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Magnetism

Magnetism. Chapter 17 WS 17.1. Objectives. Introduce basic properties of magnetism Describe induced magnetism Explore magnetic behaviour of iron, steel and other metals Construct magnetic field lines Differentiate uses of temporary versus permanent magnets. Drawing relationships.

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Magnetism

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  1. Magnetism Chapter 17 WS 17.1

  2. Objectives • Introduce basic properties of magnetism • Describe induced magnetism • Explore magnetic behaviour of iron, steel and other metals • Construct magnetic field lines • Differentiate uses of temporary versus permanent magnets

  3. Drawing relationships Static Electricity Magnetism Unlike poles attract, like poles repel. A magnet can attract a non-magnet as long as material is ferromagnetic An object can be conclusively deduced to be magnetisedwhen it is repelled by a magnet. • Unlike charges attract, like charges repel. • A charged object can attract an uncharged object. • An object can be conclusively deduced to be charged when it is repelled by a charged object

  4. How was magnetism discovered? • Ancient Chinese first discovered lodestone(which today we call magnetite ), which would always point in the north-south direction

  5. What causes magnetism? • The orbital motion of electrons around the nucleus of an atom produces some magnetic effect, making every atom an atomic magnet • Many atomic magnets form a magnetic domain. • When magnetic domains within a material point in one direction, there will be ‘free’ poles on the two ends of the materials • These form the North and South poles of a magnet! (Note that every magnet MUST have a North pole and a South pole on its two ends). • The magnetic effect of a magnet is strongest at its poles.

  6. Induced magnetism • A magnet can induce an initially un-magnetisedferromagnetic object to become a magnet. • Magnetic domainsof the object can be aligned by the magnet. • The object will now have a north and a south pole. • An opposite pole is always induced on the un-magnetised object. (example)

  7. Try it now! WS 17.1 Q1

  8. Materials Magnetic (Ferromagnetic) Non-magnetic Copper Aluminum Etc… (everything else) • (Natural) Magnetite • Iron • Neodymium • Steel • Alnico • Alcomax

  9. Ways to magnetise • Stroking method • Electrical method using a direct current

  10. Ways to demagnetise • Heating • Hammering • Electrical method using an alternating current

  11. Magnetic Field • Recall an electric field! • Magnetic fields are also regions of space where magnetic materials will experience a force. • Magnetised materials will set up a magnetic field around itself • We represent magnetic fields with magnetic field lines!

  12. Recall and compare… Electric Field Lines Magnetic Field Lines Points away from North towards South Closer together (stronger) near the poles Cannot intersect each other Mutually repulsive Prefer to go through a magnetic material • Points away from positive towards negative • Closer together (stronger field) near the charge • Cannot intersect each other • Mutually repulsive • Cannot exist inside a conductor

  13. Single bar magnet N S

  14. From simpler to more complex ones. WS 17.1 Q2 (c, d, a, B)

  15. Recall PR 7… • Think about the magnetic field lines you plotted using a compass • Think about the iron filings you have tried to interpret • Now we are telling you how all these lines should look like!

  16. Mystery of PR 7 revealed! WS 17.1 Q3, Q4

  17. Uses of magnets Temporary Permanent (hard magnetic materials) Moving-coil ammeters and speakers Magnetic door catch DC Motor (soft magnetic materials) • Cores of transformers • Magnetic shielding

  18. Checklist After this lesson, I am able to… • State the properties of magnets • Describe test of magnetism • List the magnetic and non-magnetic materials • Construct the magnetic field lines for simple magnets • Construct the magnetic field lines in set-ups involving other materials. • Describe how magnetism is induced • Describe some uses of temporary magnets • Describe some uses of permanent magnets

  19. Continuing from here… • Read the textbook to find out about: • How is the N-pole of a magnet defined? How is it related to pointing to the Earth’s North? • So does the arrow of a compass point to the North of the earth? Or the south pole? • Explain the term “Magnetic saturation” • How are magnets stored and why are they stored that way? Term 3 Week 1 • Do Part 2 of Chapter 17, which is on Electromagnetism • Do AS 17.1 • Questions from practice questions or past year block tests!

  20. During the holidays… • Go through answers for AS 16.1. • Go through answers for the last quiz on Thermistor (I will return you your scripts in Term 3). • REVISE FOR BLOCK TEST! • Do past year BT paper • Do Practice Questions for BT2 • Arrange to meet me if you have questions! I will be in school on third and fourth week of June Holidays!

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