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MAGNETISM Chapter 21. Magnetic Levitation Train (Shanghai) Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it.

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Presentation on theme: "MAGNETISM Chapter 21. Magnetic Levitation Train (Shanghai) Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it."— Presentation transcript:

1 MAGNETISM Chapter 21

2 Magnetic Levitation Train (Shanghai) Maglev uses powerful magnets to hold a train a few millimeters from the track and propel it with little noise or vibration.

3 Earth’s Magnetic Field

4

5 Properties of Magnets 1.North and South Poles

6 Properties of Magnets 2.Law of magnetic poles Unlike poles attract

7 Properties of Magnets 2.Law of magnetic poles Like poles repel

8 LAWS OF ATTRACTION LIKE POLES REPEL UNLIKE POLES ATTRACT N SS N N S

9 LAWS OF ATTRACTION LIKE POLES REPEL

10 Properties of Magnets Testing a magnet 1.Bring one end of an unknown object to a suspended magnet. 2.If repulsion occurs, then the unknown object is a magnet since repulsion occurs between like poles.

11 Magnetic Induction Magnetic Induction is the process of inducing magnetism in an unmagnetised ferromagnetic material without any contact with the magnet.

12 Induced Magnetism - Making of magnetic materials (e.g. steel and Iron) into Magnets - Inducing magnetism in an ordinary piece of magnetic material -Induced magnetism is a temporary process

13 Induced Magnetism The two iron nails become induced magnets and show a repulsion between the far ends

14 Induced Magnetism Repulsion between the two iron nails increases as a bar magnet is brought towards the nails

15 MAGNETIC POLES A magnet is a dipole (two pole) A magnet is a dipole (two pole) There is no monopole (single pole) There is no monopole (single pole)

16 MAGNETIC POLES A magnet is a dipole (two pole) A magnet is a dipole (two pole) There is no monopole (single pole) There is no monopole (single pole)

17 Magnetic Materials Most materials are NOT magnetic Most materials are NOT magnetic Naturally occurring magnetic materials are called FERROMAGNETIC (Iron-Like) Naturally occurring magnetic materials are called FERROMAGNETIC (Iron-Like) FERROMAGNETIC MATERIALS are FERROMAGNETIC MATERIALS are Iron, Steel Iron, Steel Cobalt Cobalt Nickel Nickel Alloys of above metals Alloys of above metals

18 Magnetic Domains These are small regions in a FERROMAGNETIC Material where all the magnetic fields of the atoms are aligned

19 Magnetization If the domains are aligned, the material is polarized and becomes magnetized If the domains are aligned, the material is polarized and becomes magnetized N S

20 Magnetization A magnetised bar Unmagnetised bar

21 Methods of Magnetisation & Demagnetisation Single Touch Divided Touch Making a magnet by stroking

22 Methods of Magnetisation & Demagnetisation Single Touch Making a magnet by stroking

23 Methods of Magnetisation & Demagnetisation Single Touch Making a magnet by stroking

24 Methods of Magnetisation & Demagnetisation Making a magnet by electrically

25 Methods of Magnetisation & Demagnetisation Making a magnet by electrically

26 Methods of Magnetisation & Demagnetisation Polarity of the magnet determined by: (i) Right-Hand Grip Rule (ii) Polarity of the magnetised specimen

27 Methods of Magnetisation & Demagnetisation Methods of demagnetising magnets 1.Heating - misalignment of “tiny” magnets, increase vibration of atoms 2. Hammering

28 Methods of Magnetisation & Demagnetisation Methods of demagnetising magnets 3.Alternating Current

29 Methods of Magnetisation & Demagnetisation Methods of demagnetising magnets 3.Alternating Current

30 Magnetic Fields & the Plotting Compass A magnetic field is the region where a magnetic force is exerted on any magnetic object placed within the influence of the field.

31 Magnetic Fields & the Plotting Compass

32 Magnetic Fields & the Plotting Compass

33 Magnetic Fields & the Plotting Compass

34 Magnetic Fields & the Plotting Compass X: neutral point

35 Magnetic Fields & the Plotting Compass X: neutral point

36 Magnetic Fields & the Plotting Compass

37 Earth’s Magnetic Field

38

39 Magnetic Properties of Iron & Steel Question: Comment on the magnetic properties of Iron & Steel

40 Magnetic Properties of Iron & Steel

41 Iron (soft Magnet) Steel (Hard Magnet) Easy to magnetize Can be magnetized by a weak magnetic field Harder to magnetize Required a strong magnetic field to magnetise Easy to demagnetizeHarder to demagnetize Used in Electromagnets, transformer cores and magnetic shields Good in making permanent magnets and bar magnets

42 Magnetic Field Near A Wire

43

44 Right-Hand Grip Rule

45 Magnetic Field Near A Wire

46

47

48 Iron Filings around a current-carrying wire

49 Magnetic Field Near A Wire

50 Magnetic Field Near A Loop Iron Filings around a current-carrying loop

51 Magnetic Field due to a solenoid

52 Iron Filings around a current-carrying coils of loops

53 Magnetic Field due to a solenoid

54 The strength of the magnetic field can be increased by: 1.Increasing the current 2.Increasing the number of turns per unit length of the solenoid 3.Using a soft-iron core within the solenoid

55 Magnetic Field due to a solenoid Electromagnets are far more useful than permanent magnets because: 1.They can be switched on and off. 2. The strength of the magnetic field can be changed, by altering the current. 3. They can easily be made into a variety of shapes and are less expensive to make

56 Uses of electromagnets The electric Bell

57 Uses of electromagnets The electric Bell -The current flows in the coil making an electromagnet -The armature is attracted to the core -As it moves, the circuit is broken at the contact screw. -The electromagnet is switched off, so the armature moves back. -The armature keeps vibrating back and forth making the hammer strike the gong.

58 Uses of electromagnets Magnetic Relay Relays are used as safety devices. A large current circuit can be switched on by a small current circuit When the small current/voltage (that is safe) in the input circuit is switched on, the electromagnet becomes magnetic and attracts the iron armature. The armature rotates towards the electromagnet, pushing the contacts together. This switches on the large current/voltage (that is dangerous) in the output circuit. This type of relay circuit is used in the ignition of a car.

59 Uses of electromagnets Magnetic Relay

60 Uses of electromagnets Magnetic Relay

61 Uses of electromagnets Magnetic Relay

62 Uses of electromagnets Magnetic Relay


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