Fix the mistake: Electricity and Magnetism

Magnets and Magnetic Fields

Magnets have fascinated people since ancient times. The invisible force that attracts iron, called a “magnetic field,” can be represented in a visible form.

Basic Knowledge

Magnets have an N pole and an S pole.
・Like poles (N and N, S and S) repel each other
・Unlike poles (N and S) attract each other
A space around a magnet in which magnetic force acts is called a magnetic field. Its direction is defined as the direction in which the N pole of a compass points.
Magnetic field lines can be used to visualize a magnetic field. Rules for magnetic field lines:
・They leave the N pole and enter the S pole
・The denser they are, the stronger the magnetic force
・Magnetic field lines do not cross
Earth is also a giant magnet. Its northern side is an S pole and its southern side is an N pole (which is why the N pole of a compass points north).

Important Terms
Magnetic field (the space in which magnetic force acts)
Magnetic field lines (imaginary lines showing the direction and strength of a magnetic field. N→S)
N pole and S pole (the two ends of a magnet. Like poles repel; unlike poles attract)
Ferromagnetic material (a substance strongly attracted to magnets. Iron, nickel, cobalt)
Magnetization (the phenomenon in which an iron nail temporarily becomes a magnet when rubbed with a magnet)
Earth’s magnetism (the magnetic field possessed by Earth. The northern side is an S pole, and the southern side is an N pole)

Further Exploration (Background and Significance)

At the atomic level, the source of magnetism is electron motion. Electrons orbit the nucleus and also have spin, which produces magnetism as tiny current loops. In many substances, the directions of electron spins are random and cancel one another. In iron, nickel, and cobalt, however, electron spins are easily aligned, so these materials show strong magnetism.
Compasses have been used in China since before the Common Era and were one of the inventions that changed navigation. Earth’s magnetic field also acts as a barrier protecting Earth from the solar wind and is essential for maintaining the biosphere.
Heating a magnet to a high temperature or giving it a strong удар causes its aligned magnetism to become disordered and the magnet to lose its magnetic force (Curie point). The Curie point of iron is approximately 770℃.
Magnets are used in many fields, including refrigerator doors, the magnetic stripes on credit cards, hard disks, linear motor trains, and MRI (magnetic resonance imaging) machines.

Key Points
  • Magnets have an N pole and an S pole
  • Like poles repel; unlike poles attract
  • Direction of a magnetic field = direction in which the N pole points
  • Magnetic field lines: N → S, do not cross, and are stronger where denser
  • The northern side of Earth is an S pole (because the N pole points north)
  • Ferromagnetic materials: iron, nickel, and cobalt
  • The source of magnetism is electron spin

Points to Note (Easily Confused)

① The northern side of Earth is an S pole because it attracts the N pole of a compass. This is opposite to intuition, so be careful. ② The direction of a magnetic field is defined as the direction in which the N pole points, so magnetic field lines also go N→S. ③ Even if a magnet is broken, both an N pole and an S pole always remain (magnetic monopoles have not yet been discovered in nature). ④ Even though they are metals, aluminum, copper, and gold do not stick to magnets because they are not ferromagnetic materials.

Practice

  1. How is the direction of a magnetic field defined?
  2. From which pole to which pole do magnetic field lines point?
  3. Give one example of a ferromagnetic material.

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