Chapter 20, Problem 90PP

To use a larger sample of bacteria, the experimenters construct a solenoid that has the same length, same type of wire, and same spacing of loops, but twice the diameter of the original solenoid. How does the maximum possible magnetic torque on a bacterium in this new solenoid compare with the torque the bacterium would have experienced in the original solenoid? Assume that the currents in the solenoids are the same. The maximum torque in the new solenoid is

1. A.    twice that in the original one,
2. B.     half that in the original one.
3. C.     the same as that in the original one.
4. D.    one-quarter that in the original one.