Magnetism Updated 2013 PDF

Chapter 20 Magnetism © 2013 Marshall Cavendish International (Singapore) Private Limited Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.1 Magnets an...

Chapter 20 Magnetism © 2013 Marshall Cavendish International (Singapore) Private Limited Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.1 Magnets and Magnetic Materials Learning Outcome At the end of this section, you should be able to state the properties of magnets. 20.1 Magnets and Magnetic Materials Properties of Magnets 1. Magnets attract magnetic materials. Magnetic materials Iron Nickel Cobalt Steel 20.1 Magnets and Magnetic Materials Properties of Magnets 2. Magnets have two poles. The magnetic effect is strongest at the poles. 3. A freely suspended magnet comes to rest in the north−south direction. 20.1 Magnets and Magnetic Materials Properties of Magnets 4. Like poles repel and unlike poles attract. 20.1 Magnets and Magnetic Materials Identifying a Magnet The only test for magnetism is repulsion between two magnets. Metal A Metal B Metal A Metal B attraction repulsion Either metal A or Metal A and metal B is a metal B are both magnet. magnets. 20.1 Magnets and Magnetic Materials Question In the set-up shown on the right, the ring magnets are found to float on top of one another. Explain why. Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.2 Magnetic Induction Learning Outcome At the end of this section, you should be able to: describe magnetic induction. 20.2 Magnetic Induction Magnetic Induction 1. What will happen to the paper clip when it is brought near the iron bar, which is fixed in position? The paper clip is attracted to the iron bar. S 2. Can you explain how this happened? The iron bar is magnetised when iron bar brought near the bar magnet. N The iron bar in turn magnetises the S paper clip. As a result, the paper clip is attracted N to the iron bar. 20.2 Magnetic Induction Magnetic induction is the process whereby an object made of a magnetic material becomes a magnet when it is near or in contact with a magnet. With reference to the previous slide, we say that the iron bar becomes an induced magnet when brought near the bar magnet. In turn, the iron bar magnetises the paper clip through induced magnetism. 20.2 Magnetic Induction Worked Example In an experiment, a piece of wood is held between the N pole of a bar magnet and two paper clips. (a) Although wood is a non-magnetic material, the two paper clips are attracted when the wood is held between the magnet and the paper clips. Suggest a reason for this. (b) It is observed that the bottom tips of the paper clips point away from each other. Why is this so? 20.2 Magnetic Induction Solution (a) The paper clips are magnetised by induced magnetism. This process does not require physical contact between the magnet and the magnetic material. The magnetic material only needs to be near the magnet. (b) The paper clips have become induced magnets. The bottom tips of the paper clips are like poles (N poles) and thus they repel each other. Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.3 Magnetisation and Demagnetisation Learning Outcome At the end of this section, you should be able to: describe methods of magnetisation and demagnetisation. 20.3 Magnetisation and Demagnetisation Theory of Magnetism cut cut If a bar magnet is cut up into three smaller pieces, each piece becomes a magnet, with an N pole and an S pole. If we carry on cutting each piece into smaller pieces, we will end up with even smaller magnets. 20.3 Magnetisation and Demagnetisation Magnetic Domains Magnets are made up of atoms. A magnetic domain consists of a group of atomic magnets pointing in the same direction. 20.3 Magnetisation and Demagnetisation Magnetisation 1 1 The magnetic domains in an unmagnetised bar point in random directions. 2 A bar magnet is brought near the unmagnetised bar. 3 The magnetic domains point in 3 the same direction, producing 2 magnetism. N and S poles of S N 5 adjacent domains cancel each other out. 4 4 The atomic magnets at both the ends are free. This 5 The atomic magnets at the ends tend to fan produces N and S poles at out due to repulsion between the like poles. the ends. 20.3 Magnetisation and Demagnetisation Magnetising Magnetic Materials 1. Stroking 20.3 Magnetisation and Demagnetisation Magnetising Magnetic Materials 2. Electrical method using direct current 20.3 Magnetisation and Demagnetisation Determining the Poles of a Magnet Method 1 Grip the solenoid using the right hand. Curl the fingers in the direction of current flow. The thumb points in the direction of the north pole. 20.3 Magnetisation and Demagnetisation Determining the Poles of a Magnet Method 2 View the left end of View the right end of the the coil as shown. coil as shown. If the current is in the If the current is in the clockwise direction, anticlockwise direction, then the magnet has a then the magnet has a south pole at this end. north pole at this end. 20.3 Magnetisation and Demagnetisation Demagnetising Magnets The main idea in demagnetisation is to cause the magnetic domains to point in random directions. This can be done in three ways. 1. Heating 2. Hammering When heated, the atoms Hammering a of the magnet will vibrate magnet in the vigorously and lose their east−west direction alignment. alters the alignment of the magnetic Letting the magnet cool in domains and causes the east−west direction will the magnet to lose ensure that the domains its magnetism. do not realign. 