AQA GCSE Physics: Magnetism and Electromagnetism Notes PDF

AQA GCSE Physics Topic 7: Magnetism and Electromagnetism Notes (Content in bold is for Higher Tier only) www.pmt.education Magnets - North and South Poles - Same Poles repel - Opposite poles attract Permanent Magnets - Always magn...

AQA GCSE Physics Topic 7: Magnetism and Electromagnetism Notes (Content in bold is for Higher Tier only) www.pmt.education Magnets - North and South Poles - Same Poles repel - Opposite poles attract Permanent Magnets - Always magnetic, always have poles Induced Magnets - Materials that are “magnetic” but do not have fixed poles - These can be made into temporary magnets by ‘stroking’ them with a permanent magnet www.schoolphysics.co.uk o These align the domains in the material all in the same direction, creating a temporary magnet o Iron, Nickel, Cobalt Magnetic Fields - Field Lines point from North to South - Strength decreases with distance from the magnet - Direction always points to south pole and away from north pole, at any point - Use Plotting Compasses o Small compasses which show the direction of the magnetic field at a certain point Earth’s Core - The core is magnetic, and creates a large magnetic field around the Earth - We know this because a freely suspended magnetic compass will align itself with the earth’s field lines and point North - It doesn’t point to the Geographic North pole – it is over North Canada - Also, the compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth’s ‘North pole’ o However this cannot be right, as like poles repel o So in fact, Earth’s magnetic pole above Canada is a magnetic South Pole! (and the geographic south astarmathsandphysics.com pole is close to the Magnetic North Pole) Current - Current produces a magnetic field around the wire - The direction is dictated by the “right hand grip rule” - Plotting compasses on a piece of paper through which a wire is pierced shows this Strength of Magnetic Field - Greater current, stronger magnetic field - Greater distance from wire, weaker field www.pmt.education Solenoid - Magnetic field shape is similar to a bar magnet - It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current - Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air - Factors that affect the strength o Size of current o Length o Cross sectional area o Number of turns (coils) o Using a soft iron core hyperphysics.phy-astr.gsu.edu The Motor Effect - Two magnets will interact, feeling a magnetic force of attraction/repulsion - So a magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and the current in the wire) will also interact o The magnetic field around a wire is circular, but the magnetic field between two magnets is straight o When the two interact, the wire is pushed away from the field between the poles (at right angles to the wire direction and the field direction) - To visualise o Fixed permanent magnets have field lines along the x axis, as the magnets are at A and B and the field lines are shown o Wire is along y axis, where current is moving up from C to D o The Force felt on the wire is at right angles to both the direction of the current and magnetic field lines  Along the z axis Fleming’s Left Hand Rule - Each direction is 90° to each other Force - Use this to work out the unknown factor out of the three (usually the direction of the force felt) - Remember current is conventional current, which moves in opposite direction to the electrons 𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭𝑭 = (𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎 𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇 𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅) × (𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄) × (𝒍𝒍𝒍𝒍𝒍𝒍𝒍𝒍𝒍𝒍𝒍𝒍) 𝑭𝑭 = 𝑩𝑩𝑩𝑩𝑩𝑩 en.wikipedia.org - Where Magnetic Flux Density is measured in Tesla o And it is the number of flux lines per metre squared www.pmt.education How Electric Motors work - Knowledge of structure is not expected - Permanent Magnets lie in fixed positions - In between, a coil of current-carrying wire lies on an axis o Force on one side moves that side up o Force on the other side (where current is flowing in opposite direction) moves down o This can be verified using Fleming’s Left Hand Rule - Hence it rotates Electromagnetic Induction (Physics only) - When there is a relative movement between a conductor and a magnetic field, a potential difference is induced across the conductor. - This happens if the magnetic field changes as well - A current flows if the conductor forms a complete circuit. - This current will produce its own magnetic field, which oppose the change inducing it How Electric Generators (dynamos) work (Physics only) - Same setup as a motor, with a coil of wire able to rotate between two permanent magnets - A turbine spins turning the coil of wire - The movement of the wire causes the wire to cut through the magnetic field - It experiences a change in magnetic field - This creates a potential difference - If the coil of wire is connected to a complete tutorvista.com circuit, an alternating current (AC) will flow – this is a basic alternator, as shown above - Direct current (DC) current is produced if the ends, A and D in diagram above, are connected to a split ring commutator - This reverses the current each half-rotation so current remains positive – this system is called a dynamo http://kids.britannica.com/eb/art-53254 www.pmt.education AC produced by Alternator: DC produced by Dynamo: http://www.bigshotcamera.com/learn/power/dynamo Transformers (Physics only) - AC in first coil creates a changing magnetic field - This changing magnetic field cuts through the secondary coil - This induces a current in the secondary coil o Which is also AC o If primary current was DC, magnetic field it produces will be constant, not inducing anything in the secondary coil - More coils on secondary: Step up transformer, as voltage will be increased, as changing field will cut through more of the secondary wire inducing a larger pd en.wikipedia.org - step down transformer - Fewer coils on secondary: Step down transformer, as smaller pd forms on secondary 𝒏𝒏𝒏𝒏𝒏𝒏𝒃𝒃𝒆𝒆𝒆𝒆 𝒐𝒐𝒐𝒐 𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄 𝒐𝒐𝒐𝒐 𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 𝒑𝒑𝒑𝒑 𝒐𝒐𝒐𝒐 𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏 𝒐𝒐𝒐𝒐 𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄 𝒐𝒐𝒐𝒐 𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔 𝒑𝒑𝒑𝒑 𝒐𝒐𝒐𝒐 𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔 - This only works with current too if the transformer is 100% efficient. Unless it states this, assume not and just use this to find voltage www.pmt.education How Dynamic Microphones Work (Physics only) - They produce a current which is proportional to the sound signal - Fixed magnet is at the centre, and the coil of wire around the magnet is free to move - Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field) - This current is then sent to a loudspeaker Loudspeakers (Physics only) - The setup is identical, working in reverse hyperphysics.phy-astr.gsu.edu - The current flows into the coil - The magnetic field from magnet and from current interact, causing the coil to move - The cone therefore moves - Producing pressure variations, making sound www.pmt.education