AQA GCSE Physics. Topic 7: Magnetism and Electromagnetism. Notes. (Content in bold is for Higher Tier only)

Transcription

1 AQA GCSE Physics Tpic 7: Magnetism and Electrmagnetism Ntes (Cntent in bld is fr Higher Tier nly)

2 Magnets - Nrth and Suth Ples - Same Ples repel - Oppsite ples attract Permanent Magnets - Always magnetic, always have ples Induced Magnets - Materials that are magnetic but d nt have fixed ples - These can be made int temprary magnets by strking them with a permanent magnet These align the dmains in the material all in the same directin, creating a temprary magnet Irn, Nickel, Cbalt Magnetic Fields - Field Lines pint frm Nrth t Suth - Strength decreases with distance frm the magnet - Directin always pints t suth ple and away frm nrth ple, at any pint - Use Pltting Cmpasses Small cmpasses which shw the directin f the magnetic field at a certain pint Earth s Cre - The cre is magnetic, and creates a large magnetic field arund the Earth - We knw this because a freely suspended magnetic cmpass will align itself with the earth s field lines and pint Nrth - It desn t pint t the Gegraphic Nrth ple it is ver Nrth Canada - Als, the cmpass is effectively a suspended Bar Magnet, with its wn nrth ple lining up with Earth s Nrth ple Hwever this cannt be right, as like ples repel S in fact, Earth s magnetic ple abve Canada is a magnetic Suth Ple! (and the gegraphic suth ple is clse t the Magnetic Nrth Ple) astarmathsandphysics.cm Current - Current prduces a magnetic field arund the wire - The directin is dictated by the right hand grip rule - Pltting cmpasses n a piece f paper thrugh which a wire is pierced shws this Strength f Magnetic Field - Greater current, strnger magnetic field - Greater distance frm wire, weaker field

3 Slenid - Magnetic field shape is similar t a bar magnet - It enhances the magnetic effect as ciling the wire causes the field t align and frm a giant single field, rather than lts f them all perpendicular t the directin f the current - Having an irn cre in the centre increases its strength as it is easier fr magnetic field lines t pass thrugh than air - Factrs that affect the strength Size f current Length Crss sectinal area Number f turns (cils) Using a sft irn cre The Mtr Effect - Tw magnets will interact, feeling a magnetic frce f attractin/repulsin - S a magnet and a wire will als exert a frce, as the tw magnetic fields (generated by the magnet and the current in the wire) will als interact The magnetic field arund a wire is circular, but the magnetic field between tw magnets is straight When the tw interact, the wire is pushed away frm the field between the ples (at right angles t the wire directin and the field directin) - T visualise Fixed permanent magnets have field lines alng the x axis, as the magnets are at A and B and the field lines are shwn Wire is alng y axis, where current is mving up frm C t D The Frce felt n the wire is at right angles t bth the directin f the current and magnetic field lines Alng the z axis Fleming s Left Hand Rule hyperphysics.phy-astr.gsu.edu - Each directin is 90 t each ther - Use this t wrk ut the unknwn factr ut f the three (usually the directin f the frce felt) - Remember current is cnventinal current, which mves in ppsite directin t the electrns FFFFFFFFFF = (mmmmmmmmmmmmmmmm ffffffff ddffmmdddddddd) (FFffFFFFFFmmmm) (ffffmmmmmmll) FF = BBBBBB Frce en.wikipedia.rg - Where Magnetic Flux Density is measured in Tesla And it is the number f flux lines per metre squared

4 Hw Electric Mtrs wrk - Knwledge f structure is nt expected - Permanent Magnets lie in fixed psitins - In between, a cil f current-carrying wire lies n an axis Frce n ne side mves that side up Frce n the ther side (where current is flwing in ppsite directin) mves dwn This can be verified using Fleming s Left Hand Rule - Hence it rtates Electrmagnetic Inductin (Physics nly) - When there is a relative mvement between a cnductr and a magnetic field, a ptential difference is induced acrss the cnductr. - This happens if the magnetic field changes as well - A current flws if the cnductr frms a cmplete circuit. - This current will prduce its wn magnetic field, which ppse the change inducing it Hw Electric Generatrs (dynams) wrk (Physics nly) - Same setup as a mtr, with a cil f wire able t rtate between tw permanent magnets - A turbine spins turning the cil f wire - The mvement f the wire causes the wire t cut thrugh the magnetic field - It experiences a change in magnetic field - This creates a ptential difference - If the cil f wire is cnnected t a cmplete tutrvista.cm circuit, an alternating current (AC) will flw this is a basic alternatr, as shwn abve - Direct current (DC) current is prduced if the ends, A and D in diagram abve, are cnnected t a split ring cmmutatr - This reverses the current each half-rtatin s current remains psitive this system is called a dynam

5 AC prduced by Alternatr: DC prduced by Dynam: Transfrmers (Physics nly) - AC in first cil creates a changing magnetic field - This changing magnetic field cuts thrugh the secndary cil - This induces a current in the secndary cil Which is als AC If primary current was DC, magnetic field it prduces will be cnstant, nt inducing anything in the secndary cil - Mre cils n secndary: Step up transfrmer, as vltage will be increased, as changing field will cut thrugh mre f the secndary wire inducing a larger pd - Fewer cils n secndary: Step dwn transfrmer, as smaller pd frms n secndary en.wikipedia.rg - step dwn transfrmer mmffmmbbeeee FFFFmmffdd pppppppppppppp nnnnnnnnnnnn FFFFmmffdd ddffffffmmddmmffdd = pppp pppppppppppppp pppp ddffffffmmddmmffdd - This nly wrks with current t if the transfrmer is 100% efficient. Unless it states this, assume nt and just use this t find vltage

6 Hw Dynamic Micrphnes Wrk (Physics nly) - They prduce a current which is prprtinal t the sund signal - Fixed magnet is at the centre, and the cil f wire arund the magnet is free t mve - Pressure variatins in the sund waves cause the cil t mve, and as it mves current is induced in the cil (because it cuts the magnetic field) - This current is then sent t a ludspeaker Ludspeakers (Physics nly) - The setup is identical, wrking in reverse - The current flws int the cil - The magnetic field frm magnet and frm current interact, causing the cil t mve - The cne therefre mves - Prducing pressure variatins, making sund hyperphysics.phy-astr.gsu.edu