Class 6 Science Notes Chapter 13 (Chapter 13) – Examplar Problem Book

Detailed Notes with MCQs of Chapter 13, "Fun with Magnets," from your NCERT Science book and Exemplar. This chapter introduces fundamental concepts about magnets which are often tested in various government exams. Pay close attention to the properties and behaviours of magnets.

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

1. Introduction to Magnets:

• Magnet: A substance that has the property of attracting certain materials like iron, nickel, and cobalt.
• Discovery (Legend): The discovery of magnets is often linked to a shepherd named Magnes in ancient Greece, who found his iron-tipped staff and nails in his sandals getting stuck to a large black rock (later identified as magnetite, a natural magnet).
• Magnetite: A natural magnetic ore (an iron oxide, Fe₃O₄).
• Artificial Magnets: Magnets made by humans from pieces of iron or other magnetic materials. They come in various shapes like bar magnets, horseshoe magnets, cylindrical magnets, and needle magnets (used in compasses).

2. Magnetic and Non-Magnetic Materials:

• Magnetic Materials: Materials that are attracted by a magnet.
  • Examples: Iron, Nickel, Cobalt. Steel (an alloy containing iron) is also magnetic.
• Non-Magnetic Materials: Materials that are not attracted by a magnet.
  • Examples: Wood, Plastic, Glass, Rubber, Copper, Aluminium, Brass, Stainless Steel (most types, though some specific types can be slightly magnetic), Paper, Cloth.
  • Exam Tip: Be able to classify common materials as magnetic or non-magnetic.

3. Poles of a Magnet:

• Magnetic Poles: The regions on a magnet where the magnetic force is strongest. These are usually located near the ends of a bar magnet.
• Two Poles: Every magnet, regardless of its shape, has two poles:
  • North Pole (N): Also called the North-seeking pole.
  • South Pole (S): Also called the South-seeking pole.
• Key Property: Magnetic 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 its own North and South pole.

4. Finding Directions using Magnets:

• Directive Property: A freely suspended magnet always aligns itself in the North-South direction when it comes to rest.
  • The North-seeking pole (N) points towards the Earth's geographic North.
  • The South-seeking pole (S) points towards the Earth's geographic South.
• Magnetic Compass: A device used for navigation that utilizes the directive property of magnets. It consists of a small magnetic needle pivoted at the centre, free to rotate within a casing marked with directions (N, S, E, W). The needle always points towards the North-South direction.

5. Attraction and Repulsion between Magnets:

• Fundamental Law:
  • Like Poles Repel: North repels North (N-N); South repels South (S-S).
  • Unlike Poles Attract: North attracts South (N-S).
• Surest Test for Magnetism: Repulsion is the surest test to determine if an object is a magnet. Attraction can occur between a magnet and another magnet (unlike poles) OR between a magnet and a magnetic material (like iron). However, repulsion only occurs between two magnets (like poles).
  • Exam Tip: This is a very important point often asked in competitive exams.

6. Making Your Own Magnet (Temporary Magnetism):

• Single Touch Method: You can temporarily magnetize a piece of iron (like a needle or an iron bar) by stroking it repeatedly with one pole of a strong bar magnet, always moving in the same direction (from one end to the other, lifting the magnet away, and starting again from the initial end). The end where the stroke finishes develops the opposite polarity to the pole used for stroking.

7. Properties and Precautions with Magnets:

• Demagnetization: Magnets can lose their magnetic properties 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, separated by a piece of wood. Soft iron pieces, called 'keepers', should be placed across the ends.
  • Horseshoe magnets should have a single soft iron keeper placed across their poles.
• Keep Magnets Away From: Cassettes, mobiles, televisions, music systems, compact disks (CDs), computers, and credit/debit cards, as the magnetic field can damage them.

Key Terms Recap: Magnet, Magnetite, Magnetic Materials, Non-Magnetic Materials, North Pole, South Pole, Magnetic Compass, Attraction, Repulsion, Demagnetization, Keepers.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 13 for your practice:

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

2. 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

3. Which action will cause a magnet to lose its magnetic properties?
   (a) Storing it with keepers
   (b) Hammering it
   (c) Cooling it
   (d) Suspending it freely

4. Which of the following materials is not attracted by a magnet?
   (a) Iron
   (b) Nickel
   (c) Cobalt
   (d) Aluminium

5. 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

6. If you break a bar magnet into three pieces, how many North poles will you have in total?
   (a) 1
   (b) 2
   (c) 3
   (d) 6

7. A device used by sailors and travellers to find direction is called:
   (a) Barometer
   (b) Thermometer
   (c) Magnetic Compass
   (d) Periscope

8. The surest test to identify if a given metallic bar is a magnet is:
   (a) Attraction by another magnet
   (b) Repulsion by another magnet
   (c) Attraction by an iron piece
   (d) Its ability to pick up paper clips

9. Where is the magnetic force strongest in a bar magnet?
   (a) Only at the North pole
   (b) Only at the South pole
   (c) At the centre of the magnet
   (d) Near both poles

10. To store bar magnets safely, they should be kept:
    (a) Separately in different boxes
    (b) In pairs with like poles facing each other
    (c) In pairs with unlike poles facing each other, separated by wood and with keepers
    (d) Wrapped tightly in plastic

Answers to MCQs:

1. (c) Magnetite
2. (c) Geographic North
3. (b) Hammering it
4. (d) Aluminium
5. (b) They repel each other
6. (c) 3 (Each piece becomes a complete magnet with N and S poles)
7. (c) Magnetic Compass
8. (b) Repulsion by another magnet
9. (d) Near both poles
10. (c) In pairs with unlike poles facing each other, separated by wood and with keepers

Study these notes thoroughly. Understanding these basic principles of magnetism is crucial. Let me know if any part needs further clarification!