SAC 3

SAC 3

1. Magnets

   • Filled with Dipoles
     • Dipoles randomly arranged
       • No magnetic effect
     • Dipoles arranged in one line
       • Magnetic effect
   • Soft or hard
     • Soft easy to de/magnetise
     • Hard hard to de/magnetise
   • Cannot seperate a N and S pole
   • Magnetic field
     • Region where a magnetic material experiences a force
       • E.g. compass
       • Force Formula
         • F=nBIL Where N = Number of wires, B = Field strength (Tesla), I = Current (amperes) and L = Length of wire in field (m) This works when perpendicular when not, F=nBILsin(theta). Where theta is the angle between current and field. If Current and Field is parallel, F=0
     • Forces Diagram
       • https://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/VFPt_cylindrical_magnet_thumb.svg/250px-VFPt_cylindrical_magnet_thumb.svg.png
     • Field lines travel from N to S
     • Earth's Magnetic Field
       • http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/_img/NewImages/Magnetic%20Susceptability/earth-magfield.jpg What we call the North Pole is actually the earth's South Pole. Due to this north magnets are attracted to it.
     • Vector
     • Magnetic fields are always apparent when current flows
       • Field Diagram and Right Hand Grip Rule
         • http://courses.ae.utexas.edu/ase463q/design_pages/fall02/wavelet/3_sour8.gif
       • Parallel Wires
         • http://www.physchem.co.za/OB11-ele/graphics/magnetic1_f8.gif
       • Right Hand Solenoid Rule
         • http://etc.usf.edu/clipart/35600/35671/rhrsole_35671_md.gif
       • Solenoid Field Diagram
         • http://www.met.reading.ac.uk/pplato2/h-flap/phys4_4f_4.png
       • When current flows field strength equals
         • B = (kI)/r Where k is a constant, I is current and r is radius.
       • If current and field are parallel, F=0
       • Right Hand Slap Rule
         • http://www.ph.unimelb.edu.au/~mkl/640162/prot/module1/image/RHruleI.gif
     • Field is stronger closer
     • On diagrams..
       • x goes into the page . goes out of the page Think of a feathered arrow. Cross at the back going away from you.
     • Field Strength (B)
       • Tesla (T)
       • Typically...
         • Permanent magnets typically are B = 10^-3 to 1T The Earth's field is B ~ 5x10^-5T
     • Force on an electron in a field
       • F= BvQ, where v = velocity and Q = charge (charge = 1.6E^-19 for electrons)
   • Flux
     • The amount of magnetic field in a region
       • Size and density
     • Wb = B x A
       • Where B = Field strength (T) and A = area (m^2)
       • Area not diameter!
         • Convert dimensions to m before calculating area
     • Amount of turns is irrelevant
     • If field and coil parallel, = 0
     • EMF
       • Voltage
       • Created by batteries, generators etc.
       • Formula
         • -nA*(Change in B/Change in time)    OR     -nB*(Change in A/Change in time)         FOR AVERAGE OVER TIME -n*(Change in flux/Change in time) N = Turns
         • Lenz's Law
           • As a magnet passes through a coil, a current is induced due to the presence of a magnetic field. This causes the coil to oppose this magnetic change by switching the direction of the current.
       • Currents are not always induced
       • Fleming's RH rule
         • http://www.diracdelta.co.uk/science/source/f/l/flemings%20right%20hand%20rule/image001.jpg
         • For coils...
           •  Can also be used for coils; do one side, then use direction to show current
     • Moving a conductor parallel to a field line changes nothing
     • To find Change in F rate, use quarter rotation, not full
     • No change in flux when completely in field

2. Motors

   • DC
     • http://electrical4u.com/electrical/wp-content/uploads/2013/03/dc-motor-parts.jpg
     • RHS can be applied
       • Results in Torque
     • Motor requires split ring commutator
       • http://fcw2.needham.k12.ma.us/~michael_h/Physics/FOV1-00112C20/FOV1-00112C2C/S045D68FC.0/1713_105349_0.png
       • Commutator repeatedly changes direction of current and force
     • To increase turning force increase n, B, I or L or use a wider motor.
   • Generators
     • DC (Dynamo)
       • "DC Motor in reverse"
       • You spin, it produces voltage/current
       • Max V when parallel to the field, Min when perpendicular

3. Transformers

   • RMS
     • AC RMS is equivalent of DC not RMS; should be equal
   • Only work on AC
     • Doesn't work on cells, batteries, etc.
     • DC can't have EMF induced as flux can' be time-varying
   • Changes number of turns on input compared to output
     • Amplifies/diminishes voltage by inducing EMF in the secondary coil
   • Power in = Power out for "ideal transformer"
     • E.g. If Voltage increases, current decreases (As P=VI)
   • Done in Ratios
     • N1/N2 = V1/V2 = I2/I1    Where N = Turns

4. Power transmission

   • Increase V to decrease I
     • P=VI
     • Less I, the more efficient
     • Power Loss Formula
       • P=I^2R
   • Heat is the mechanism by which we lose Power
   • Voltage Drop experienced across cables
     • V=IR (IR OF CABLES)
   • As power usage increases, so does current flow
     • Delivered voltage falls