Magnetism levels 4C to 8A

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  • Created by: Meghna
  • Created on: 13-04-10 12:07

Magnetic Fields

-A magnetic field is the area around the magnet where the magnetic force works

-Iron-filings can show the magnetic field around a magnet

-The poles of a magnet are the strongest parts

-A magnet has a north and south pole

-Steel, Iron, Nickel and Cobalt (SINC) are magnetic elements

-Like poles repel while unlike attract

-You can only show that an object is a magnet if it repels a known magnet

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Earths Magnetic Field

-The earth acts like a big magnet

-The magnetic force around the earth protects us from cosmic rays and solar winds

-Because the north pole of a compass points towards a magnetic south pole, earth's magnetic field acts as though there is a giant magnetic south pole at the geographical north pole

-Scientist believe that the earth has at its centre a dense fliud outer core that surrounds a solid inner core, both of which have iron-rich material in them

-The inner core is under such high pressure that it remains colis

-THe outer core is so hot that it remains molten

-But this iron core cannot act as a permanent magnet because iron looses magnetism as it is heated

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The Geodynamo Theory

-This suggests that it is the movement of the molten iron in the outer core caused by the earth's rotation) combined with convection currents (caused by the heat of inner core) which creates a magnetic field in the molten iron

-Earth's North ans south poles attract charged particles from the sun

-These charged particles collide with gas particles in the earths atmosphere which creates the Aurora Borealis( the northern lights) at the arctic circle

-Recent evidence from studies of ancient rocks and seafloors suggests that the earths north and south poles actually swap round, these reversels have occurred at intervals of about 300 000years

-Analysis suggests we are overdue one as the lst one occurred about 780 000 years ago

-Earths magnetic feeled has also weakend by about 10% in the last 200 years

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Explaining Magnetism

-Some materials are easily magnetised

-Just by holding a permanent magnet near them will turn them into magnets

-These materials are called soft magnetic materials and soft iron is one of them

-Iron can become weak magnets when they are placed near stronger ones

-Although iron is easier to magnetise steel stays magnetic for longer

-A steel rod can be made into a magnet by stroking it with a permanent magnet in the same direction a number of times

-Placingiron or steel rod in a coil of wire or solenoid which has a direct current passing through it

-Direct current flows in only one direction

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Explaining Magnetism

-Little groups of atoms in a magnetic material (SINC) behave as tiny magnets

-Each of these groups are called magnetic domains

-In unmagnetised materials the magentic domains are randomly placed and face all direction, they cancel each other out

-In magnetised materials they all face the same direction

-This makes on end of the magnet stronger, north or south poles are formed

-The better domains are aligned the dtronger the magnet

-When atoms are orientated randomly exposure to a magnet can align them

-Substances that do this easily can be magnetised easily

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Explaining Magnetism

-A piece of magnetised iron attracts a unmagnetized nail because it makes more of the domains that are present in the nail line up

-When the magnet is removed most of the domains return to random orientation, the nail becomes demagnetized

-Non-Magnetic materials are those in which when placed in a field domains to not change and perform straight lines

-Magnetism is easy to remove:

-When a magnet is dropped or hammered it becomes weaker

-Heating a magnet

-Stroking a magnet with another in a random fashion, sometimes the magnetic field can be to strong for this to workd

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-Passing current thorugh a conductin wire creates a magnetic field

-When the current is turned of the magnetic field is turned off

-Electromagnets can be made stronger by:

-increasing the current

-Making the wire a coil

-Increasing the number of coils

-placing an iron core in the centre

-Most electromagnets are made by coiling wire around an iron core, when a current passes through it the iron core concentrates the magnetic field and becomes magnetized

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Using Electromagnets

-The main advantage of an electromagnet is that it can be switched off

-This makes them easy to control

-The main disadvantage is that they need to be continuously supplied with electricity

-Large electromagnets are used in scrap yards and in iron and steel foundries

-The electric bell works by using the electromagnet to attract a soft iron ba called the armature

-The switch is closed and current flows, the soft iron core of the ELECTROMAGNET becomes magnetized. Then the armatureis atrracted to the electromagnet and the hammer strikes the gong. The spring steel strip then moves away and breaks the cicuit. Now the electromagnet is demagnetized. Finally the strip touches the contact screw again completing the circuit and switching on the electromagnet so that the cycle can repeat.

