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Name: ________________ Class: _________________ Index: ________________

N. NW. NE. E. W. SW. SE. S. Magnetism. Name: ________________ Class: _________________ Index: ________________. Objectives --state properties of magnets --describe induced magnetism --describe electrical methods of magnetisation and demagnetisation

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Name: ________________ Class: _________________ Index: ________________

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  1. N NW NE E W SW SE S Magnetism Name: ________________ Class: _________________ Index: ________________

  2. Objectives --state properties of magnets --describe induced magnetism --describe electrical methods of magnetisation and demagnetisation --describe the plotting of magnetic field lines with a compass --draw the magnetic pattern around a bar magnet and between the poles of two bar magnets --distinguish between the magnetic properties and uses of temporary magnets (e.g. iron) and permanent magnets (e.g. steel)

  3. Discovery of the phenomenon About 900 years ago, the Chinese found that a dish carrying a certain type of rock known as magnetite would constantly float in water in a North-South direction. Magnetite

  4. Magnetic materials • Magnetite consists of an iron oxide. • Natural magnet attracts certain materials: • cobalt • nickel • iron • steel • alloys of any of the above • These materials are called • Magnetic materials. Alloy of Nickel and Cobalt

  5. Non-Magnetic materials • Natural magnet cannot attract other materials. • These include: • copper • brass • wood • plastics • materials other than iron, steel, cobalt, nickel • These materials are called Non-Magnetic materials. Copper/Brass plated pipes

  6. Properties of magnets • All magnets exhibit the following properties: • Attract magnetic materials

  7. Properties of magnets • All magnets exhibit the following properties: • They have 2 magnetic poles; the North and South seeking poles. These are the strongest parts of the magnets. The poles are found very near (but not at) the ends of the magnet.

  8. Properties of magnets • All magnets exhibit the following properties: • If allowed to swing freely a magnet will come to rest with one end pointing towards the Earth’s North pole, the other end pointing towards the Earth’s South pole. Hence, a magnet can be used as a compass for navigational purposes. N NW NE E W SW SE S cylindrical rod made of non-magnetic material

  9. Properties of magnets • All magnets exhibit the following properties: • Law of magnetic poles: • Take a look at the following actions taken during an experiment. What can you conclude ? (Step 1) (Step 2)

  10. Properties of magnets • All magnets exhibit the following properties: • Law of magnetic poles: • Take a look at the following actions taken during an experiment. What can you conclude? (Step 3) (Step 4) N N S N S S S N

  11. Properties of magnets • All magnets exhibit the following properties: • Law of magnetic poles: • Conclusion: Like poles repel, Unlike poles attract.

  12. A Y Y X Properties of magnets Question 1: In an experiment conducted to test if an object is a magnet, one end of this object (A) is brought near one end (X) of a suspended bar magnet. Attraction occurs. Can you conclude that the object is a magnet?

  13. Properties of magnets Question 1: In an experiment conducted to test if an object is a magnet, one end of this object (A) is brought near one end (X) of a suspended bar magnet. Attraction occurs. Can you conclude that the object is a magnet? Answer: Not yet. The object could have been a magnet with end A an opposite pole to that of end X of the magnet; or The object could just have been an ordinary magnetic material (unmagnetised yet). A Y Y X

  14. Properties of magnets • Question 2: • In the same experiment, • The same end of this object (A) is brought near • the other end (Y) of a suspended bar magnet. • If attraction occurs again, can you conclude now that the object is a magnet? • If repulsion occurs instead, can you conclude now that the object is a magnet? A Y Y X

  15. Properties of magnets • Question 2: • In the same experiment, • The same end of this object (A) is brought near • the other end (Y) of a suspended bar magnet. • Answer: • If attraction occurs again, can you conclude now that the object is a magnet? • No, the object is just an unmagnetised magnetic material as • no repulsion between the object & bar magnet was observed. • If repulsion occurs instead, can you conclude now that the object is a magnet? A Y Y X

  16. Properties of magnets • Question 2: • In the same experiment, • The same end of this object (A) is brought near • the other end (Y) of a suspended bar magnet. • Answer: • If attraction occurs again, can you conclude now that the object is a magnet? • No, the object is just an unmagnetised magnetic material as • no repulsion between the object & bar magnet was observed. • If repulsion occurs instead, can you conclude now that the object is a magnet? • Yes, it is a magnet. • Since only like poles repel (A repels Y) • & unlike poles attract (A attracts X), • the object is indeed a magnet. A Y Y X

  17. Properties of magnets: Repulsion is the only true test for polarity. Floating magnets

  18. Induced Magnetism When a non-magnetised magnetic material is brought near to (or touches) a magnet, the material itself will become a weak magnet. This is called induced magnetism (which means the material has magnetism induced in it).

  19. Induced Magnetism Notice that magnetic induction, an opposite pole is always induced. In other words, 2 unlike poles facing each other is observed during magnetic induction.

  20. induced magnet permanent magnet N S N S Induced Magnetism If placed sufficiently near to each other, attraction occurs between the permanent & induced magnets. Induced magnetism in magnetic materials is the reason that these non-magnetised objects are able to be attracted to magnets.

  21. Induced Magnetism Here is another example of induced magnetism.

  22. Induced Magnetism Using the theory of induced magnetism, explain how it is possible to get several iron nails to stick together (as shown in the diagram below).

  23. Magnetisation Making a material permanently magnetic is called magnetisation. There are several ways to magnetise materials. Magnetisation by Stroking (Single-Touch) This method is derived from applying the lessons learnt on magnetic induction. Note the polarities of both the permanent magnet & steel bar that is to be magnetised. This form of magnetism gained is weak but permanent.

