The field strength of an alternator can be adjusted by varying what?

The field strength of an alternator can indeed be adjusted by varying the rotating field. In an alternator, the rotor (which is the rotating part) is usually equipped with electromagnets. By changing the amount of current flowing through these electromagnets, you can increase or decrease the magnetic field strength. This, in turn, affects the voltage generated by the alternator because the voltage output is directly proportional to the strength of the magnetic field and the speed of rotation.

The concept relies on the principles of electromagnetic induction, where the changing magnetic field interacts with the stator windings to induce alternating current. Therefore, manipulating the current in the rotating field provides a controllable means to adjust the output voltage and overall performance of the alternator.

The other choices relate to different components of the alternator that do not directly change the magnetic field’s intensity. Pole pieces are fixed components that help shape the magnetic field but do not vary it. The stator remains stationary and only produces electricity when the magnetic field changes around it. Similarly, the stator windings are part of the power generation but do not directly adjust the magnetic field strength. Hence, varying the rotating field is the effective way to control the alternator's output characteristics.