Magnets

1. Magnets

2. Magnets • More than 2,000 years ago Greeks discovered deposits of a mineral that was a natural magnet. • The mineral is now called magnetite. • In the twelfth century Chinese sailors used magnetite to make compasses that improved navigation.

3. Magnetism • Properties and interactions of magnets • Magnetic Force • interaction between two magnets • Magnetic Force increase as the magnets move closer • Magnetic Field • caused by magnetic force • Area surrounding a magnet and is strongest closest to the magnet

4. Magnetic Poles • Region of a magnet where the magnetic force is the strongest • All magnets have a north and a south pole • Like poles repel – unlike poles attract • The Earth has magnetic poles

5. Magnetic Field • Magnetic field runs from the north to south poles and is always strongest at the poles

6. Magnetic Field Lines • The magnetic field can be represented by lines of force, or magnetic field lines. • A magnetic field also has a direction. The direction of the magnetic field around a bar magnet is shown by using arrows.

7. Compass • First use of the compass was in China around 1100 AD • It used a magnetized needle floating in a bowl of water

8. Compass • A compass needle is a small bar magnet that can freely rotate • When a compass is brought near a magnet, the compass needle rotates until it lines up with the magnetic field

9. Compass • The arrow of a compass points in the direction of the magnetic field. • This direction is always away from a north magnetic pole and toward a south magnetic pole.

10. Earth’s Magnetic Field • A compass needle always points north on the Earth • This is because Earth acts like a giant bar magnet and is surrounded by a magnetic field that extends into space.

11. Earth’s Magnetic Field • Earth’s south magnetic pole is located in northern Canada about 1,500 km from the geographic north pole. • The magnetic poles move slowly over time so sometimes the magnetic south pole is the same as the geographic south pole

12. Magnetic Materials • Only some metal is magnetic • Iron, Cobalt and Nickel

13. Magnetic Domains • The magnetic field created by each atom exerts a force on nearby atoms • Magnetic domains contain groups of atoms with aligned magnetic poles

14. Permanent Magnets • In a magnet, the like poles of all the domains point in the same direction • Permanent magnets are made by placing a magnetic material in a strong magnetic field, forcing a large number of magnetic domains to line up

15. Temporary Magnets • Some items can only be made magnetic in the presence of a magnetic field. • The magnetic domains flip and align when they are near a magnet but move out of alignment when the field is removed

16. Isolating Poles • Magnetic poles can not be isolated because all materials are made of atoms and each atom has its own magnetic domain so each atom acts like a magnet with a north and a south pole

17. Electricity and Magnetism • Moving charges, like those in an electric current, produce magnetic fields • The magnetic field around a current – carrying wire forms a circular pattern about the wire

18. Electricity and Magnetism • The direction of the field depends on the direction of the current • The strength of the magnetic field depends on the amount of current flowing in the wire

19. Electromagnet • An electromagnet is a temporary magnet made by wrapping a wire coil carrying a current around an iron core. • When a current flows through a wire loop, the magnetic field inside the loop is stronger than the field around a straight wire

20. Electromagnet • Electromagnets are temporary magnets because the magnetic field is present only when current is flowing • One end of the electromagnet is a north pole and the other end is a south pole

21. Electromagnet • Changes Electrical to Mechanical Energy • Parts of Electromagnet • Wire • Iron • Power source • Increase strength • Increase turns of wire coil • Increase iron core • Increase current

22. Uses of Electromagnets • Maglev trains – levitated trains in Japan – float on a magnetic field, eliminates friction so the trains can obtain speeds up to 343 mph • Toasters – current creates an electromagnet in bottom of toaster which will then attract the tray, when the toast is done, current cuts off and toast pops up

23. Uses of Electromagnets • Galvanometer – device that uses an electromagnet to measure electric current • Gauges in cars

24. Electric Motors • Motor – converts electrical energy to mechanical energy • Contain • Rotor – disk that contains magnets around the edge with alternating north and south poles • Electromagnet – outside of the disk to attract magnets in the disk • Commutator – changes the current in the electromagnet so the poles of the electromagnet switch

25. Electric Motors • An electromagnet that is free to rotate between the poles of a permanent, fixed magnet. The coil in the electromagnet is connected to a source of electric current • When a current flows, a magnetic field is produced • Changing the direction of the current keeps the coil rotating

26. Electric Motors • Controlling rotation speed • Vary the amount of current in the coil • More current makes the magnet stronger, the magnetic force between the coil and the magnet increases and the coil turns faster

27. Electromagnetic induction • Using a magnet to generate electricity • Instead of an iron core – a magnet • Magnet is pulled through a coil of wire or a coil of wire is rotated inside of a magnet • The magnets motion causes the current to flow • Change of direction of the magnet changes direction of the current • Mechanical energy to electrical energy • Generators in power plants

28. Electromagnetic Induction

29. Generators • Use electromagnetic induction • Generates alternating current • A coil wrapped around iron and placed between poles of a permanent magnet • Coil is rotated by mechanical energy • Magnets in the rotor are alternating N-S so the current changes twice each revolution

30. Turbine wind • a rotating machine used to spin a rotor – uses air power, gas, oil, coal or nuclear energy gas water

31. Hydropower

32. Direct and Alternating Current • Direct current is current that flows in only one direction through a wire • Alternating current reverse the direction of the current flow in a regular way • In North America, generators produce alternating current at a frequency of 60 cycles per second or 60 Hz • A 60 Hz alternating current changes direction 120 times each second

33. Transformers • Change voltage of the electricity to send it over power lines or for use in our homes and buildings • Made of two coils wrapped around the same iron core • Changing current in primary coil induces current in secondary coil

34. Step-Up Transformer • Changes low voltage to a high voltage – when electricity leaves a power plant to carry it through power lines • Secondary coil has more turns of wire than the primary coil

35. Step-Down Transformer • Changes the high voltage of the power lines to the low voltage sent to homes and buildings • Power in power lines is reduced by step-down transformers to household voltage of 120 V