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MagnetsChapter 8MagnetsMore 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.

MagnetismProperties and interactions of magnets

Magnetic Force interaction between two magnetsMagnetic Force increase as the magnets move closer

Magnetic Field caused by magnetic forceArea surrounding a magnet and is strongest closest to the magnetMagnetic Poles

Region of a magnet where the magnetic force is the strongestAll magnets have a north and a south poleLike poles attract unlike poles repelThe Earth has magnetic poles

Magnetic FieldMagnetic field runs from the north to south poles and is always strongest at the poles

Magnetic Field LinesThe 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.

CompassFirst use of the compass was in China around 1100 ADIt used a magnetized needle floating in a bowl of water

CompassA compass needle is a small bar magnet that can freely rotateWhen a compass is brought near a magnet, the compass needle rotates until it lines up with the magnetic field

CompassThe 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.

Earths Magnetic FieldA compass needle always points north on the EarthThis is because Earth acts like a giant bar magnet and is surrounded by a magnetic field that extends into space.

Earths Magnetic FieldEarths 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

Magnetic MaterialsOnly some metal is magnetic

Iron, Cobalt and Nickel

Magnetic DomainsThe magnetic field created by each atom exerts a force on nearby atomsMagnetic domains contain groups of atoms with aligned magnetic poles

Permanent MagnetsIn a magnet, the like poles of all the domains point in the same directionPermanent magnets are made by placing a magnetic material in a strong magnetic field, forcing a large number of magnetic domains to line up

Temporary MagnetsSome 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

Isolating PolesMagnetic 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

Electricity and MagnetismMoving charges, like those in an electric current, produce magnetic fieldsThe magnetic field around a current carrying wire forms a circular pattern about the wire

Electricity and MagnetismThe direction of the field depends on the direction of the currentThe strength of the magnetic field depends on the amount of current flowing in the wire

ElectromagnetAn 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

ElectromagnetElectromagnets are temporary magnets because the magnetic field is present only when current is flowingOne end of the electromagnet is a north pole and the other end is a south pole

ElectromagnetChanges Electrical to Mechanical EnergyParts of ElectromagnetWireIronCurrent

Increase strengthIncrease turns of wire coilIncrease iron coreIncrease current

Uses of ElectromagnetsMaglev 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

Uses of ElectromagnetsGalvanometer device that uses an electromagnet to measure electric currentGauges in cars

Electric MotorsMotor converts electrical energy to mechanical energyContainRotor disk that contains magnets around the edge with alternating north and south polesElectromagnet outside of the disk to attract magnets in the diskCommutator changes the current in the electromagnet so the poles of the electromagnet switch

Electric MotorsAn 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 currentWhen a current flows, a magnetic field is producedChanging the direction of the current keeps the coil rotating

Electric MotorsControlling rotation speedVary the amount of current in the coilMore current makes the magnet stronger, the magnetic force between the coil and the magnet increases and the coil turns faster

Electromagnetic inductionUsing a magnet to generate electricityInstead of an iron core a magnetMagnet is pulled through a coil of wire or a coil of wire is rotated inside of a magnetThe magnets motion causes the current to flowChange of direction of the magnet changes direction of the currentMechanical energy to electrical energyGenerators in Power plants Electromagnetic Induction

GeneratorsUse electromagnetic inductionGenerates alternating currentA coil wrapped around iron and placed between poles of a permanent magnetCoil is rotated by mechanical energyMagnets in the rotor are alternating N-S so the current changes twice each revolution

Turbinea rotating machine used to spin a rotor uses air power, gas, oil, coal or nuclear energy

windwatergasHydropower

Direct and Alternating CurrentDirect current is current that flows in only one direction through a wireAlternating current reverse the direction of the current flow in a regular wayIn North America, generators produce alternating current at a frequency of 60 cycles per second or 60 HzA 60 Hz alternating current changes direction 120 times each secondTransformersChange voltage of the electricity to send it over power lines or for use in our homes and buildingsMade of two coils wrapped around the same iron coreChanging current in primary coil induces current in secondary coil

Step-Up Transformer

Changes low voltage to a high voltage when electricity leaves a power plant to carry it through power linesSecondary oil has more turns of wire than the primary coilStep-Down Transformer

Changes the high voltage of the power lines to the low voltage sent to homes and buildingsPower in power lines is reduced by step-down transformers to household voltage of 120 V