Fun with Magnets

Magnets are used in refrigerator doors, pencil boxes, toys and compasses. In this chapter you will hear the story of the shepherd Magnes, whose iron-tipped stick stuck to a rock containing a natural magnet called magnetite. You will learn that materials attracted by a magnet, such as iron, nickel and cobalt, are magnetic materials, while plastic, wood, rubber and glass are non-magnetic. When a bar magnet is rolled in iron filings, most filings stick near its ends, showing that the attraction is strongest at the two poles. The chapter explains that a freely suspended bar magnet always comes to rest in the north-south direction. The end pointing north is the north-seeking or north pole, and the other is the south pole. This property is used in a compass to find directions. You will learn to make a magnet by stroking an iron strip with a bar magnet, observe that like poles repel and unlike poles attract, and learn how to store magnets in pairs with keepers so they do not lose their magnetism.

Detailed Worksheet: Fun with Magnets Section A - Definitions (10 marks) 1. What is magnetite? How was it discovered according to the popular story? (2 marks) 2. Define magnetic and non-magnetic materials with two examples of each. (2 marks) 3. What are the poles of a magnet? Where is the attraction of a magnet strongest? (2 marks) 4. What is a compass? State the principle on which it works. (2 marks) 5. What is meant by repulsion between magnets? When does it occur? (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify the following as magnetic or non-magnetic: iron nail, plastic scale, steel spoon, rubber eraser, paper clip, aluminium foil, wooden pencil, nickel coin, glass marble. How many are magnetic? (3 marks) 7. Predict attraction or repulsion when (i) the N pole of one magnet is brought near the N pole of another (ii) N is brought near S (iii) S is brought near S. State the rule. (3 marks) 8. A student finds that a bar magnet picks up 40 pins at each end and only 2 pins at the middle. (i) How many pins did it pick up in total at the two ends? (ii) What does this show about the poles? (iii) What would happen if the magnet were broken into two pieces? (3 marks) 9. To make a magnet, an iron strip is stroked 30 to 40 times with one pole of a bar magnet, always in the same direction. (i) Why must the direction stay the same? (ii) How can you test whether the strip has become a magnet? (iii) Name one more method to make a magnet. (3 marks) 10. Give one reason for each precaution: (i) magnets should not be hammered or dropped (ii) magnets should be kept away from mobile phones and computers (iii) bar magnets are stored in pairs with unlike poles on the same side, separated by wood, with soft iron keepers at the ends. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a bar magnet rolled in iron filings, showing where the filings cling. Label the north pole, south pole and the middle region. (4 marks) 12. Draw a labelled diagram of a freely suspended bar magnet hung by a thread from a stand. Show the direction in which it comes to rest. (3 marks) 13. Draw a diagram showing the correct way to store two bar magnets with keepers. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. You are given two identical-looking iron bars, one of which is a magnet. (i) How can you identify the magnet using only the two bars? (ii) Why does this method work? (iii) How could you identify it using a thread? (iv) How could you find its north pole? (5 marks) 15. A sailor's compass is placed near a large iron object on a ship and gives wrong readings. (i) Why does this happen? (ii) Why should compasses be kept away from magnets and iron? (iii) How did early travellers use magnets to find direction? (iv) Name one modern use of magnets. (5 marks) 16. Paheli brings a magnet near a bowl of mixed sand and iron filings. (i) What happens? (ii) How can this property be used to separate materials? (iii) Where is this method used in industry? (iv) Will the magnet attract aluminium cans in a scrap yard? Explain. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Mark the poles of magnets clearly as N and S.