KS3 Revision. 8J Magnets and Electromagnets

Transcription

1 KS3 Revision 8J Magnets and Electromagnets 1 of 29 Boardworks Ltd 2007

2 Contents 8J Magnets and Electromagnets Magnetic materials Magnetic fields Electromagnets Summary activities 2 of 29 Boardworks Ltd 2007

3 Magnetic materials Which of the metals below are magnetic metals? silver (Ag) 3 of 29 Boardworks Ltd 2007

4 Magnetic materials A magnetic material is attracted to a magnet. Only iron (Fe), nickel (Ni) and cobalt (Co) are magnetic. Ag 4 of 29 Boardworks Ltd 2007

5 Magnetic or non-magnetic? 5 of 29 Boardworks Ltd 2007

6 Contents 8J Magnets and Electromagnets Magnetic materials Magnetic fields Electromagnets Summary activities 6 of 29 Boardworks Ltd 2007

7 Forces between magnets experiment 7 of 29 Boardworks Ltd 2007

8 What is a magnetic field? The region around a magnet where it has a magnetic effect is called its magnetic field. When a magnetic material is placed in a magnetic field it will experience a force. S N The iron filings feel the effect of the magnetic field and show the direction of the forces in this region. 8 of 29 Boardworks Ltd 2007

9 Shape of a magnetic field What is the shape and direction of the lines of force in the magnetic field around a bar magnet? strongest field at poles N S strongest field at poles weakest field further away from poles Where is the magnetic field strongest? 9 of 29 Boardworks Ltd 2007

10 Viewing magnetic fields: N poles together Bring the north poles of two bar magnets together. S N N S What happens to the magnets? Next, bring the two north poles as close to each other as possible and place a piece of paper on top of the magnets. Carefully scatter iron filings onto the paper. Draw the pattern created by the iron filings. 10 of 29 Boardworks Ltd 2007

11 Magnetic field pattern: N poles together What do you notice about the pattern of the lines of force in the region between the two north poles? S N 11 of 29 Boardworks Ltd 2007

12 Viewing magnetic fields: N and S poles together Bring the north and south poles of two bar magnets together. S N S N What happens to the magnets? Next, put the north and south poles close to each other, without letting them touch, and place a piece of paper on top. Carefully scatter iron filings onto the paper. Draw the pattern created by the iron filings. 12 of 29 Boardworks Ltd 2007

13 Magnetic field pattern: N and S poles together What do you notice about the pattern of the lines of force in the region between the north and south poles? S N S N How does this pattern compare with the pattern between the two north poles? 13 of 29 Boardworks Ltd 2007

14 Magnetic fields summary 1. When two like poles (e.g. two north poles or two south poles) are put together, they repel each other. 2. When two unlike poles (e.g. a north and a south pole) are put together, they attract each other. 3. Scattering iron filings around a bar magnet makes it possible to see the lines of force of the magnetic field. 14 of 29 Boardworks Ltd 2007

15 Making a magnet A magnet can be made by magnetizing a material which is attracted to a magnet, e.g. a paper clip. There are three methods that can be used to make a magnet: Stroke a magnet along the paperclip from one end to the other and then starting from the same place, repeat the movement. The more times this is done, the more magnetic the clip becomes. Hold a nail in a magnetic field and hit it with a hammer. Put a magnetic material in a strong magnetic field. 15 of 29 Boardworks Ltd 2007

16 Contents 8J Magnets and Electromagnets Magnetic materials Magnetic fields Electromagnets Summary activities 16 of 29 Boardworks Ltd 2007

17 Making an electromagnet When electricity is passed through a coil of wire, the coil has a magnetic field around it. This is called an electromagnet. If the coil of wire is wrapped around a piece of iron, such as an iron nail, the magnetic field gets stronger. 17 of 29 Boardworks Ltd 2007

