Each Magnet has a north and a south pole. The field lines are always from N to S outside the magnet & S to N inside the magnet.

Transcription

1 Each Magnet has a north and a south pole. The field lines are always from N to S outside the magnet & S to N inside the magnet. Magnets Field about a bar magnet A magnet is a piece of metal that pulls other metal objects towards itself. The magnet can hold those objects, or even make them move. The pulling force of the magnet is called magnetism. Magnetism is found naturally in iron and steel, which are used to make many different kinds of magnets. 1

2 Magnets have amazing properties and these are often used in the following pieces of apparatus: Fridge doors ATM cards Electric motors Uses of magnets Make a list of other examples. Computer CD Rom discs Identify substances that are magnetic. Fe, Ni & Co 2

3 Electrons in an atom behave in such a way that it is thought that every atom in certain elements like iron, actually behave like a magnet. In iron, groups of these atoms form a domain and these groups of atoms behave like a small magnet. In un-magnetised iron & non magnetic materials, these domains face in all directions and thus cancel each other out. Domains If all the domains face in the same direction as happens when a steel knitting needle is stroked, it will now become magnetic. 3

4 Arrangement of domains in unmagnetised piece of iron. S N Arrangement of domains in magnetised piece of iron thus setting up a N and S pole. Draw a magnet showing 3 paper clips being lifted (end to end) off the desk by the magnet and indicate the polarity in the magnet and each of the clips. 4

5 North with a stick A compass needle is merely a magnet that can turn freely about its centre point. It s N pole is the end that points towards the North Pole of the earth. Properties of magnets Alternatively, North of a suspended magnet will point 5 in the N-S direction in the Earth s magnetic field. N

6 By cutting a magnet in half, you now have 2 smaller magnets each with a North & South pole. It is not possible to have an isolated N or S pole. Like poles of magnets repel, while unlike poles of magnets attract each other. Like poles repel Unlike poles attract 6

7 THE DISCOVERY OF MAGNETISM The Ancient Chinese and Ancient Greeks knew of a rock called lodestone that attracted iron. If you tied a string around it and held it suspended, it turned towards the north. By the Middle Ages, people were using lodestones as compasses, to help them to find their way on long journeys. Lodestone is a form of iron ore - a compound of iron and oxygen called magnetite. String Suspended lodestone Monster lodestone 7 N

8 This illustration shows the magnetic field lines around two sets of bar magnets. Each set of magnets has its north and south poles represented by different colours (red and green). We can see the effects of attraction in the top diagram and repulsion in the lower. Like poles repel, while unlike poles attract. 8

9 Place a magnet between 2 books and cover with a piece of paper Using iron filings and/or small compasses, establish the shape of the magnetic field around a single magnet and then 2 magnets close together with like and then opposite poles near each other. Draw the shapes of each of the different fields. 9

10 Single bar magnet Horse shoe magnet Combination Field lines Opposite poles attract Like poles repel 10

11 N S Suspend a magnet by means of cotton. Bring another magnet near to the suspended magnet and explain what you see. Reverse the polarity and explain what happens. 11

12 Each straight magnet has a North and a South pole. The poles attract other substances equally strongly. The strength of the magnetic field decrease as you move further from the poles. Attraction can act over a distance, through paper, thin plastic, water and other non-magnetic substances. Like poles repel each other, while unlike poles attract each other. Iron can be made magnetic. Iron that easily loses its magnetism is called soft iron, while iron that keeps its magnetism is called hard iron. 12

13 Arrows are drawn on the magnetic field lines to indicate their direction. Magnetic field lines never cross. Field is strongest where lines are close together. Field is strongest at the poles. Field is weakest where lines are far apart. 13

14 Making a magnet Stroke a steel knitting needle with a magnet as indicated in the sketch. Do this at least 20 times using the same motion. The needle will become magnetic and will keep its magnetism. 14

15 Geographic N The Earth behaves as if there was a huge magnet situated inside the earth as in sketch. Earth's magnetic field 15 Establish the difference between true N and magnetic N.

16 It can be located hundreds of miles away from the geographic North Pole lies the magnetic North Pole. Presently 15 0 away. Movement of magnetic N pole from However, this location is not fixed and is moving continually, even on a daily basis. The geographic N pole is the point about which the earth rotates on it s axis. Northern lights Aurora Borealis The angle between the geographic and magnetic North poles is called the angle of declination. Northern lights occur when particles are trapped in magnetic field. Van Allen belts 16

17 Draw the magnetic field about each of the above situations. (Red = N & Green = S poles) Field about bar magnet Unlike poles 17