1. Magnets
Chapter 8

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 attract – unlike poles repel
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 north 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
Current
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
a rotating machine used to spin a rotor – uses air power, gas, oil, coal or nuclear energy
wind
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 oil 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