18 Investigating an electromagnet An iron core at the centre of a coil of wire increases the strength of an electromagnet. Two experiments can be carried out to investigate the other factors that can affect the strength of an electromagnet: 1. Investigate how the number of coils affects the number of paper clips attracted to an electromagnet keep the current the same in this experiment. 2. Investigate how the size of the current affects the number of paper clips attracted to an electromagnet keep the number of coils the same in this experiment. 18 of 29 Boardworks Ltd 2007

19 Investigating an electromagnet results 1 Number of coils Number of paper clips attracted Remember keep the current the same throughout this experiment! of 29 Boardworks Ltd 2007

20 Investigating an electromagnet results 2 Current (A) Number of paper clips attracted Remember keep the number of coils the same throughout this experiment! 20 of 29 Boardworks Ltd 2007

21 Investigating an electromagnet graph 1 Graph to show how the number of coils affects the strength of an electromagnet. number of clips attracted number of coils 21 of 29 Boardworks Ltd 2007

22 Investigating an electromagnet graph 2 number of clips attracted Graph to show how the current affects the strength of an electromagnet current (A) 22 of 29 Boardworks Ltd 2007

23 Using electromagnets scrap yards A large electromagnet is used in a scrap yard to pick up and move heavy pieces of scrap metal. Which metals would the electromagnet attract? What advantages does an electromagnet have over a permanent magnet? 23 of 29 Boardworks Ltd 2007

24 Using electromagnets door bells The circuit for a door bell includes an electromagnet. 24 of 29 Boardworks Ltd 2007

25 Using electromagnets relay Lifts, cars and other large electrical machines use high currents. A relay, which includes an electromagnet, is used to allow a small current in one circuit to control a large current in another circuit. 25 of 29 Boardworks Ltd 2007

26 Label the diagram electric bell 26 of 29 Boardworks Ltd 2007

27 Label the diagram - relay 27 of 29 Boardworks Ltd 2007

28 Electromagnets summary 1. When electricity is passed through a coil of wire, the coil behaves like a magnet and has a magnetic field around it this is an electromagnet. 2. There are three ways to make an electromagnet stronger: wrap the coil of wire around an iron core; increase the number of coils; increase the size of the current. 3. An electromagnet can be easily turned on and off. This is why electromagnets can be used in scrapyards and as switches in electrical devices. 28 of 29 Boardworks Ltd 2007

29 Contents 8J Magnets and Electromagnets Magnetic materials Magnetic fields Electromagnets Summary activities 29 of 29 Boardworks Ltd 2007

30 Glossary attraction The force that pulls things together, e.g. opposite poles of two magnets. electromagnet A magnet made by passing electricity through a coil of wire, which often has a core inside. magnet An object that has a magnetic field and can attract magnetic materials. magnetic field The area around a magnet where its magnetic force can be felt. magnetic materials Materials that are attracted to a magnet, e.g. iron, cobalt and nickel. magnetism The non-contact force of a magnetic field. other, e.g. like poles of two magnets. 30 of 29 Boardworks Ltd 2007 poles The parts of a magnet where its magnetic field is

31 Anagrams 31 of 29 Boardworks Ltd 2007

32 Magnetism true or false? 32 of 29 Boardworks Ltd 2007

33 Multiple-choice quiz 33 of 29 Boardworks Ltd 2007

34 KS4 34 of 29 Boardworks Ltd 2007

35 35 of 29 Boardworks Ltd 2007

36 Electromagnetism and movement What is the link between movement, magnetism and electric current? 36 of 29 Boardworks Ltd 2007

37 Wire in a magnetic field 37 of 29 Boardworks Ltd 2007

38 Changing the direction of the force The direction of the force acting on a wire in an electromagnetic field can be reversed by: reversing the current reversing the magnetic field The direction of the force is therefore relative to both the direction of the magnetic field and the current. 38 of 29 Boardworks Ltd 2007

