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# Magnetism science physics e learning

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• To explain the principles of magnetism
• How do we see the field? How can we plot the magnetic field lines? In the last lesson, you have learned about the use of compass as a means of telling direction. One can interpret these lines as indicating the direction that a compass needle will point if placed at that position.
• Are there finite nos of magnetic lines? Would the pattern differ when another magnet is brought near? Is the pattern identical for all magnets? No, depending on the strength of the magnet but the general pattern would be the same. What assumption(s) has/have been made? Ignore earth’s magnetic field.
• ### Magnetism science physics e learning

1. 1. Overview…• Properties of magnets• Magnetic & non-magnetic materials• Testing of a magnet• Induced magnetism• Theory of magnetism• Methods of magnetisation & demagnetisation• Plotting / Drawing of magnetic field lines
2. 2. Testing of Magnet One end of specimen The other end of Specimen brought near to N specimen brought pole near to N pole Bar Magnet Attraction occurs Repulsion occurs (or repulsion occurs) (or attraction occurs) Soft Iron Rod Attraction occurs Attraction occurs(magnetic material) Wooden Rod Nothing happens Nothing happens
3. 3. Testing of magnetThus• If attraction occurs for both ends of the material, then it must be a soft iron (or any magnetic material)• If repulsion occurs for one side of its end, then it must be a magnet.• Only repulsion between a specimen and a magnet allows us to conclude that the specimen is a magnet
4. 4. Induced Magnetism• When a piece of unmagnetised magnetic material (iron or steel) touches or is brought near to a permanent magnet, it is attracted and becomes a magnet itself. The material is said to have magnetism induced into it.• Materials that has magnetism induced into it are called induced magnet.• The process by which the unmagnetised magnetic materials become magnets is called magnetisation.
5. 5. Induced Magnetism
6. 6. Induced Magnetism Temporary. They lose their magnetism once the permanent magnet is drawn away from it.
7. 7. Background knowledgeTheory of Magnetism• In an unmagnetized state, the domains all point in different directions.• Their magnetic effects cancel each other.
8. 8. Background knowledgeTheory of Magnetism• All the domains of a fully magnetized material point in the same direction.
9. 9. Theory of Magnetism S N
10. 10. Method of magnetization• The process of magnetization converts a piece of steel into a permanent magnet.• Note: Methods of magnetisation produces permanent magnet, unlike induced magnets. Thus, steel must be used for all magnetisation and not iron
11. 11. Method of magnetization• Magnetization by stroking  Single stroke method  Double touch method• Magnetization using an electric current.
12. 12. SS S SS NN NNNN Steel Bar S  The pole produced at the end of the magnetized steel is of the opposite polarity of the stroking pole.
13. 13. N S S N Steel Bar What are the poles induced in the steel bar above? Click on either of the diagrams below. N S S N
14. 14. The steel bar to beN S magnetized is placed inside a solenoid. When a direct current (d.c.) is passed through the solenoid and then turned off, Use the Right-Hand the steel bar becomes Grip Rule to determine magnetized when the polarity of the magnetized steel. removed from the solenoid.
15. 15. Ways of increasing the Magnetism• 1) Increase the current (ie Use a stronger battery)• 2) Increase the number of coils / turns in the solenoid
16. 16. Methods of Demagnetization• The methods of demagnetisation removes all magnetism from a permanent magnet. – Heating – Hammering (east-west direction) – Using an A.C current.• Note:We only demagnetised permanent magnets. So all specimen described here are either permanent bar magnets or magnetised steel and not iron
17. 17.  Hammer magnet vigorously.By heating, the molecules vibrate vigorouslythus causing the magnetic alignment to be lost.This is a very quick way to remove magnetism
18. 18.  Heat magnet to red-hot. Then cool.As in heating, the molecules are set to vibration,causing the magnetic alignment to be lost.
19. 19. solenoid • Place a magnet or magnetized steel inside a 12 V a.c. solenoid connected to an alternating current (a.c)• Turn on the current and slowly remove themagnet 2 to 3 metres away from the solenoid.
20. 20. Magnetic Field Magnetic field lines were introduced by Michael Faraday (1791 – 1867) who named them lines of force.
21. 21. Plotting Magnetic Field Pattern Use of compass.
22. 22. Magnetic FieldIt is a region around a magnet whereother magnetic objects experience amagnetic force. Lines used to represent the direction of magnetic field pattern.
23. 23. Magnetic Field Lines Magnetic field lines are similar to electric field lines1) The lines can never cross each other.2) The more closely spaced the lines, the stronger the force and the further apart the lines, the weaker the force.
24. 24. Magnetic Field Lines (I)
25. 25. How would the magnetic field patterndiffer if there are two magnets withlike poles facing each other?
26. 26. It is the region between two magnets where there are no magnetic field lines. lines This is because the field due to onemagnet cancels out that due to the other. X is known as the neutral point. No magnetic field lines at X.
27. 27. Magnetic Field Line (II) X X
28. 28. Magnetic Field Line (II)
29. 29. Magnetic Properties of Iron and Steel Iron SteelIt gets easily magnetized It is hard to get magnetizedIt gets easily It is hard to getdemagnetized demagnetizedUsed for temporary Used to make permanentmagnets such as induced magnetsand electromagnetsKnown as soft-magnetic Known as hard magneticmaterial material