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Chapter 24 Magnetic Fields (24.3 (Cyclotrons (Particle Accelerators, Trap…
Chapter 24
Magnetic Fields
24.1
Magnetic field
- force field surrounding a magnet or a current carrying wire that acts on any of the magnet or current carrying wire placed in the field.
Lines close together represent strong magnetic forces
Field lines point north to south and form close loops
Flemings LHR
Motor effect
- current carrying wire placed in a field at non-zero angle to field will experience for due to field
Force is
Greatest when wire perpendicular to field
Zero when parallel to field
Thumb= Motion (F) First Finger= Field (B) Second Finger= Current (I) (Flows +ve to -ve)
Force increases if current increases (Force
current)
Force increases if a large length of wire is in the field (Force
length of wire)
Equation
F= BIL
Where B= magnetic flux density, I= current, L= length of wire in the field
only applies when at 90° to field
Couple on a coil in a field
Long sides of the coil of a vertical and perpendicular to the field- F=BIL - Form a torque as forces are in opposite directions
Perpendicular distance between them, d=wcosα
Torque= Fd sub in F=BIL and d=wcosα you get BILwcosα
Lw= Area (A) n=number of loops giving
Torque= BIAncosα
When α=0 cosα=1 (perpendicular), when α=90 cosα=0 (parallel)
Right hand thumb rule
Point thumb in direction of current (+ve to -ve) and fingers wrap round in direction of field
Electrons flow -ve to +ve (OPPOSITE TO CURRENT)
Circuit diagrams for current direction-
24.3
Charged particles move in a circle when travelling perpendicular to a magnetic field
Kinetic energy is unchanged and no work is done by the field as the force always acts perpendicular
Force causes centripetal acceleration because it is perpendicular to the velocity
Combine F=BQv and F=mv^2/r to get
r=mv/BQ
(mv =momentum)
R is proportional to V and m
R is inversely proportional to B and Q
Cyclotrons
Particle Accelerators
Trap particles in circular paths with ever increasing radii using magnetic fields
2 hollow d-shaped electrodes that the particles travel between
Dees produced magnetic field perpendicular to their combined surface
Particles enter centre of Dees and move in circles as a result of the magnetic field
Between gap of dees there's an electric field to accelerate the particles as they jump across
Field swaps every half cycle (ensures always accelerated)
As they accelerate radius increases until radius of Dee
Mass Spectrometry
They exploit movement of charged particles in magnetic fields to analyse particle mass
Particles with a larger specific charge will have a smaller radii
Steps
1)
Vaporise sample
2)
Accelerate ions using electric field (confine them to the same velocity)
3)
Fire them through a magnetic field
4)
Light ions deflect more- small radius
Uses
1)
Painting and tapestry analysis
2)
Mars science laboratory
24.2
Moving charged particles in magnetic field
Combine Q=It, F=BIL and V=L/t to get
F=BQv
When F=force, B=magnetic flux density and V=velocity
Hall Probe
Used to measure magnetic flux density
Contains a slice of semiconducting material
Pass current through it, electrons experience a force and deflected
Excess electrons build up on once side of slice inducing a pd
Creates a uniform electric field in opposite direction to magnetic field
When electric field and magnetic field are equal, electron won't be deflected
Electron Beams
Beams of electrons can be produced using a thermionic filament and potential
Electrons produced at filament and accelerated into a beam using the pd
If magnetic field directed at plane of screen, electrons are deflected due to force from magnetic field
Beam follows a circular path as force is perpendicular to motion
