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# Alvl P2: Magnetic Fields (ch16) Cheat Sheet by MostAncientDream

aqa A-level physics year 2 (yr13) Magnetic Fields : chapter 16

### Defini­tio­ns/­Equ­ations

 motor effect F = BIL a current carrying wire in a magnetic field will a force magnetic flux flux = BA a measure of how much magnetism passes through an area flux linkage N x flux = BAN(cos0) faradays law E = -(N) Δflux/ Δt (in a moving wire) E = BLv derrived from E = flux/t

### rules/laws

 right hand rule for finding direction of current in a wire thumbs up, thumb represents direction of current while curled fingers are the direction of the field FLH rule directions for force current and mag field first 3 fingers 90' to each following from the thumb as FBI Faradays Law induced emf is propor­tional to the rate of change of flux linakage Lenz's Law the direction of the induced emf is such that it opposes the change that caused it

### Cyclotrons

 Cyclotrons use F = BQv to produce a beam of charged particles for example: for proton therapy magnetic fields causes protons to be emitted by the source in the centred to undergo circular motion inside the metal Dees they use altern­ating currents as the oppositely charged dee causes the protons to be accele­rated across and then back again after flipping the charge again, increasing their velocity and therefore radius. >every half a cycle the polarity of the dees must reverse in order for the protons to be contin­uously accele­rated across again. as f is indepe­ndant of r, all protons have the same frequency and time period regardless of radius > the frequency of AC applied to the dees must match this

### transf­ormers

 used to change the voltage (reduces current and therefore power lost to heat in national grid cables) - AC primary coil induces altern­ating magnetic field in sort iron core (easily (de)ma­gne­tised) - this induces a current in the secondary coil - the side with the most turns (N) has the greater pd (N ∝ v) equations: VpIp = VsIs Ns/Np = Vs/Vp efficiency = useful­/total x100 rms = Xo/21/2 Energy losses: problem- --- heat is produce in copper coils when a current flows causing heat loss solution- --- use thicker wires (creates lower resist­ance) problem- --- some mag flux doesnt pass through the iron core reducing the flux lin of the secondary coil solution- --- reduced by keeping coils close/­wound together problem- --- eddy currents are induced, due to the mag flux created in coils, opposing the charge that produced it (Lenz's law) causing heat loss in the coil solution- --- laminating with insulating material --- using thin sheets so smaller emfs are induced

### Charged particles in a field

 a charge in a magnetic field has to be moving to experience a force. a free charged particle will undergo circular motion in this field. this is given by F = BQv for proving frequency is indepe­ndant of radius: BQv = mw2r BQwr = mw2r w = BQ/m 2pi.f = BQ/m therefore indepe­ndant

### Generators

 generators consist of a spinning coil in a magnetic field. -when the coil is parallel to the field there is no induced emf -when the coil is perpen­dicular to the field there is induced emf (the constantly spinning coil allows the induced emf to remain for longer as the field is constantly changing.) Peak EMF: E = BANw = 2BLv EMF at any time: E =Eo sin(wt) = BANwsi­n(wt) overhead cables are made from aluminium (light) with steel core (strong). copper would be too expensive

### induction

 an emf (and current) will be induced in a wire thats part of a loop if it expere­inces a changing field. lenz's law > demons­trated by dropping a magnet downa copper pipe. an eddy current is induced which produces a force that opposes the magnets motion (therefore slowing down as the magnet wants to accelerate down due to gravity the eddy currents create a force upwards slowing it down) for a moving wire: - must move perpen­dicular to the field lines flux = BA therefore flux now = BLd where L is the length of wire in the field and d is the distance in the perpen­dicular direction for moving loop: - emf is only induced as it enters­/leaves as this is where there is a change in magnetic field (change in magnetic flux) - its constant while inside the field for static coils: - B must be changed as A is not - this is done by using an AC current - if DC is used then the current will only be induced for a short amount of time

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