  24. Magnetisation by Stroking (Double-Touch) This method is also derived from applying the lessons learnt on magnetic induction. 2 permanent magnets are used in this method, as compared to one being used in the single-touch stroking method. Note the polarities of both permanent magnets. Once again, do take note of the polarities of the permanent magnets & their induced ends of the steel bar. This form of magnetism gained is also weak but permanent.

  25. Magnetisation by Heating & Hammering A magnet can be made by first placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools. This can be done by laying the magnet in a North-South direction in the Earth’s magnetic field. However, the magnet produced is not very strong but permanent.

  26. Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid steel rod Place the steel object inside a coil of wire (a solenoid). Pass a direct current (d.c.) through the solenoid for a few seconds. A magnetic field is produced on the solenoid. As such, the steel rod is now placed inside a magnetic field. When the current is turned off the steel rod is found to be magnetised. Note: the d.c. flows through the solenoid. It does not flow through the steel rod. direct current

  27. Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid steel rod direct current The polarity of the newly-formed magnet can be determined using the Right-hand Grip Rule. (Fingers coiled round & following the direction of the flow of d.c. in the solenoid; the thumb will point in a direction indicating the end which becomes the N-pole).

  28. Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid Looking through end A Looking through end B steel rod clockwiSe direction flow of d.c. aNticlockwise direction flow of d.c. direct current The polarity of the newly-formed magnet can also be determined using the method: Take a look at which way the d.c. is flowing at each end. If the direction of flow is anticlockwise, the end is a N-pole. If the direction of flow is clockwise, the end is a S-pole.

  29. steel rod direct current Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid The magnetism produced using this method is strong & permanent.

  30. Demagnetisation by Heating & Hammering Heat a magnet. Then hammer it as it is allowed to cool in the absence of a magnetic field i.e. facing East-West .

  31. Demagnetisation by the use of an Electrically-generated magnetic field of a Solenoid (700 turns) magnet withdrawn to a few metres

  32. Demagnetisation by the use of an Electrically-generated magnetic field of a Solenoid Place magnet in a solenoid. Pass an alternating current (a.c.) through the solenoid (not through the magnet). Slowly remove the magnet from the solenoid with the a.c. supply still on. Remove to a great distance. Repeat the procedure for as many times as it is necessary. Each time it is done, the magnet’s strength weakens. Finally, it is completely demagnetised. (700 turns) magnet withdrawn to a few metres

  33. Magnetic Field A magnetic field is the region where a magnetic force is exerted on any magnetic objects placed within the influence of the field. Showing the Magnetic Field Using Iron a Filings One method to observe the shape of the magnetic field is by sprinkling iron filings onto a piece of paper placed on top of the magnet.

  34. Plotting Compass A compass is a freely suspended. A compass is normally drawn with the N-pole shown as an arrowhead. It can be used to find the direction of a magnetic field. Remember the N-pole of the compass points to the Earth’s N-pole. The Earth’s magnetic field is produced by electric currents at its core. It is similar to the field that would be due to an imaginary large bar magnet in the Earth’s centre.

  35. What do you think are the directions that the compass would point in if placed in the ten different points around a strong permanent magnet. N S

  36. N S Did you get them all correct?

  37. Magnetic Field Lines Magnetic field lines are imaginary & represent the direction of the magnetic field. Magnetic field lines are also known as lines of force because if magnetic objects are placed in the region of the field lines, the magnetic objects will experience a magnetic force directed along the same lines. By convention, the magnetic field line is the path along which an imaginary “free” N-pole will move if placed along this line.

  38. Neutral Point Whenever a point in space has no magnetic field the magnetic field due to one magnet cancels out that due to another magnet, this point is known as a Neutral Point. neutral point

  39. Plotting Magnetic Field Lines With A Plotting Compass The lines can be investigated to find their path and direction using a plotting compass. Place a plotting compass at point A. Note the direction it points at. Mark a 2nd point next to the N-pole of the plotting compass. 2

  40. Plotting Magnetic Field Lines With A Plotting Compass The lines can be investigated to find their path and direction using a plotting compass. Place a plotting compass at point A. Note the direction it points at. Mark a 2nd point next to the N-pole of the plotting compass. These steps are repeated as shown. The points are all joined using a pencil. All these steps are repeated for other points next to the N-pole of the magnet.

  41. Examples of Magnetic Fields (i) A permanent bar magnet. (ii) 2 opposite poles facing each other

  42. Examples of Magnetic Fields (iii) 2 like poles facing each other (e.g. 2 N-poles)

  43. Neutral Point Earth’s Magnetic field N If a plotting compass is placed at the neutral point (i.e. X), how will it point?

  44. Neutral Point It will point in the same direction as that of earth’s magnetic field. Earth’s Magnetic field N

  45. Properties of Field Lines ·        Lines always start and end on the magnet. ·        The lines travel from the N-pole to the S-pole. ·        The lines never cross or touch each other. ·        The closer the lines the stronger the field.(More magnetic field lines do not necessarily mean stronger magnetic field)

  46. Try labelling the poles of the following magnets. (Some books draw magnetic field lines as broken lines)

  47. These are possible answers. N S N N S N N S S N N N S N S S

  48. These are also possible answers. S N S S N S S N N S S S N S N N

  49. Magnetic Shielding Magnetic fields are sometimes not wanted and can damage delicate equipment such as watches, televisions, computer disks, etc.. Shielding is achieved by surrounding the object with soft iron. The lines of magnetic force concentrate in the soft iron.

  50. Although iron and steel are both magnetic materials, their properties are different. • Two unmagnetised rods have a magnet placed on top of them. • Iron filings are supported from the induced magnets • The permanent bar magnet is then removed. • Note that the iron bar no longer has any iron filings attracted to it. • The steel bar, however, still has some iron filings attracted to it.

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