39 Fleming s left-hand rule It is possible to predict the direction of the force acting on a wire its motion if the direction of the current or the magnetic field are known. Fleming s left-hand rule is used to do this. thumb = Motion First finger = magnetic Field second finger = Current 39 of 29 Boardworks Ltd 2007

40 Increasing the size of the force 40 of 29 Boardworks Ltd 2007

41 Coil in a magnetic field 41 of 29 Boardworks Ltd 2007

42 The motor effect: true or false? 42 of 29 Boardworks Ltd 2007

43 43 of 29 Boardworks Ltd 2007

44 What are electric motors? How many items do you own that contain an electric motor? An electric motor is a device that converts electrical energy into mechanical energy to produce a turning effect. Most motors are powered using direct current (DC), which is produced by cells and batteries. Motors powered by mains electricity use alternating current (AC). These motors use electromagnets rather than permanent magnets. 44 of 29 Boardworks Ltd 2007

45 How does an electric motor work? 45 of 29 Boardworks Ltd 2007

46 DC electric motor simulation 46 of 29 Boardworks Ltd 2007

47 How do we increase motor strength? Would the same strength motor be used in both of these? How can the strength of an electric motor be increased? increase the current flowing through the coil increase the strength of the magnet increase the number of turns on the coil 47 of 29 Boardworks Ltd 2007

48 48 of 29 Boardworks Ltd 2007

49 Inducing current in a wire 49 of 29 Boardworks Ltd 2007

50 Fleming s right-hand rule It is possible to predict the direction of the induced current produced by a generator if the direction of the force (or motion) or the magnetic field are known. Fleming s righthand rule is used to do this. thumb = Motion First finger = magnetic Field second finger = Current 50 of 29 Boardworks Ltd 2007

51 What is electromagnetic induction? When current flows through a wire held in a magnetic field, a force is created that moves the wire. The opposite is also possible: if a wire is moved across a magnetic field, a current is produced. This is called electromagnetic induction. Induction also occurs if a magnet is moved in a coil of wire, or if a coil of wire rotates in a magnetic field. In all these methods of inducing a current, the wire and magnetic field move perpendicular to each other. If they move parallel to each other, no current is induced. 51 of 29 Boardworks Ltd 2007

52 Inducing current in a coil 52 of 29 Boardworks Ltd 2007

53 What are generators? A generator is a device that converts mechanical energy into electrical energy. It is the opposite of an electric motor. Power stations use generators to produce electricity on a large scale. Mechanical energy is provided by rotating turbines that can be powered by: high-pressure steam in coal, oil, gas and nuclear power stations wind in wind turbines falling water in hydroelectric power stations 53 of 29 Boardworks Ltd 2007

54 How do AC generators work? 54 of 29 Boardworks Ltd 2007

55 AC generator simulation 55 of 29 Boardworks Ltd 2007

56 Increasing the size of the induced current How can the size of an induced current be increased? increase the speed at which the coil rotates increase the strength of the magnetic field increase the number of turns in the coil increase the total area of the coil. In a power station generator, an electromagnet is often used as this can provide a stronger magnetic field than is possible with a permanent magnet. 56 of 29 Boardworks Ltd 2007

57 Factors affecting induced current 57 of 29 Boardworks Ltd 2007

58 Induction: true or false? 58 of 29 Boardworks Ltd 2007

59 59 of 29 Boardworks Ltd 2007

60 Glossary alternating current A current that constantly changes direction. It is produced by most electrical generators. commutator The part of a motor that enables the coil to rotate using direct current. direct current A current that always flows in the same direction. It is produced by cells and batteries. generator A device that converts mechanical energy into electrical energy. induction Generating a current in a wire by moving the wire in a magnetic field, or by moving a magnet inside a coil. motor A device that converts electrical energy into mechanical energy. slip rings The parts of a generator that enable the rotating coil to produce alternating current. 60 of 29 Boardworks Ltd 2007

61 Anagrams 61 of 29 Boardworks Ltd 2007

62 Multiple-choice quiz 62 of 29 Boardworks Ltd 2007