20.3 Magnetisation and Demagnetisation Demagnetising Magnets 3. Electrical method using an alternating current Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.4 Magnetic Fields Learning Outcomes At the end of this section, you should be able to: draw the magnetic field patterns of interacting magnets; describe how to plot the magnetic field lines of a magnet using a compass. 20.4 Magnetic Fields A magnetic field is the region surrounding a magnet, in which a body of magnetic material experiences a magnetic force. When a paper clip is brought near to, but not touching a bar magnet, it is attracted to the bar magnet. There is a field around the bar magnet, which exerts a force on magnetic materials placed in it. The field can be visualised as magnetic lines of force. A group of magnetic lines of force forms a magnetic field pattern. 20.4 Magnetic Fields Magnetic Field Patterns There are two ways to show the magnetic field pattern of a magnet. Sprinkling iron filings around a magnet (watch the video to see how this is done) URL Plotting the magnetic field pattern with a compass (next slide) 20.4 Magnetic Fields Magnetic Field Patterns 3. Place the compass at X and mark the S and N ends of the compass needle. 4. Move the compass to Y and repeat. 1. Place a bar magnet 5. Repeat the process of at the centre of the marking the points 7. Repeat the paper. until the S pole is process by starting reached. from different 2. Ensure its N pole points near the points to the Earth’s 6. Join the dots with a N pole until N pole. single line. several lines are drawn. 20.4 Magnetic Fields Magnetic Field Patterns The magnetic field plot obtained should be similar to the one on the left. Conclusions Magnetic field lines are directed outwards from the N pole towards the S pole. Magnetic field lines do not cross or intersect one another. Field lines are drawn closer to represent stronger fields and vice versa. 20.4 Magnetic Fields Magnetic Field Patterns between Magnets 20.4 Magnetic Fields Question Can you draw the magnetic field patterns of the following pairs of magnets? 20.4 Magnetic Fields Magnetic Shielding Magnetic field lines from the N pole pass through the iron container instead of through the air. The magnetic field lines will exit from the right- hand side of the iron container and enter the S pole. This will protect equipment (that are sensitive to magnetic fields) when they are placed within the container. 20.4 Magnetic Fields Magnetic Keepers Magnets tend to become weaker after some time if not stored properly. This is because the free poles at the ends of the magnet repel one another. Soft iron keepers ensure that the magnetic domains remain aligned in the same direction.  20.4 Magnetic Fields magnets Question When a piece of iron is inserted between the two cardboard paper clips pieces of cardboard, some of the paper clips When a piece of wood is fall off. Why? inserted between the two pieces of cardboard, the paper clips do not fall off. 20.4 Magnetic Fields Question (a) Can you draw the magnetic field lines between the pair of magnets? (b) Indicate the direction that the compass needle points in. compass S N N S Chapter 20 Magnetism 20.1 Magnets and Magnetic Materials 20.2 Magnetic Induction 20.3 Magnetisation and Demagnetisation 20.4 Magnetic Fields 20.5 Temporary and Permanent Magnets 20.5 Temporary and Permanent Magnets Learning Outcome At the end of this section, you should be able to: distinguish between temporary magnets (e.g. iron) and permanent magnets (e.g. steel) in terms of their properties and uses. 20.5 Temporary and Permanent Magnets Comparing the Magnetic Properties of Iron and Steel Iron is a stronger induced magnet 1 than steel. Iron loses its 2 magnetism easily, but steel does not. 20.5 Temporary and Permanent Magnets Iron is a soft magnetic material and steel is a hard magnetic material. Their properties are summarised below: Magnetic material Soft Hard Easy to magnetise Difficult to magnetise Properties and demagnetise and demagnetise Steel, alnico, Example(s) Iron neodymium Used to make Temporary magnets Permanent magnets 20.5 Temporary and Permanent Magnets Uses of Magnets Permanent magnet Temporary magnet Electromagnets are used to separate magnetic materials from non-magnetic materials in a scrapyard. The magnetic door catch of a refrigerator ensures it is airtight. 20.5 Temporary and Permanent Magnets Uses of Magnets Permanent and temporary magnets 1. Moving-coil loudspeakers 1. Moving-coil ammeters  Chapter 20 Magnetism Magnetism induced magnetism Magnetic materials Magnets (e.g. iron, steel, cobalt) Magnetic fields Hard magnetic Soft magnetic materials materials Properties: Magnetisation through Demagnetisation through Attract magnetic stroking; heating; materials electrical method hammering; Demagnetisation Have N pole and using direct electrical method using S pole current. an alternating current. Rest in N−S direction when freely suspended Permanent magnets Temporary magnets Obey laws of magnetism used in used in used in Magnetic door Moving-coil loudspeakers Electromagnets catches Moving-coil ammeters Chapter 20 Magnetism The URLs are valid as at 3 May 2018. Acknowledgements (slides 1−46) lodestone © Ryan Somma | Wikimedia Commons | CC-BY-SA-2.0 (slide 8) floating ring magnets © Marshall Cavendish International (Singapore) Private Limited (slide 43) refrigerator © Chepe | Dreamstime.com (slide 43) scrapyard magnet © Dvande | Dreamstime.com