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Simple Phenomena Of Magnetism

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Apuntes de la lección

Magnetic and Non-Magnetic Materials

  • Only iron, cobalt, nickel, and steel (an alloy containing iron) are magnetic.
  • Magnetic materials are attracted to a magnet; non-magnetic materials are not.
  • A magnetic material (not a magnet) is attracted to both poles of a permanent magnet.
  • To test a material: if repelled by a known magnet, it is a magnet; if attracted, it is a magnetic material; if neither, it is non-magnetic.

Magnetic materials

Magnetic materials

Forces Between Magnetic Poles

  • Every magnet has a north pole and a south pole; magnetic forces are strongest at the poles.
  • Like poles repel: north–north or south–south push apart.
  • Opposite poles attract: north–south pull together.
  • The force between a magnet and a magnetic material is always attractive due to induced magnetism.

Poles of a Magnet

Poles of a Magnet

Induced Magnetism

  • An induced magnet is a material (e.g., soft iron) that becomes a magnet temporarily in a magnetic field.
  • The end of the material nearest the magnet acquires the opposite pole to the magnet’s pole.
  • When removed from the field, the material loses its magnetism quickly.
  • Induced magnetism always causes attraction between the magnet and the material.

Inducing magnetism in a magnetic material

Inducing magnetism in a magnetic material

Permanent and Temporary Magnets

  • Permanent magnets (e.g., steel) produce their own magnetic field and do not lose magnetism easily.
  • Temporary (induced) magnets (e.g., soft iron) are magnetic only while in an external field.
  • An electromagnet consists of coils of wire around a soft iron core; it can be switched on/off.
  • Strength of an electromagnet can be increased by: increasing current, increasing number of turns, or adding a soft iron core.

Electromagnet diagram

Electromagnet diagram

Uses of Magnets

  • Permanent magnets are used in compasses, fridge magnets, toys, and school experiments.
  • Electromagnets are used in MRI scanners, speakers, recycling (lifting scrap metal), and Mag-Lev trains.

Uses of permanent magnets

Uses of permanent magnets

Magnetic Fields

  • A magnetic field is a region where a magnetic pole experiences a force.
  • Field lines go from north to south; arrows show the direction of force on a north pole.
  • Field is strongest at the poles (lines are closer together) and weakens with distance (lines spread out).
  • The direction of the field at a point is the direction of the force on a north pole placed there.

The magnetic field around a bar magnet

The magnetic field around a bar magnet

Diapositivas

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Preguntas de práctica

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  1. 1.Which of the following metals is magnetic?

    Easy
    • ACopper
    • BAluminium
    • CNickel
    • DZinc
  2. 2.Two magnets are brought close together. The north pole of one magnet is placed near the north pole of the other magnet. What happens?

    Easy
    • AThey attract each other
    • BThey repel each other
    • CNo force is exerted
    • DThey become demagnetised
  3. 3.A bar magnet is brought near a piece of metal. The metal is attracted to the magnet. Which of the following could be the metal?

    Easy
    • AAluminium
    • BCopper
    • CSteel
    • DZinc
  4. 4.Magnetic materials are attracted to both poles of a permanent magnet.

    Easy

    True or false?

  5. 5.An induced magnet can be switched on and off.

    Medium

    True or false?

  6. 6.State the two magnetic metals (other than steel).

    Easy
  7. 7.Explain why a magnetic material is always attracted to a magnet, regardless of which pole is held close to it.

    Medium
  8. 8.Complete the sentence:

    Easy

    Like magnetic poles ____ and opposite magnetic poles ____.

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