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Magnetic field of magnets

Magnetic field of magnets. Warm-up. 1 Three ring magnets are inserted into a vertical rod on a wooden stand. Which of the following results is/are reasonable?. Red & blue sides denote the 2 poles of a magnet. C. A. B. Warm-up.

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Magnetic field of magnets

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  1. Magnetic field of magnets

  2. Warm-up 1 Three ring magnets are inserted into a vertical rod on a wooden stand. Which of the following results is/are reasonable? Red & blue sides denote the 2 poles of a magnet. C A B

  3. Warm-up 2 4 objects in bar shape are wrapped with paper so that they look the same. 2 of them are bar magnets, 1 is an iron bar & 1 is a copper bar. Without using any other tools, how can you identify all of them?

  4. 0 Introduction Lodestone (磁石) • a naturally-occurring ore • attracts iron objects • points north-south when freely hung • has been used to make compass for centuries

  5. 1 Poles of a magnet Paper clips are attracted to bar magnets. The magnetic force seems to come from 2 ends. poles (極) How are they called?

  6. 1 Poles of a magnet • If a magnet is hung by a piece of thread, it swings until it lies pointing north-south. North Pole north pole (N-pole) south pole (S-pole)

  7. Poles of a magnet If 2 bar magnets are brought together… attraction repulsion unlike poles attract like poles repel Like poles repel each other; unlike poles attract each other.

  8. Poles of a magnet • A magnet attracts some metallic objects but not all. These objects are called magnetic objects. • A magnet exerts a force on nearby magnetic objects even if they are not in contact. Magnetic force, like electrostatic force, is an action-at-a-distance force. • We say that a magnet sets up a magnetic field(磁場), or B-field for short.

  9. Experiment 16a Magnetic fields of permanent magnets Investigate the magnetic fields produced by permanent magnets. iron filings perspex board 2 slab-shaped magnets on steel yoke bar magnet on underside of perspex board perspex board iron filings

  10. Magnetic fields Iron filings are attracted by magnet, settle into a special pattern. These patterns show the magnetic fieldsof the magnets.

  11. Magnetic field patterns Field lines run from the N-pole round to theS-pole. S N • When field lines are closely-spaced, field is strong. • When field lines are widely-spaced, field is weak.

  12. Magnetic field patterns N N N S neutral point  field lines between unlike poles field lines between like poles Neutral point: the field from one magnet cancels the field from another (No magnetic field exists)

  13. Magnetic field patterns What is its magnetic field pattern? S N uniform magnetic field • The slab-shaped magnets produce a strong & uniform magnetic field.

  14. The Earth's magnetic field geographic North Pole magnetic South Pole 11.5 The earth has a weak magnetic field. • It is like the field around a huge bar magnet. Note: N-pole of compass needle points north  S-pole of the 'earth magnet' is actually in the north!

  15. Example 1 Bar magnets 2 bar magnets placed side by side with like poles facing each other. (a) Mark in the diagram the positions of all neutral points. P  Q 

  16. Example 1 Bar magnets (b) Explain briefly why there are neutral points in these positions. At P, the magnetic field due to the 2 N-poles & the 2 S-poles cancel each other out. The same explanation applies to the neutral point at Q. P  Q 

  17. Example 1 Bar magnets (c) What happens when a plotting compass is put in these positions? P  Q  The magnetic field at P and Q is zero. The needle of the compass will lie in whatever direction it settles down to.

  18. Q1 In which direction does a... In which direction does a magnet point if it is hung freely at the North Pole? A The N-pole of the magnet points downwards. B The S-pole of the magnet points downwards. C The S-pole of the magnet points horizontally. D Any direction.

  19. Q2 True or false: Every magnet... True or false: Every magnet has a north pole and a south pole, just as every coin has 2 sides, a 'head' & a 'tail'. If you break a magnet into 2, you get 2 smaller magnets, each again having both a north pole & a south pole. (T/F)

  20. Q3 Using iron filings, we may... pattern strength Using iron filings, we may be able to figure out the __________ and the ___________, but not the __________ of a magnetic field in a region. (strength/direction/pattern) direction

  21. Q4 In a region where a... parallel evenly In a region where a magnetic field is uniform, the field lines must be _________ (convergent/parallel/divergent) and __________ (evenly/unevenly) spaced. Draw a uniform magnetic field in the box.

  22. Q5 A strong magnet attracts... A strong magnet attracts a paper clip with a certain force. (a) Does the paper clip exert a force on the magnet? (Yes/No) (b) If no, why not? If yes, does it exert as much force on the magnet as the magnet exerts on it? Why? Yes, because they are an action-reaction pair.

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