriented.