Class 6 Notes

Class 6 Science Notes Chapter 13 (Fun with Magnets) – Science Book

Philoid

Philoid

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Science
Detailed Notes with MCQs of Chapter 13, 'Fun with Magnets'. This is an important chapter, and understanding the basic concepts of magnetism is crucial, not just for your class but also for various competitive government exams where basic science is tested. Pay close attention to the definitions and properties.

Chapter 13: Fun with Magnets - Detailed Notes for Exam Preparation

1. Discovery of Magnets:

  • Legend: The discovery is often linked to a shepherd named Magnes in ancient Greece. His wooden stick, which had an iron tip, supposedly got stuck to a rock containing magnetite. His iron-nailed sandals also got stuck.
  • Magnetite: This naturally occurring rock (an ore of iron, Fe₃O₄) possesses magnetic properties. These natural magnets were the first ones discovered.
  • Magnet: Any material that attracts objects made of iron, nickel, or cobalt is called a magnet. Substances attracted by magnets are called magnetic materials.

2. Magnetic and Non-Magnetic Materials:

  • Magnetic Materials: Materials that get attracted towards a magnet.
    • Examples: Iron, Nickel, Cobalt. Objects like pins, nails, keys (if made of iron), almirahs, etc.
  • Non-Magnetic Materials: Materials that are not attracted towards a magnet.
    • Examples: Wood, Plastic, Rubber, Glass, Aluminium, Copper, Stainless Steel (most common types), Leather, Paper, Sand, etc.
    • Exam Tip: Be careful with 'steel'. While steel is an alloy primarily of iron, some types (like stainless steel used in utensils) are largely non-magnetic due to other components. However, for Class 6 level, generally assume steel items might be attracted if the question doesn't specify 'stainless'.

3. Poles of a Magnet:

  • Observation: When iron filings are brought near a bar magnet, they stick mostly to the ends of the magnet. These ends are where the magnetic strength is concentrated.
  • Definition: The regions on a magnet where the magnetic force is strongest are called its poles.
  • Types of Poles: Every magnet, regardless of its shape (bar, horseshoe, cylindrical, ball-ended), has two poles:
    • North Pole (N): Also called North-seeking pole.
    • South Pole (S): Also called South-seeking pole.
  • Existence: Poles always exist in pairs. You cannot isolate a single North pole or a single South pole. If you break a magnet into two pieces, each piece will become a complete magnet with both a North and a South pole.

4. Finding Directions using Magnets:

  • Directive Property: A freely suspended magnet (e.g., a bar magnet tied to a thread and hung so it can rotate freely) always aligns itself in a nearly North-South direction when it comes to rest.
  • North-Seeking Pole (N): The end of the magnet that points towards the Earth's geographic North direction.
  • South-Seeking Pole (S): The end of the magnet that points towards the Earth's geographic South direction.
  • Application: This property is used to make magnetic compasses.
  • Magnetic Compass: A small device consisting of a magnetized needle pivoted at its center, free to rotate within a small box with a glass cover. The dial is usually marked with directions (N, S, E, W). The needle's North pole always points towards the geographic North. It's used for navigation.

5. Making Your Own Magnet (Temporary Magnetism):

  • Method (Single Touch Method):
    1. Take an iron bar or needle.
    2. Place one pole (say, North pole) of a strong bar magnet near one end of the iron bar.
    3. Without lifting the magnet, stroke the iron bar from one end to the other.
    4. Lift the magnet at the end, bring the same pole back to the starting point, and repeat the stroking process in the same direction about 30-40 times.
    5. The iron bar will acquire temporary magnetic properties.
  • Important: Stroking must be done consistently in one direction with the same pole. Random rubbing will not magnetize the object.

6. Attraction and Repulsion between Magnets:

  • Fundamental Law:
    • Like poles repel: North pole repels North pole (N ↔ N); South pole repels South pole (S ↔ S).
    • Unlike poles attract: North pole attracts South pole (N →← S).
  • Test for Magnetism: Repulsion is the surest test for magnetism. A magnet attracts magnetic materials (like iron), but it only repels another magnet (when like poles face each other). An unmagnetized iron bar will always be attracted by either pole of a magnet.

7. Properties of Magnets - Summary:

  • Attractive Property: Attracts magnetic materials like iron, nickel, cobalt.
  • Directive Property: A freely suspended magnet aligns itself in the North-South direction.
  • Poles Exist in Pairs: Magnetic poles cannot be isolated.
  • Like Poles Repel, Unlike Poles Attract.

8. Care and Storage of Magnets:

  • Magnets can lose their properties (demagnetization) if they are:
    • Heated strongly.
    • Hammered repeatedly.
    • Dropped from a height.
    • Stored improperly.
  • Proper Storage:
    • Bar Magnets: Should be stored in pairs with their unlike poles facing each other. Wooden pieces should separate them, and soft iron pieces (called keepers) should be placed across the ends.
    • Horseshoe Magnet: A single piece of soft iron (keeper) should be placed across its poles.
  • Purpose of Keepers: Keepers provide a path for the magnetic field lines, helping to preserve the magnet's strength over time.

Multiple Choice Questions (MCQs) for Practice:

  1. Which of the following is a natural magnet?
    (a) Bar magnet
    (b) Horseshoe magnet
    (c) Magnetite
    (d) Magnetic needle

  2. Which of these materials will NOT be attracted by a magnet?
    (a) Iron nail
    (b) Cobalt piece
    (c) Aluminium foil
    (d) Nickel coin

  3. When a bar magnet is suspended freely, its North pole points towards the:
    (a) Geographic East
    (b) Geographic West
    (c) Geographic North
    (d) Geographic South

  4. What happens when the North pole of a magnet is brought near the North pole of another magnet?
    (a) They attract each other
    (b) They repel each other
    (c) There is no effect
    (d) They first attract and then repel

  5. Where is the magnetic strength of a bar magnet maximum?
    (a) At the center
    (b) Only at the North pole
    (c) Only at the South pole
    (d) At both the poles

  6. Which action can cause a magnet to lose its magnetic properties?
    (a) Cooling it
    (b) Stroking it with another magnet
    (c) Hammering it
    (d) Storing it with keepers

  7. To make a temporary magnet from an iron rod using the single touch method, you should:
    (a) Rub it randomly with a magnet
    (b) Stroke it consistently in one direction with one pole of a magnet
    (c) Heat the iron rod and cool it near a magnet
    (d) Simply keep it near a strong magnet for a long time

  8. What is the surest test to identify if a given object is a magnet?
    (a) Attraction towards an iron piece
    (b) Attraction towards another magnet
    (c) Repulsion from another magnet
    (d) It points North-South when suspended

  9. Poles of a magnet always exist:
    (a) Singly (isolated)
    (b) In pairs
    (c) In groups of three
    (d) Only at the ends of a bar magnet

  10. Soft iron pieces placed across the ends of bar magnets during storage are called:
    (a) Insulators
    (b) Conductors
    (c) Keepers
    (d) Separators

Answer Key for MCQs:

  1. (c)
  2. (c)
  3. (c)
  4. (b)
  5. (d)
  6. (c)
  7. (b)
  8. (c)
  9. (b)
  10. (c)

Study these notes carefully. Remember the definitions, properties, and examples. Understanding attraction, repulsion, and the directive property is key. Good luck with your preparation!

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