Magnets Chapter 8

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.

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

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

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

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.

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

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

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.

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.

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

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

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

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

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

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

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

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

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

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

Electromagnet Changes Electrical to Mechanical Energy Parts of Electromagnet Wire Iron Current Increase strength Increase turns of wire coil Increase iron core Increase current

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

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

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

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

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

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

Electromagnetic Induction

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

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

Hydropower

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

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

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

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