# MAGNETISM AND STARS

Physics

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• Created by: katie
• Created on: 28-05-12 16:14

## magnetic fields

The magnetic field round a current carrying wire: 1) there is a magnetic field around a straight current carryng wire 2) the field is made up of concentric circles with the wire in the centre

The magnetic field round a solenoid:

• inside a solenoid (a coil of wire) magnetic field is strong and uniform
• outside its just like the one round a bar magnet
• ends of solenoid act like the north and south pole of a bar magnet
• if direction of the current is reserved, the N and S poles will swap ends
• you can increase the strength of the magnetic field around a solenoid by adding a magnetically "soft" iron core through the middle of the coil = then called an ELECTROMAGNET
• a magnetically soft material magnetises and demagnetises very easily, so as soon as you turn off the current through the solenoid the magnetic field dissapears - so the iron doesnt stay magnetised

Iron, steel and nickel are magnetic:

• dont forget that all other common metals are not magnetic at all so a magnet wont stick to aluminium ladders or copper kettles or brass trumpets or gold rings or silver spoons
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## the motor effect

A current in a magnetic field experiences a force:

• the force gets bigger if either the current or the magnetic field is made bigger
• you can always predict which way the force will act using FLEMINGS LEFT HAND RULE
• THUMB= MOTION
• FIRST FINGER = FIELD
• SECOND FINGER = CURRENT
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## the simple electric motor

THE SIMPLE ELECTRIC MOTOR:

4 factors which speed it up

• more current
• more turns on the coil
• stronger magnetic field
• a soft iron core in the middle

Electric motors are used in: CD players, food mixers, fan heaters, fan printers, drills, hairdriers and cement mixers...

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## electromagnetic induction

Electromagnetic induction:

• the creation of a voltage (and maybe current) in a wire which is experiencing a change in magnetic field
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Four factors affect the size of the induced voltage:

• the strength of the magnet
• the area of the coil
• the number of turns on the coil
• the speed of the movement
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## generators

AC generators - turn the coil and theres a current

• generators rotate a coil in a magnetic field
• their construction is pretty much like a motor
• as the coil spins a current is induced in the coil (this current changes direction every half turn)
• instead of a split-ring commutator, generators have slip rings and brushes so the contacts dont swap every half turn
• this means they produce AC voltage
• faster revs produce not only more peaks but higher overall voltage too

Dynamos - you turn the magnet instead of the coil

• dynamos are slightly different type of generator - they rotate the magnet instead of the coil
• causes the field through the coil to swap every half turn so the output is just the same as for the generator
• you get same CRA traces
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## transformers

Transformers change the voltage - but only AC voltages: Step-up transformers and step-down transformers - both have two coils, the primary and secondary joined with an iron core

Step up transformers step the voltage up. They have more turns on the secondary coil than the primiary coil

Step down transformers step the voltage down. Thet have more turns on the primary coil than the secondary coil

Transformers work by electromagnetic induction:

Primary coil prouces a magnetic field which stays within iron core which means nearly all of it passes through the secondary coil and hardly any is lost this is because there is an alternating current (AC) in the primary coil and the field in the iron core is always changing direction, this changing magnetic field is felt by the secondary coil and induces an alternating current in the secondary coil

The iron core carries magnetic field, not current:

1) the iron core is purely for transferring the changing magnetic field from the primary coil to the secondary 2) no electricity flows round the ion core

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## transformers

The transformer equation -

Primary voltage/secondary voltage = number of turns on primary/number of turns on secondary

Transformers are used on the national grid:

• you get both step up and step down transformers on the national grid
• to transmit a lot of poweryou need either high voltage or high current
• formula for power loss due to resistance = P = I(squared)R
• I(squared) bit, current is 10x bigger, losses will be 100x bigger
• much cheaper to boost the voltage up to 400,000V and keep current low
• requires transformers as well as big pylons with huge insulators but it is still cheaper!
• transformers have to step the voltage UP at one end for efficient transmission then bring it back DOWN to safe, useable levels at other end
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## stars and galaxies

Stars and solar systems form from clouds of gas and dust: - stars form from clouds of gas and dust which spiral together due to gravitational attraction, the gravity compresses the matter so much that intense heat develops and sets off nuclear fusion reactions and the star emits light and other radiation, at the same time the star is forming other lumps may develop around the cloud which form planets which orbit around the star

Our sun is in the milky way galaxy: - the sun is one of many billions of stars with form the milky way galaxy Gravity is the force which keeps the stars together in a galaxy and all galaxies rotate, our sun is about 2/3 of the way out towards the end of one of the spiral arms of the MWG

The whole universe has billions of galaxies: 1) galaxies themselves are often millions of times further apart than the stars are within a galaxy 2) so the universe is mostly empty space and is really really big

The early universe contained only hydrogen: - at the very beginning after the big bang there was only hydrogen (helium forming soon after), as universe expanded these atoms clumped together to form stars, hydrogen nuclei smash together to form helium nuclei which is called nuclear fusion. Once hydrogen runs out, helium nuclei fuses to form heavier elements which call happens in red giant stars. Eventually helium i the core runs out and some of the carbon, oxygen and neon combine silicon and keep combining until they form iron and at the end of their lives massive stars explode, in these explosions heavy nucleai combine with each other and with neutrons to make all the elements in the universe. The dust and gas from these supernova explosions can form new stars and planets (like ours).

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## the life cycle of stars

• stars intially form from clouds of DUST and GAS
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• the force of gravity makes the gas clouds come spiralling in and as they do, gravitational energy in coverted into heat energy and the temperature rises
• when temp gets igh enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei which gives out massive amounts of heat and light. In stable period its called a MAIN SEQUENCE STAR because of balanced forces to stop everything exploding
• eventuallt the hydrogen begins to run out and the star swells into a RED GIANT. It becomes red because the surface cools
• a small star like our sun will begin to cool and contract into a WHITE DWARF and then finally as light fades completely it comes a BLACK DWARF
• big stars start to glow brightly again as they undergo fusion and expand and contract several times forming heaver elements in various nuclear reactions so eventually they will explode in a SUPERNOVA
• the explodind supernova throws the outer layers of dust and gas into space leaving a very dense core called a NEUTRON STAR or if the star is big enough a BLACK HOLE
• the dust and gas thrown off by the supernova will form into SECOND GENERATION STARS like our sun
• CLOUDS OF DUST AND GAS -> PROTOSTAR -> MAIN SEQUENCE STAR -> RED GIANT (-> small stars -> red dwarf -> black dwarf) -> SUPERNOVA (-> new planetary nubela .. and a new solar system) -> NEUTRON STAR ... or BLACK HOLE
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