【People's Education Press】High School Physics - Elective Compulsory, Volume 2: Starting from the Hand-Cranked Generator

When we turn the generator handle in the lab, the flickering of the bulb reveals a profound physical mystery: current does not always flow in one direction.Alternating Current (AC)its fundamental characteristic is thatits magnitude and direction vary periodically over time. This fundamentally differs from direct current (DC), which provides a constant direction of flow, as supplied by dry batteries.Direct Current (DC)has a fundamental difference.

1. Generator Structure and Classification

Modern generators primarily consist ofa coil (armature) that generates induced electromotive forceanda magnet that produces a magnetic fieldcomprising:

Rotating Armature Type: The coil rotates while the poles remain stationary. Suitable for low-voltage, small-scale devices.
Rotating Magnetic Pole Type: The magnetic poles rotate while the coil remains fixed. Capable of generating high voltage and strong current, this is the dominant design in modern large-scale power plant units.

2. Physical Mechanism: Starting from the Neutral Plane

By applying the right-hand rule to analyze the coil’s rotation, we identify two key positions:

Neutral Plane (coil plane ⊥ magnetic field lines): At this point, the magnetic flux through the coil is maximum, but the rate of change of flux is zero, so the induced current is zero.
Perpendicular to the Neutral Plane (coil plane ∥ magnetic field lines): At this moment, the magnetic flux is zero, but the effective speed at which the coil sides cut across the magnetic field lines is fastest, resulting in peak induced current.

QUESTION 1

[Short Answer] [Reading Context: Assuming the coil rotates uniformly counterclockwise, as shown in Figure 3.1-3. ...3. At what position is there no current in the coil, and at what position is the current maximum? During the process of the coil rotating from position A to position B shown in the diagram, in which direction does the current flow in segment AB?]

QUESTION 2

[Multiple Choice] In each of the following scenarios, the coil rotates uniformly with angular velocity ω about the axis shown in Figure 3-1. Which configurations can generate alternating current? Briefly explain your reasoning.

A [The coil rotates about an axis parallel to the magnetic field direction]

B [The coil rotates about a central axis perpendicular to the magnetic field direction]

C [A triangular coil rotates about its base within a uniform magnetic field]

D [A triangular coil rotates about its apex within a uniform magnetic field]

QUESTION 3

In the hand-cranked generator experiment, two diodes (LEDs) are connected in reverse parallel. When the rotation speed is very slow, observing the diodes lighting up alternately proves what characteristic of alternating current?

Current magnitude remains constant

Current direction changes periodically over time

Current voltage is very high

No change in magnetic flux inside the coil

QUESTION 4

Which of the following statements about the 'neutral plane' is correct?

The magnetic flux is zero when the coil passes through the neutral plane

The induced electromotive force is maximum when the coil passes through the neutral plane

The direction of the induced current changes once every time the coil passes through the neutral plane

The neutral plane refers to the position where the coil plane is parallel to the magnetic field

QUESTION 5

What type of generator do large power stations typically use to deliver powerful electrical output?

Rotating armature generator

Rotating magnetic pole generator

DC battery pack

Stationary electromagnet