1. Mr. Lajos Papp The British International School, Budapest 2011/2012

2. UNITS AmpereA VoltV WattW

4. Magnets and magnetic materials Magnets attract objects made of magnetic materials. Magnetic materials include the elements iron, nickel, cobalt, alloys containing some of these such as steel. stainless steel cobalt nickel iron

5. Magnetic poles Magnetic poles are the parts of a magnet that exert the greatest force. A magnetic field is a region around the magnet where magnet materials experience a force. Magnetic poles occur in pairs called north (N) and south (S) Iron filing are attracted mostly to the poles of a magnet

6. Magnetic fields All magnets are surrounded by magnetic fields. The shape of a magnetic field can be shown by iron filings or plotting compasses.

7. The shape of the magnetic field around the magnet is shown by lines. Arrows on the lines point away from North and towards South to show the direction of the magnetic field. The lines of magnetic force do not cross each other. The closer together the lines are, the stronger the field is.

8. Magnetic field around a bar magnet magnetic field line Arrows on the field lines show the direction of the force on a free to move north pole The stronger the magnetic field the denser the magnetic field lines.

10. Magnetic fields between two bar magnets

13. Unlike poles attract, like poles repel This means that two North poles will repel each other, two South poles will repel each other, one North pole and one South pole will attract each other. The further apart the poles, the weaker the forces between them. The lines of magnetic force between the poles are parallel.

14. Producing a uniform magnetic field A uniform magnetic field exerts a constant force over a region. Such a field will consist of parallel equally spaced magnetic field lines. This type of field can almost be found between a north and south magnetic pole.

16. Electromagnetism In 1820 Hans Ørsted noticed that a wire carrying an electric current caused a compass needle to deflect. No current, compass points to north Current, compass deflected

17. Magnetic field patterns around a straight wire The magnetic field consists of concentric circles centred on the wire. The magnetic field is strongest near the wire. This is shown by the field lines being closest together near to the wire. The strength of the field increases if the electric current is increased.

18. Electromagnets An electromagnet consists of a current carrying coil wrapped around an iron coil.

19. When a current flows through a conductor, it produces a magnetic field. The shape of the magnetic field depends on the shape of the conductor. The magnetic field around a straight wire is circular, at right angles to the wire.

20. Permanent magnets are needed for electric motors and loudspeakers. An electromagnet is used in the loudspeaker. http://www.youtube.com/watch?v=hjDrqsy6kH0

21. Electric motor Electric motors have a wide variety of uses. The catapult effect (motor effect) is used to make a simple electric motor. The wire is pushed in the opposite direction if the direction of the current through it is reversed. In a motor, the wire is wound around a central block called an armature. A spindle through the armature allows it to rotate. The current flows in opposite directions on each side of the armature, so one side is pushed while the other is pulled. This makes the armature rotate.

22. After the armature has rotated through half a turn (180°), then the side of the armature being pushed upwards in the above picture is now on the left and the side being pulled down on the right. The armature would be trying to turn in the opposite direction. For the armature to continue to spin in the same direction, the direction of the current flowing through the wire must be reversed every half turn. This is achieved using a split - ring commutator.

23. Electric Motor. The Split - Ring Commutator. A split - ring commutator (sometimes just called a commutator) is a simple and clever device for reversing the current direction through an armature every half turn. The commutator is made from two round pieces of copper, one on each side of the spindle. A piece of carbon (graphite) is lightly pushed against the copper to conduct the electricity to the armature. The carbon brushes against the copper when the commutator spins. As the motor rotates, first one piece of copper, then the next connects with the brush every half turn. The wire on the left side of the armature always has current flowing in the same direction, and so the armature will keep turning in the same direction. The pieces of copper are held apart in the centre and do not touch each other. They look like a ring of copper which is split down the middle This is why it is called a split -ring commutator.

25. Loudspeaker The loudspeaker uses a coil which can slide backwards and forwards over the central pole of a circular permanent magnet. The coil is joined by the brown bars to a paper cone, shown below. The wire from the amplifier carries an alternating current which makes the coil (and the paper cone) move backwards and forwards at the same frequency as the changing current. The paper cone then moves the air backwards and forwards which creates the sound.

27. Induced Current Just as a current flowing through a wire will produce a magnetic field, so a wire moving through a magnetic field will have a current flowing through it. This is called electromagnetic induction and the current is called induced current. The same effect occurs in a stationary wire in a changing magnetic field. It does not matter if the wire is moved near to a magnet or a magnet is moved near a wire, so long as one is moving in relation to the other. A stationary wire in a magnetic field which is not changing will not have a current induced in it. You will sometimes see this effect described as induced voltage. Strictly speaking, you will only get an induced current in the wire if it is part of a complete circuit. A wire which is unconnected at both ends will have a difference in voltage between the ends (a potential difference) but current can only flow when the wire is in a circuit. Induced current is used in electricity generation and transformers.

28. Induced Current in a Coil. When a magnet is moved towards and inside a coil of wire, a current is induced inside the wire. This can be shown by connecting the coil to a very sensitive ammeter called a galvanometer.inducedammeter The size of the induced current can be made bigger by 1. Using a stronger magnet. 2. Moving the magnet at a faster speed. 3. Using more turns of wire on the coil. This would result in the pointer on the galvanometer moving further to the right. The direction of the current can be reversed by 1. Moving the magnet in the opposite direction. 2. Using a magnet facing the opposite way round (with North becoming South). This would result in the pointer on the galvanometer moving to the left.

29. Induced Current in a Generator. The effect of inducing a current in a coil by moving a magnet inside it is used for the large scale generation ofelectricity in power stations.coillarge scalepower stations There are two types of generator or dynamo. Both turn rotational energy into electrical energy. 1. One type involves rotating a coil inside a magnet. 2. The other type involves rotating a magnet inside a coil (like a dynamo found on a bicycle). Both types produce alternating current. 1. 2.bicyclealternating current It is possible to make agenerator without a permanent magnet. The generator used on cars (called an alternator) uses an inner set of coils to make an electromagnet which turns inside an outer set of coils.permanent magnetelectromagnet

30. Generator. A simple generator is similar to an electric motor. With a motor, we put electrical energy in and get rotational energyout, with a generator we put rotational energy in and get electrical energy out.generatormotorrotational energy As with the motor, thecurrent direction changes with each half turn of thegenerator. The generator produces alternating current because slip rings are used in place of a split - ring commutator.motoralternating currentsplit - ring commutator The slip rings keep a continuous connection with the wire around the armature (continued).continued If a simple electric motor with a split - ring commutator is used to generate electricity, you do not get alternating current. A different type of electrical outputis produced.different type