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Motors and Generators

-In 1831 the english physicist Michael Faraday found that if a wire carrying current was placed in a magnetic field then the wire would move

-The wire moves because the magnetic field around the wire repels the magnet field around the wire

-This called the motor effect and is used in all electric motors

-Every electrical device that transfers kinetic energy to kinetic energy uses an electric motor

-When two magnets are placed close together their two magnetic fields will interact and either attract or repel

-So if a wire is placed in the field and electric current is passed through it, there are now two magnetic fields: one from the permanent magnet and one from the wire

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Explaining the Motor Effect

-If there is no current there is no movement

-If the direction of current is reversed then the movement will also reverse

-The amount of movementof the wire can be increased by: Increasing the current

-increasing the size of the magnetic field

-making the straight wire a coil

-increasing the number of coils

-Commercial electric motors incorporate all these improvements and build a circuit with a variable resistor to control the size of current and speed of the motor

-Motors change magnetic energy into kinetic energy

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The Generator Effect

-Generators use non-contact magnetism to convert kinetic energy into electrical energy

-A generator is like the motot effect in reverse

-When a magnet is pushed into a coil made from conducting wire which is part of a complete circuit the current flows in the wire (a induced current)

-There is ony a current when the magnet is moving, the current can be increased by: using a stronger magnet, making more coils in the wire, moving the magnet faster

-The current changes direction if the pole on the magnet is reversed

-To keep the circuit flowing the magnet must be moving

-One of the ways to to this is to spin the magnet, machines that do this are called dynamos, some bike lights use dynamos

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Power Stations

-Most of the power we use is generated in power stations

-There are many different types of power stations but they all work in the same way, by burning fossil fuels

-The 7 Steps in a Power Station:

-1) The fuel is supplied, 2) The fuel is burnt in a furnace, 3) The heat given out is used to heat water and turn it into steam, 4) The steam is piped to a turbine where it forces the blades to turn, 5) The turbine drives a generator, 6) The generator produces electricity, 7) the steam use to drive the turbines condenses back into water and is recycled back to the boiler and reused.

-A power station is not very efficient at converting the stored energy into electrical energy

-Power station produce large amounts of water vapour and carbon dioxide

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Power Stations

-These hot gasses leave the power station through the towers

-Carbon dioxide cannot be seen whilst water vapour can

-The water vapour can be seen and as it is cooled it condenses

-At least 60% of the fuels energy is lost as heat through the towers

-Power stations also loose a lot of heat because of the generation of electricity

-Modern power stations are more efficient than older ones

-The electricity they produce is carried through underground cables or suspended between pylons

-All power stations produce alternating current which is electricity at the same frequency


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Power Stations

-This means all power stations are connected together in a countrywide distribution called the National Grid

-Burning large amounts of fossil fuels adds to the green house effect because of the carbon dioxide produced

-Sulphur dioxide is produced when low grade fuels are burnt, this causes acid rain

-Large amounts of water are used in the cooling process

-If coal is burnt, ash is produced

-To reduce the effect of some of these drawbacks, powerstations are often near coasts or large rivers

-Coal burning power stations are near coal fields

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Burning Problems

-The supply of fossil fuels will run out

-Burning causes pollution

-Carbon Dioxide: green house gas leads to global warming. Sulphur dioxide leads to acid rain, Soot leads to breathing problems, deposited buildings, blocked air filters. Others lead to: allergies and smog

-One of the biggest risks with using oil as a fuel is the danger of pollution during its transportation

-Pipeline can leak, tankers sink, realeses of crude oil

-Power Stations use large amounts of water when they generat electricity

-Sometimes chemicals get into the water and pollute it

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Renewable Energy

-To make our fuels last longer we need to use less and reduce the amount of pollution

-One of the best ways is hydroelectric power (HEP) it uses the energy from falling water to turn generators that make electricity

-Other renewable energy sources are: water, wave wind, tidal, solar, geothermal, bio-fuels

-Most of these energy sources can only contribute to a small amount of our needs

-This is because they are less concentrated, they have low energy densities

-Problems: inaccessible, difficult to maintain, difficult to connect to national grid, need specific conditions to produce electricity, expensive

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Nuclear Powe

-Nuclear power is new and developing technology very dangerous,

-First used in 1950s

-Nuclear power stations use the heat from nuclear reactions and produce steam used to drive turbines which generators turn into electricity

-Nuclear power generate about 5% of the worlds power

-Advantages: Small amount of uranium needed, lasts a long time, produces efficient electricity, does not pollute

-Disadvantages: Non-Renewable, Nuclear radiation causes death and cancer, expensive technology required, very hard to dispose of

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Potential Dangers

-The same materials are used to make a nuclear bomb, uranium may fall into wrong hands

-Produces leathally toxic waste which remains active for thousands of years

-Difficult to find good places to build nuclear power stations

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