【Shanghai Textbook Edition】Junior High School English, Grade 9, Semester 1: The Nature of Magnetic Fields and Magnetization Phenomena

A magnetic field is not a mere imagination—it is an objective reality existing around magnets.special substanceThough invisible and intangible, the attraction and repulsion between magnets are transmitted through this medium.

Core Physical Principles

Magnetization: The process by which an originally non-magnetic object (such as steel or iron) acquires magnetism.Hard magnetic materials(e.g., steel) retain their magnetism easily after magnetization;Soft magnetic materials(e.g., iron) lose their magnetism easily after magnetization.
Curie Point: When a magnet's temperature rises to a critical threshold, intense molecular thermal motion disrupts the orderly alignment of magnetic domains, causing the magnetism to vanish completely.
Earth's Magnetic Field: Earth is a massive magnet, but note thatthe geomagnetic South Pole is near the geographic North Pole, and the geomagnetic North Pole is near the geographic South Pole (Figure 8-1-14).

⚠️ Common Misconception

Magnetic field lines are aphysical model, not actual physical lines. Furthermore, real magnetic fields exist throughout three-dimensional space, not just as two-dimensional patterns on paper.

QUESTION 1

The interaction between magnets occurs through ______.

air

magnetic field lines

magnetic field

gravitational waves

QUESTION 2

Assuming you're given a bar magnet and an iron rod with identical appearances (non-magnetic), which method can identify the magnet fastest without external tools?

Bring them close parallelly and observe if they attract

Touch one end of one object to the middle of the other and check for attraction

See which rod points south

QUESTION 3

Given several steel beads and one magnet, can you string them into a beautiful necklace without using needles or thread?

No, the beads are too heavy and will fall off

Yes, by magnetizing the beads to make them magnetic

QUESTION 4

Why does the door of a household refrigerator close tightly with just a light push?

Because the door hinge has a spring

Because the door seal contains permanent magnets

Because of pressure differences between inside and outside

QUESTION 5

If you heat the pole of a bar magnet with an alcohol lamp, how will its ability to attract paperclips change as the temperature increases?

Strength increases due to faster molecular motion

Strength decreases, possibly even disappearing completely

Remains unchanged

Comprehensive Investigation: Macroscopic and Microscopic Perspectives on Magnetic Fields

Analyze Patterns of Magnetic Field Distribution and Engineering Applications

In the physics lab, the teacher demonstrated the magnetic field line distribution when two like poles are placed opposite each other, as shown in Figure 8-1-18. Then, he posed preview questions about electromagnetic devices such as generators and motors. Please answer based on today’s lesson and prior knowledge:

1. Observing Figure 8-1-18, if a small compass is placed at points A, B, C, and D, what common feature do the directions of their north poles share? Also, indicate the direction of the magnetic field lines.

Answer:
The magnetic field lines should emerge outward from both N poles, forming a repulsive pattern. The direction of the north pole of each small compass at these points corresponds to the tangent direction of the magnetic field line at that location. Specifically, since field lines diverge from the N pole, the compass needle’s north pole will also point away from the nearest N pole.

2. [Generator Preview] How does a generator achieve energy conversion? What types of generators are you familiar with?

Answer:
A generator is a device that converts mechanical energy (or other forms of energy) into electrical energy. Common types include: hydroelectric generators (potential/kinetic energy → electricity), wind turbines (wind/kinetic energy → electricity), thermal power generators (thermal energy → kinetic energy → electricity), and nuclear power generators. Their core principle typically involves relative motion between magnetic fields and conductors.

3. [Water Level Automatic Alarm] Analyze the working principle shown in Figure 8-2-12: When the water level reaches the metal block, how does the circuit use magnetism to trigger an alarm?

Answer:
When the water level rises and contacts the metal block, the control circuit closes, giving the electromagnet magnetism and pulling down the armature. This closes the red-light branch (or triggers a bell) in the operating circuit, thus achieving automatic water level alarm. This demonstrates the property that magnetism is controlled by electric current.